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Open Access
Issue
A&A
Volume 691, November 2024
Article Number A158
Number of page(s) 21
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202449659
Published online 11 November 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model.

Open Access funding provided by Max Planck Society.

1. Introduction

Gamma-ray bursts (GRBs) are high-energy signatures of the death of massive stars (long-duration sub-class) or the merger of two neutron stars (short-duration sub-class); however, outliers to these associations have also been reported. While the prompt high-energy emission offers information about the activity of the central engine and the energy dissipation mechanism, the ensuing afterglow emission carries information about the GRB surroundings. Moreover, they can be used to probe the host galaxy and the Universe at large. Due to this versatility of the GRB afterglows, they have been the focus of GRB research for more than a decade, before Fermi observations have put the prompt gamma-ray emission again in the centre of attention and the gravitational-wave detection of GRB 170817A has given a boost to multi-wavelength observations of short GRBs.

Afterglow emission is detectable in high-energy gamma-rays (MAGIC Collaboration 2019) for a few hours, in the X-ray to the optical/near-infrared for days up to weeks, and in the radio band up to years after the gamma-ray burst. The Neil Gehrels Swift observatory (Gehrels et al. 2004) has pioneered systematic X-ray/UV/optical afterglow observations. An early surprise of these observations was the finding that the X-ray afterglow does not just fade exponentially, as the early predictions and observations suggested. Instead, the ‘canonical’ X-ray light curve consists of at least five different segments (Nousek et al. 2006). While, in X-rays, basically every GRB afterglow was detected whenever Swift slewed immediately, the detection rate in the optical/UV was only about 25% (Roming et al. 2009); thus, Swift’s optical afterglow light curve data set is less extensive. Despite this, Swift/UVOT (as well as many ground-based observations) have shown that the optical light does not trace the behaviour of the X-ray afterglow during the first hours of observations, which has spawned a number of investigations (Panaitescu et al. 2006; Ghisellini et al. 2009). Today, a large part of this mismatch posits a challenge in our understanding of the GRB afterglow phenomenon. The latest Swift/UVOT afterglow catalogue (Roming et al. 2017) is the largest coherent optical/UV sample of afterglows, with 538 GRBs observed. Due to its short-wavelength coverage, it is biased against dust extinction and high-redshift. Thus, higher sensitivity and near-infrared (NIR) coverage was in demand. Both of these improvements could be provided by GROND at the 2.2 m telescope. In this work, we describe and summarise the dedicated optical/near-infrared GRB afterglow observing program executed with GROND between May 2007 (commissioning) and September 2016 (when the MPE directorate terminated this program).

2. GROND and the afterglow observing strategy

GROND, a simultaneous seven-channel optical/near-infrared imager (Greiner et al. 2008) mounted at the 2.2 m MPI/ESO telescope at La Silla (ESO, Chile), was designed and developed to rapidly identify GRB afterglows and measure their redshift via the drop-out technique, using seven filter bands between 0.4–2.4 μm (g′r′i′z′JHKs). GROND was commissioned at the MPG 2.2m telescope over April and May 2007, and the first gamma-ray burst followed up on was GRB 070521 (Greiner et al. 2007). For the first few months (until end of September 2007), follow-up observations depended on the willingness of the scheduled observers to share observing time. Thereafter, a general override permission and a 15% share of the total telescope time allowed us to follow every well localised GRB that was visible from La Silla (weather permitting), with only a few exceptions (see statistics in the next but one section).

The GROND observations of GRBs within the first 24 hr were fully automated (see Greiner et al. 2008 for more details). Typically, each GRB was scheduled to be observed for its full visibility period on the first night. For subsequent nights, a human decision process kicked in, based on the results of the previous night. In general, nightly observations were performed for the first two to four nights, after which the coverage decreased depending on the brightness of the afterglow. In general, we attempted to follow each GRB with detected afterglow until it was no longer detectable with GROND in a 2-hr exposure. For nearby GRBs, the coverage was enhanced again after 10×(1+z) days to search for the GRB-supernova. In many cases, single late-time observations (after months to years) were performed, motivated by the search for the underlying host galaxy.

GROND data have been reduced in the standard manner (Krühler et al. 2008) using pyraf/IRAF (Tody 1993; Küpcü et al. 2008b). The optical/NIR imaging was calibrated against the Sloan Digital Sky Survey (SDSS)1 (Eisenstein et al. 2011), Pan-STARRS12 (Chambers et al. 2016), or the SkyMapper (SM) Survey3 (Wolf et al. 2018) catalogues for g′r′i′z′, and the 2MASS catalogue (Skrutskie et al. 2006) for the JHKs bands. This leads to typical absolute accuracies of ±0.03 mag in g′r′i′z′ and ±0.05 mag in JHKs. Since the GROND dichroics were designed to match the Sloan and 2MASS filter systems (Greiner et al. 2008), the colour terms are very small, below 0.01 mag, except for the i′ band (for details see Greiner et al. 2021).

Photometric redshifts were derived with the publicly available hyperZ code (Bolzonella et al. 2000), which minimises the χ2 from synthetic photometry of a template spectrum against the observed data. In addition to the built-in handling of the Lyman absorption according to Madau (1995) and several default reddening templates, the following features have been added: (i) a number of power-law spectra with different spectral index to account for the synchrotron afterglow spectra of GRB afterglows; (ii) neutral hydrogen absorption following the description of Totani et al. (2006) to account for damped Ly-α absorbers (DLA); (iii) a reddening law according to Maiolino et al. (2004); and (iv) the filter responses of GROND and Swift/UVOT, including all optical components between the primary telescope mirror and the detector, along with its quantum efficiency (Greiner et al. 2008; Poole et al. 2008). In a similar approach to the nominal hyperZ usage, we carried out a fitting for four variables simultaneously: power-law slope, redshift, host extinction, and normalisation (fixing the Galactic foreground extinction). Simulations show that photometric redshifts of GRB afterglows with the 12 filters of Swift/UVOT and GROND result in Δz/(1 + z) ≲ 0.1; this is substantially more accurate than for galaxies or active galactic nuclei (AGNs) due to the simple power-law shape of afterglow SEDs (Krühler et al. 2011a). A full description of the procedure, the simulations of the redshift-dependent error, comparisons with spectroscopic redshifts, and applications to the first sample of UVOT/GROND detected afterglows is given in Krühler et al. (2011a).

3. Observations

A total of 1018 GRB triggers with localisation errors smaller than 20′ occurred between May 2007 and September 20164. Out of these, 255 GRBs were at declinations north of +37 . d $ \overset{\text{d}}{.} $5 which is the northern-most declination reachable with GROND due to a minimum 22° horizon distance requirement of the 2.2m telescope (the northern-most burst observed with GROND is GRB 101008A at +37 . d $ \overset{\text{d}}{.} $06). Another 104 and 4 were too close to the Sun and Moon, respectively, to be observable. Out of these 655 observable GRBs, 519 (79%) were actually observed with GROND. The reasons for the 21% (136 GRBs) non-observations split into the following sub-groups: 8.5% (56 GRBs, = 41% of the non-observed) bad weather, 5.3% (35, 26%) GROND being off due to instrument or telescope (M1 coating) maintenance, 2.7% (18, 13%) missing override permission (during Chilean or MPIA time), 1.4% (9, 7%) technical problems of the telescope, and 1.2% (8, 6%) being purposely not observed (see Table 1), and 1.5% (10, 7%) due to positions available more than 48 hr after the event (mostly Swift/XRT-follow-up of IPN, AGILE, MAXI, or Fermi/LAT positions).

Table 1.

Eight GRBs purposely ignored for GROND observations.

For the subset of 829 Swift-detected GRBs with immediately (up to a few hours) well localised Swift/XRT afterglow positions during the period considered here, 533 were observable for GROND and 434 were actually observed. This implies a follow-up efficiency of these well-localised Swift-GRBs of 81%.

Out of the 519 observed triggers, 2 were later re-classified as galactic X-ray transients and, thus, they are not covered here any further; for 3 further GRBs (100225A, 110426A and 130310A), the original error boxes were much larger than the FOV of GROND, but nevertheless observed with multiple pointings in the hope that follow-up X-ray observations would better constrain the afterglow position. This didn’t happen for those three GRBs, so we continue with a sample of 514 GRBs. The distribution of these 514 GRBs in equatorial coordinates is shown in Fig. 1, separated into long- and short-duration (465/49) GRBs and labelled differently for detected (257 long and 15 short) and non-detected afterglows.

thumbnail Fig. 1.

Sky distribution in equatorial coordinates of the GROND GRB sample. We note the lack of extinction bias towards the Galactic plane, made evident based on the ratio of detected and non-detected afterglows. The apparent ‘concentration’ of observed GRBs towards the South Pole is due to a 2σ lack of triggers in the −40° < Dec < −50° range: 32 versus a mean of 50 (between −70° and +20°). These results are updated from Greiner (2019).

For 363 of these 514 observed GRBs, preliminary results of the first observing sequences were quickly reported via the GRB Circular Network (GCN), while 29 were published in refereed journals or conference proceedings without a GCN. In total, GROND afterglow measurements for 123 GRBs have thus far been published in refereed journals, namely, 94 GCN-notified afterglows have a follow-up paper.

For the remaining 122 previously unpublished GRBs, Table A.2 gives the detections or upper limits in all seven bands, with two changes: (i) two GRBs are omitted: GRB 100707A (the GROND follow-up was focussed on two PTF-detected candidates) and GRB 120522A (IPN error box covered with three GROND pointings), for which the lack of a second GROND epoch prohibits any statement on objects fainter than the SM and 2MASS catalogues; (ii) three GRBs (080703, 130211A, 140719A) were added to clarify (or improve) earlier GCN notifications (see text in Appendix A.1.6). Table A.2 includes ten new afterglow identifications, namely: GRBs 081127, 101017A, 120215A, 120302A, 130211A, 130725A, 140412A, 140619A, 140710B, and 150518A (marked in boldface in Table A.2) and one candidate (GRB 120328A), as well as new host detections and new or updated photometric redshifts. The majority of the other 27 GRBs with GROND-detected afterglows (Table A.2) include those where the GROND observations were either late or not did not go deeper (or more informative) than measurements already published in GCNs. The upper limits are not necessarily the very earliest GROND observations, but compromised for depth (either with higher photometric accuracy or deeper limits). Therefore, there are often stacks of multiple OBs where single outliers in seeing or ellipticity have been removed.

For selected GRBs with afterglow detections and/or interesting upper limits, the appendix provides context and more GROND observing details. Table A.1 lists the GRBs with new afterglow detections, new host identification, and new or corrected redshift estimates.

4. Statistics

4.1. Delay time and detection statistics

With a dedicated instrument, an automated telescope response on GRB triggers and (for most of the time) a generous override permission, it is interesting to look at the distribution of response times. For a total of 69 (out of 514) GRBs we managed to start observations immediately, within 30 min after the GRB trigger. This 13% fraction is to be compared against the expectation of about 30% (for a mean 8 hr duration of dark time per day) minus the losses due to weather, technical issues, and satellite constraints. In particular, the latter is a large factor for observations from South America: for about 40% of the time (6 out of 15 orbits per day), GRB detection is largely impossible from satellites in low-Earth orbit (like Swift or Fermi), while flying over South America, due to the passage of the South-Atlantic anomaly. Thus, our success rate for immediate follow-up is close to what is possible from La Silla (Chile).

The other important quantity is the detection rate of the optical/NIR afterglows of GRBs, which is detailed in Fig. 2 and Table 2. Not surprisingly, the discovery fraction is significantly higher when observations started during the first 30 min after a GRB. However, compared to our first counting after 3 years of operation (Greiner et al. 2011a), this is down from 91% to 81%. This can partially be explained with our increasingly more aggressive follow-up even at bad observing conditions, which increased the likelihood of non-detections due to lower sensitivity. However, the dominant factor for the overall decreased afterglow detection fraction is likely just small number statistics in the beginning (Greiner et al. 2011a). Also not surprisingly, the discovery fraction is substantially larger for long- than for short-duration GRBs. This is readily explained by the generally brighter afterglows of long GRBs. Finally, we note that due to our use of NIR filter bands, there is no detection bias with respect to extinction towards the Galactic plane, namely, the ratio of detected and non-detected afterglows is independent of the Galactic latitude (Fig. 1).

thumbnail Fig. 2.

Histogram of the time between GRB alert and start of the GROND observation in 1 hr bins, showing all 514 GRBs (black), and those with optical afterglow detection (red). The inset shows a zoom of the distribution during the first hour, in 5 min bins. The fastest reaction time was 78 s (for GRB 130514A).

Table 2.

GROND afterglow detection fraction, separately for the 465 long- and 49 short-duration GRBs, as a function of time delay of the start of the observation after the GRB trigger, based on a total of 514 bursts.

Somewhat unexpected, however, is the fact that our discovery fraction for long GRBs declines only slowly over the next hours (Table 2). Since our sensitivity (limiting magnitude) does not change with the delay time to the GRB, this suggests that other effects play a role. Partly, this is an observational bias: initially bright afterglows can be detected for longer times than faint ones, independent of their decay slope. But plateaus (080129 (Greiner et al. 2009c), 091127 (Filgas et al. 2011b), 121217A (Elliott et al. 2014), 191016A (Pereyra et al. 2022), 191221B (Zhu et al. 2024)), or even re-brightening episodes (081029 (Nardini et al. 2011), 100621A (Greiner et al. 2013), and 100814A (Nardini et al. 2014)) seem to play a larger than expected role at least during the first 24 hr of a GRB. This is also suggested by our sample of the 60 best-covered (with GROND) afterglow light curves, where about one third have only decayed by about 2-3 magnitudes over the first 12 hours (see future part IV of this publication series).

For later times, namely, beyond 24 hr after a GRB, we caution that there is a technical bias: Fermi/LAT positions have typically been reported after one day, delaying early observations. Since, on average, those GRBs are more energetic (McBreen et al. 2010; Cenko et al. 2011), with correspondingly brighter afterglows, their detection rate certainly enhances our late-time detection fraction.

It might also be interesting to compare against the nominal UVOT detection rate of 25% (Roming et al. 2009). For our sample of 62 GROND-observed (within 30 min) long-duration GRBs, we excluded 3 GRBs without and 6 GRBs with late Swift slews. From the remaining 53 GRBs, UVOT detected 22 afterglows, corresponding to a 41% fraction, while GROND detected 45 (85%). Out of the 23 GROND- but not UVOT-detected afterglows, 9 have a redshift > 3, 9 are below the typical UVOT sensitivity of B ∼ 22.0 mag (incl. 4 with a redshift < 3), 3 suffer from large (AV > 1 mag) foreground absorption, 1 was reported with a 2.5σ detection at the GROND afterglow position, and 1 should have been detectable with a different filter exposure distribution. Thus, from this limited sample, we derived the following fractions for the three reasons of UVOT-non-detection (each with at least a 5% error): 39% due to high redshift, 13% due to (foreground) dust extinction, and 48% due to simply not enough sensitivity.

A comparison with the gamma-ray fluence is made in Fig. 3, showing the histograms of the Swift/BAT 15–150 keV gamma-ray energy fluence distribution of the GRB prompt emission (Lien et al. 2016), computed for simple power-law fits, for detected and non-detected afterglows, separately for long- and short-duration GRBs. The nearly identical distributions demonstrate the well-known lack of correlation between prompt emission and afterglow detection rate. Figs. 4 and 5 finally provide a distribution of observed GRBs over satellite origin, and a visual comparison of the detection rates in X-rays, optical/NIR, and radio based on our sample of 514 GRBs.

thumbnail Fig. 3.

Histogram of the 15–150 keV GRB fluences as measured with Swift/BAT for the detected (black) and non-detected (red) afterglows. For each category, we show the long- (dashed lines) and short-duration (solid lines, hashed) GRBs separately. There is only a weak tendency for bright GRBs to be better detectable than faint GRBs.

thumbnail Fig. 4.

Pie diagram of the satellite-origin of the 514 GROND-observed GRBs: the outer circle represents the overall ratio of the missions providing the trigger, the inner circle depicts the ratio of detected (darker colour) vs. non-detected (lighter colours) optical/NIR afterglows. The thin slice above MAXI is the optically identified iPTF14yb transient, which later was related to GRB 140226A based on a Mars/Odyssey, Konus-Wind and INTEGRAL data. The vast majority of GROND-observed are from Swift and thus have XRT-positions and spectra.

thumbnail Fig. 5.

Venn diagram of the detected afterglows out of the sample of 514 GROND-observed GRBs visible from Chile between 21 May 2007 and 1 Oct 2016, i.e. 458 of the 514 GROND-observed GRBs have an immediate Swift/XRT detection, 272 a GROND detection, 268 a combined GROND and Swift/XRT detection, and so on. Radio detections are taken from https://www.mpe.mpg.de/~jcg/grbgen.html.

4.2. Peak brightness distribution and dominance of X-ray over optical flaring

The brightness of the GRB afterglow at the first detection with GROND is shown in Fig. 6 for all observations starting within 4 hr of the GRB trigger. We chose to show the J(AB) magnitudes as a compromise between minimising the effect of dust extinction (AV) with respect to the optical bands and optimising GROND’s sensitivity.

thumbnail Fig. 6.

Sample of 122 long GRB afterglows observed with GROND within the first 4 hr, coloured according to redshift. For bright afterglows, these are the first 4-min exposures; for fainter afterglows, stacking of multiple exposures has been applied (as indicated by horizontal ‘error’ bars) and a compromise between amount of stacking and time after the GRB has been adopted. The sensitivity limit of GROND@2.2m is given as dashed line, assuming a start of the exposure at 150 s and an exposure of 60 s, thereafter improving as time progresses: at early times, this is close to the typical t−1 afterglow fading rate. For those afterglows with J-band upper limits but z′-detections, we plotted an open square at the expected J-band value (using a mean colour of z − J = 0.15 mag), connecting it with the downwards arrow with a dotted line (GRB 120328A at 6534 s has z′ = 23.79±0.35, outside of the plot). The sky background limits our sensitivity to J(AB) ≲ 22 mag, irrespective of exposure duration.

A noteworthy aspect of this distribution is the difference in the dynamic range of the optical/NIR versus X-ray afterglows. Figure 6 shows the former to be of the order of a factor 600 (7m) during the first 900 s. In contrast, for the sub-sample with Swift/XRT afterglows, the difference between brightest and faintest X-ray emission is a factor 104 (for this estimate, we have omitted the first 100 s interval in order to not be biased by the prompt-to-afterglow transition and the so-called tail emission). This large diversity in the X-ray emission is consistent with earlier estimates (e.g. Berger et al. 2003), but larger than that of D’Avanzo et al. (2012)5. Late-peaking optical forward shock emission is rare beyond a few hundred seconds and, thus, cannot be the cause of this difference. The missed afterglows are obviously fainter, but would have to be at least 3 mag below our sensitivity line to make up the difference, corresponding to J(AB) ≳ 24–25, or r′(AB) ≳ 26–27 mag. Similarly unlikely are unaccounted for host-intrinsic absorption/extinction effects in the detected optical/NIR afterglows: while not being corrected for, very substantial amounts of dust are needed, given our use of the J-band fluxes with AJ = 0.29AV. One possible explanation could be the prevalence of X-ray flaring which is not accompanied by proportionally increased optical emission (e.g. Krühler et al. 2009a; Uehara et al. 2010).

4.3. Low incidence of bright reverse shocks

The brightness distribution of the early afterglow emission (Fig. 6) is remarkable for another reason, namely that very bright afterglows with < 13 mag (AB) are very rare. GRB 080319B, the first naked eye afterglow, is well-known in the community, partly because it still is only one out of a handful afterglows brighter than the above limit after the detection of about 900 optical afterglows6. This is consistent with wide-field searches for afterglows in the 1990s (Greiner et al. 1996; Hudec et al. 1996) and corroborated by the low discovery rate of orphan afterglows in the wealth of present-day all-sky monitoring observations. Allowing a reverse shock decline with t−2…t−3 (Kumar & Panaitescu 2000; Nakar & Piran 2004) during the first 1000 s, and the canonical t−1 at later times, back-extrapolating the first readings of our 122 GRB afterglows results in only 6 to rise above 13 mag (AB) at time < 300 s after the GRB. Such a 5% rate of observable reverse shocks is already higher than the about 20 suggested RS interpretations (out of 900 afterglows) in the literature (e.g. Oganesyan et al. 2023), but consistent within the statistics and in line with theoretical considerations (McMahon et al. 2006).

4.4. Correlated optical and radio detection probability

For a check of the relative detection probabilities in the optical and radio bands, we used the Swift/XRT sample. Among the 272 optically detected afterglows, 32 (12%) have a reported radio detection. In contrast, only 6 (2.5%) radio detections are reported for the 242 optically non-detected afterglows. In their review of radio afterglow detections, Chandra & Frail (2012) have shown (their Fig. 18) that for optically bright afterglows the radio detection rate is substantially higher than for optically faint afterglows. While our statistics is lower than that of Chandra & Frail (2012), our finding is consistent with this correlation.

4.5. The dark and high-z fraction

The distribution of Fig. 6 for the 2007–2010 GROND operation phase had been used to estimate the fraction of dark bursts to 25–40% depending on definition (Greiner et al. 2011a). Since that sample had a afterglow detection rate of 90% and a redshift (z) completeness of 92%, the fraction of high-redshift GRBs could be separated off, revealing 5.5 ± 2.8% of GRBs at z > 5 (Greiner et al. 2011a). The present sample, more than 3x larger, unfortunately does not reach these completeness levels: the detection rate is 71% (87/122), and the redshift completeness is 56% (69/122). Thus, the dark and the high-z fraction cannot be improved.

5. Conclusions

The dedicated and systematic GRB follow-up program with GROND at the 2.2 m telescope has substantially increased the detection rate of GRB afterglows, by, for instance: (i) being first in reporting the afterglow detection in 75 cases (only surpassed by Swift/UVOT with 92 afterglows, and counting) or (ii) by providing afterglow identifications for 32 GRBs (corresponding to 12% of all GROND-detected afterglows), which no other group has detected. This is particularly true for fainter afterglows, beyond the reach of robotic telescopes (typically < 1 m) or dust-extinguished GRBs. The GROND program also helped in identifying high-redshift GRBs. While GROND’s record-breaking (photometric) redshift GRB 080913 (Greiner et al. 2009b) lasted for only a few months (when the z = 8.2 GRB 090423 took over), the number of identified 3 < z < 6 redshifts, easily seen with GROND as g′- or r′-drop-outs, has substantially decreased since 2017. GROND’s seven channels were very useful for photometric redshift estimates, allowing for the selection of particularly interesting GRBs for spectroscopic follow-ups. The lack of a filter bluer than 400 nm has limited the frequent application to redshifts ≳3, however, with contemporaneous Swift/UVOT observations (Krühler et al. 2011a), this method could be extended down to z ≈ 1, thereby covering the peak of the GRB redshift distribution.

The multi-channel coverage of GROND results in a SED from 400–2400 nm, which offers additional benefits for various applications; for instance, in identifying the afterglow among many candidates due to the power-law shape of the SED and measuring of dust extinction (local as well as host-intrinsic), with increased confidence in low-significance objects by requiring detections in more than one spectral band. More details on these benefits, examples, and non-GRB applications are given in Greiner (2019).

In terms of detection fraction, a 2 m class telescope on the ground is still not sufficient to identify all afterglows, even in the NIR bands, as we still miss the ≈20% faintest ones. For a higher rate of follow-up of GRBs detected from low-Earth orbit, an observing site away from the longitudes of the South-Atlantic Anomaly would be beneficial. Alternatively, a NIR multi-channel camera on a small satellite could substantially increase the detection rate: with a 50 cm diameter telescope, a detection fraction close to 100% in the J or H band is feasible (Thomas et al. 2022; Greiner & Laux 2022).

Data availability

Tables A.2 and B.1 are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/691/A158

4

See https://www.mpe.mpg.de/~jcg/grbgen.html for a complete list.

5

The reasons for this mismatch could be, among others, (1) it actually provides luminosities, not fluxes; the folding with the distances changes the flux distribution (statistically, many nearby GRBs are sub-luminous); (2) their sample is based on a brightness-cut of the γ-ray emission, i.e. fainter GRBs are purposely omitted; (3) their sample is a factor 10 smaller, implying that a lesser amount of extremes (at both ends of the distribution) were sampled.

Acknowledgments

This publication is dedicated to David Alexander Kann who passed away in 2023 at the age of 46, just a day after some of us had the chance to discuss recent GRB news with him. Being a member of the GROND team since 2011, Alex was our encyclopedic dictionary for nearly every GRB mentioned herein. He was eagerly anticipating this manuscript, but we were too late. We deeply miss him and will always remember him. SK acknowledges support by DFG grants Kl 766/11-3, 13-1 and 13-2. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. Facilities: Max Planck:2.2m (GROND), Swift.

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Appendix A: Notes on individual sources I

This section provides previously unpublished GROND information on either new afterglow discoveries, new host detections or new redshifts as summarised in Table A.1. We refrain from reviewing the full afterglow observation history for each source and just mention what is important for the context of the GROND observation and the afterglow characterisation.

All GROND magnitudes (including JHK) are in the AB system and are not corrected for foreground extinction, unless specifically mentioned. Upper limits are at a 3σ confidence.

Table A.1.

Summary of the new findings in this study.

A.1. Newly discovered afterglows or candidates

A.1.1. GRB 081127

For this Swift/BAT GRB, a bright, fading X-ray afterglow was immediately found (Mao et al. 2008), but no UVOT detection (Holland & Mao 2008a). There are no reports of any optical or radio observations. GROND started observing about 18 hrs after the GRB, and a source is found in the g′r′i′z′ bands within the 1 . ″ $ \overset{\prime \prime }{.} $8 XRT error circle. Photometry in the bluer bands (Table A.2) might be affected by a bright star about 10″ north of this source (Fig. A.1). A second epoch on Aug. 26, 2016 at better seeing conditions shows no further emission, implying fading by at least 2.5 mag in the two bluest filters bands (g′ > 25.9 mag, r′ > 25.7 mag, i′ > 24.8 mag, z′ > 24.3 mag). Thus, we propose this source, at RA (J2000) = 22:08:15.43, Decl.(J2000) = +06:51:02 (±0 . ″ $ \overset{\prime \prime }{.} $3), to be the optical afterglow.

thumbnail Fig. A.1.

GROND z′-band image of GRB 081127 from the 55 min co-add of the first night (left), showing the optical afterglow within the 1 . ″ $ \overset{\prime \prime }{.} $8 XRT error circle (drawn here with a 3″ radius for better visibility), and the second epoch from 26 Aug. 2016 (right).

A.1.2. GRB 101017A

The XRT/UVOT observations of this long and bright Swift-detected GRB started 81 s after the trigger, providing an X-ray position and a UVOT counterpart at 20th mag (Siegel et al. 2010). GROND observations started immediately after the end of twilight, on 2010-10-18 00:13 UT and lasted for 2 hrs. A second observation was obtained on 2012-05-21, at better seeing conditions. This clearly reveals extended emission very close to the UVOT position, which we suggest to be the host galaxy of GRB 101017A (right panel of Fig. A.2). In stacked images of the first epoch, we can also identify emission in excess of the host emission (left panel of Fig. A.2), which is about 3 mag fainter than the UVOT detection. We measured RA(J2000) = 19h 25m 32.​​s52 and Decl.(J2000) = -35°08′ 40.​​s9 (±0 . ″ $ \overset{\prime \prime }{.} $3), slightly north but consistent with the UVOT position. For the host galaxy, we measured g′ = 23.6±0.1 mag, r′ = 22.9±0.1 mag, i′ = 22.3±0.2 mag, z′ = 22.0±0.2 mag. The U-band detection with Swift/UVOT (Siegel 2010) suggests a redshift of < 2.5.

thumbnail Fig. A.2.

GROND images from the co-add of g′r′i′ stacks of the afterglow (left, red circle) and the host galaxy (right) of GRB 101017A. The circle denotes the GROND position of the afterglow, slightly north but consistent with the UVOT position.

thumbnail Fig. A.3.

GROND r′i′-band images from 2 days after GRB 120215A (left), and 7 months later (right) with the XRT error circle (black) and the GROND afterglow (red).

A.1.3. GRB 120215A

Based on Skynet observations starting 69 sec after the Swift-detected GRB, LaCluyze et al. (2012) reported the lack of an immediate counterpart detection, but a later (20 min post-burst) faint source at B = 20.32 − 0.27 + 0.36 $ ^{+0.36}_{-0.27} $ mag and I = 19.10 − 0.23 + 0.29 $ ^{+0.29}_{-0.23} $ mag. No significant variability was found. GROND observations only commenced 2 days later and revealed a faint source at the edge of the Swift/XRT error circle (Evans et al. 2012), at RA (J2000) = 02:00:11.42, Decl. (J2000) = 08:48:08.6, (± 0 . ″ $ \overset{\prime \prime }{.} $2) (Fig. A.3). The results of the forced photometry are given in Table A.2. Assuming that this is the same source as reported by LaCluyze et al. (2012) (their report does not provide a position), this suggests fading by about 3 mag. A (longer) GROND observation at better seeing on 2012-09-20 provides upper limits about 2 mag deeper, thus establishing fading of at least 5 mag, and thus the very likely afterglow nature: g′ > 25.4 mag, r′ > 25.0 mag, i′ > 24.6 mag, z′ > 24.0 mag, J> 21.2 mag, H> 20.7 mag, and K> 20.0 mag.

Table A.2.

Measurements of either unpublished GROND-observed GRBs or those with updates relative to published GCNs. Only GRBs with particular importance (and described in the appendix in detail) are listed here; the full table is available at CDS.

A.1.4. GRB 120302A

This Swift-BAT detected GRB was found in automated ground analysis (Sakamoto et al. 2012) and the first GROND observation covered the later-derived X-ray afterglow position only in the JHK channels. The second GROND observation revealed the newly found NIR source at the X-ray position at a similar brightness level (Elliott et al. 2012b), as seen in Table A.2, but given that (i) the source was not visible in the SDSS, albeit brighter than the SDSS depth, and (ii) the SED slope was a GRB afterglow-typical power law with a slope of 1.4, we had argued for this source as the optical counterpart candidate (Elliott et al. 2012b). A third GROND epoch was taken three weeks later (starting March 26, 00:52 UT) which provided clear evidence for fading, thereby securely identifying the optical afterglow: g′ = 25.00±0.22 mag, r′ = 24.79±0.25 mag, i′ > 24.1 mag, z′ > 23.8 mag, J > 21.5 mag, H > 20.9 mag, and K > 20.2 mag.

thumbnail Fig. A.4.

GROND r′-band images from 2 days after GRB 120302A (left) and 3 weeks later (right) The black circle denotes the Swift/XRT position of the afterglow, while the red circle shows the afterglow and host.

A.1.5. GRB 120328A

The bright X-ray afterglow of this Swift-detected long-duration GRB was rapidly found with Swift/XRT observation (Pagani et al. 2012). Skynet observations at 4 min after the GRB did not reveal an optical afterglow, with 3-sigma limiting magnitudes of V = 17.92 mag, R = 18.73 mag, I = 18.15 mag (Haislip et al. 2012). GROND observations started about 2.5 hrs after the GRB. In stacked observations we detect a faint source in r′ (Table A.2) within the 1 . ″ $ \overset{\prime \prime }{.} $4 Swift/XRT error box (Beardmore et al. 2012). This source is marginally visible in r′i′z′ (Fig. A.5), for which the forced photometry, r′ = 25.75±0.32 mag, gives i′ = 24.60±0.40 mag, and z′ = 23.79±0.35 mag. No second epoch was obtained, so no statement on fading can be made. While the SED looks consistent with a red afterglow, we suggest caution with respect to the substantial foreground galactic of AV = 2.3 mag.

thumbnail Fig. A.5.

GROND image from the co-add of r′i′z′ stacks of the afterglow candidate (red circle) of GRB 120328A. The black circle denotes the Swift/XRT position of the X-ray afterglow.

A.1.6. GRB 130211A

GROND observations of this Swift-detected GRB (Oates et al. 2013) started within 30 min under poor weather conditions, but the optical source detected in the Swift/XRT error circle (Knust et al. 2013) turned out to be constant (Sudilovsky & Greiner 2013). Later re-analysis of the GROND data using the UK Swift/XRT repository listing of the UVOT-enhanced X-ray position (which had shifted by 8″) revealed a fading optical source within the XRT localisation, at RA(J2000) = 09h 50m 08.​​s66, Decl.(J2000) = -42°20′ 38 . ″ $ \overset{\prime \prime }{.} $0, ±0 . ″ $ \overset{\prime \prime }{.} $3 (Fig. A.6). During the first 25 min observations (mid-time 4:10 UT), this object is only seen in the r′i′ bands, at r′ = 23.79±0.28 mag, i′ = 22.88±0.21 mag. Stacking of several OBs at later times (longer exposure and better conditions) leads to detections in five bands, with the magnitudes as reported in Table A.2. A late epoch was taken on Jan 10th, 2016, resulting in no detection in any band, with upper limits of r′ > 25.8 mag, i′ > 24.9 mag, z′ > 24.4 mag, providing evidence of > 2 mag fading. Despite the large foreground reddening of E(B − V) = 0.53 mag, there is indication for a Ly-α drop of the g′-band, suggesting a redshift of z ≈ 3 (Fig. A.7). The above deep upper limits for any host emission at and around the afterglow position are consistent with such an interpretation.

thumbnail Fig. A.6.

GROND image from the 94 min i′ stack (left) and a later epoch (right), demonstrating the fading of the afterglow. The large circle denotes the Swift/XRT afterglow error circle, the small red one encircles the afterglow position.

thumbnail Fig. A.7.

Fit to the GRB 130211A afterglow SED, using the foreground extinction-corrected GROND data with SMC-type host-intrinsic extinction.

A.1.7. GRB 130725A

Due to observing preference given to GRB 130725B, the Swift-detected GRB 130725A was observed with GROND only one night later. Weather conditions were mediocre, but a source was successfully detected within the 1 . ″ $ \overset{\prime \prime }{.} $8 Swift/XRT error circle (Zhang 2013) in the g′r′ bands (the i′z′ bands suffered from a temporary electronics problem, so no data are available). This is about 3 mag fainter than the R = 20.8±0.2 mag candidate reported by Kuroda et al. (2013) for their observation at 2 hrs after the GRB, thus establishing this to be the afterglow. We measured RA(J2000) = 15h 20m 07.​​s71, Decl.(J2000) = +00°37′ 39 . ″ $ \overset{\prime \prime }{.} $8, with an error of ±0 . ″ $ \overset{\prime \prime }{.} $2.

thumbnail Fig. A.8.

GROND image from the co-add of g′r′ stacks of the two OBs. The circle denotes the Swift/XRT afterglow position.

A.1.8. GRB 140412A

GROND observations started about 1 hr after this Swift-detected GRB, at the end of evening twilight, under very good conditions. No fading source is seen within or close to the Swift/XRT error circle of the fading X-ray afterglow (D’Elia et al. 2014; Goad et al. 2014). Observations continued for the whole night and subsequent 2-hr long observations were carried out on the second and third night. A stack of 194 min exposure towards the end of the first night reveals a faint source in the z′-band (z′ = 24.22±0.24 mag) at the edge of the XRT error circle. A stacking of the first four exposures (totalling ≈25 min) uncovers this source at z′ = 23.32±0.32 mag (see Table A.2), and a stack of the third night (totalling ≈100 min) finds it at z′ = 24.25±0.33 mag (Fig. A.9). While formally consistent with each other within 3σ, we propose the early emission to be the afterglow, while the faint emission, being constant over 48 hrs, to stem from the host galaxy. The unusual GROND non-detection in the somewhat more sensitive r′-band can be explained by the substantial Galactic foreground extinction of AV = 0.57 mag (Schlegel et al. 1998).

A.1.9. GRB 140619A

thumbnail Fig. A.9.

GROND images from the co-add of 25 min from the first night (left, afterglow) and 100 min of the third night (right, host). The black circle denotes the Swift/XRT position of the X-ray afterglow of GRB 140412A, and the red circle marks the position of the afterglow and host.

GROND observations started about 22 hrs after the Swift GRB trigger (De Pasquale et al. 2014), and a source consistent with the Swift/UVOT position (Siegel et al. 2014) was detected in the g′r′i′z′ bands. This fading of 3 mag since the UVOT epoch demonstrates its afterglow nature. Another GROND observation was performed two months later, on Aug. 18, 2014, at good seeing conditions. We still detected emission at this position (Fig. A.10), about 0.6 mag fainter in r′i′z′, suggesting the detection of the host and proposing that the emission seen during the first GROND epoch is mostly afterglow (except in g′). We also obtained a marginal J-band detection, at J = 21.31±0.32 mag. If confirmed, the very red z′-J = 2.4±0.5 could be interpreted as the Ca-HK break, suggesting a redshift of 1.9±0.5.

thumbnail Fig. A.10.

Swift/UVOT white (left) and GROND g′ image (right) of GRB 140619A. The ≈3 mag fading confirms the afterglow nature of the UVOT detection, and the GROND detection two months later is likely the GRB host galaxy. The circle (radius 1 . ″ $ \overset{\prime \prime }{.} $4) is to guide the eye; the Swift/XRT error circle (1 . ″ $ \overset{\prime \prime }{.} $7) is omitted for better legibility.

A.1.10. GRB 140710B

GROND observations of this INTEGRAL-detected GRB (Götz et al. 2014) started about 80 min after the GRB trigger and continued for 4 hrs. A second epoch was taken on the following night. Despite the crowded low galactic latitude field, we identified (in all bands except g′) a fading source within the 1.​​′7 INTEGRAL/IBIS error box (Fig. A.11), at coordinates RA(J2000) = 13h 38m 31.​​s68, Decl.(J2000) = −58°35′ 01 . ″ $ \overset{\prime \prime }{.} $3, ±0 . ″ $ \overset{\prime \prime }{.} $3. After correcting for the substantial Galactic foreground extinction of AV = 2.7 mag (Schlegel et al. 1998), the SED is a straight power law with a slope of 1.4±0.1. The combined evidence of fading nature and the afterglow-typical SED suggests it to be the afterglow of GRB 140710B.

thumbnail Fig. A.11.

GROND z′-band image from the first 10 min OB from the first night (top left; mid-time 2014-07-10T23:09), the stack of two 30 min OBs from the next night (top right’ mid-time 2014-07-12T01:13), and the GROND light curve with a best-fit slope of 1.2±0.1 (bottom). The 1.​​′7 INTEGRAL error circle is outside this figure, but the combined evidence of clear fading and typical power-law SED identifies this source without doubt as the afterglow of GRB 140710B.

A.1.11. GRB 150518A

Due to missing override permissions, GROND observations of the Swift/XRT-detected X-ray afterglow (Sbarufatti et al. 2015) of this MAXI-detected GRB (Kawamuro et al. 2015) could only start 30 hrs after the GRB trigger. We clearly detect the unresolved afterglow plus host as first reported by Xu et al. (2015a). Our g′-band detection is still 0.3 mag brighter than the corresponding SDSS g′-magnitude, while in the other visual filters, we see an excess emission of only 0.1-0.2 mag. A second GROND epoch was done on May 22nd, during which we could only see marginal g′-excess relative to SDSS. However, we did see significant fading in the JHK-bands (down to J = 20.66±0.12 mag) between our two GROND epochs (Fig. A.12), providing additional evidence for the afterglow being situated within the SDSS J153648.25+161946.9 galaxy.

thumbnail Fig. A.12.

GROND J-band images from the co-add of two 20 min OBs each from the May 20th (left), and May 22nd (right), respectively, of GRB 150518A. The red circle is the 2 . ″ $ \overset{\prime \prime }{.} $6 XRT error circle. We also note the slight shift of the centroid of the emission.

A.2. Newly detected host galaxies

Information on GRBs 101017A, 140412A, and 140619A is given in Sect. A.1.

A.2.1. GRB 070917

This GRB was detected with Swift/BAT at 07:33:56 UT, with automatic slewing of Swift disabled (Cummings et al. 2007). Swift/XRT observations started 35 ks after the trigger, and localised the X-ray afterglow to 8″ accuracy (Evans et al. 2007). Optical observations with Gemini South revealed a faint source inside the XRT error circle at R ∼ 22.0 mag, i′ ∼ 21.6 mag (Cenko 2007), which was found to fade with NOT observations, finding R = 23.1 mag (Fynbo et al. 2007). GROND observations started at 23:48 UT, more than 16 hrs after the burst, lasting about 4 hrs (see Fig. A.13 for a finding chart). A second epoch was done on the following night. We confirm fading in i′z′ between our first two epochs by about 0.5 mag, amounting to about 1.3 mag fading relative to the early observations of Cenko (2007). We had no detection in g′ (note the large Galactic foreground reddening of E(B-V) = 0.45 mag) and did not see fading in r′, suggesting a contribution from the host galaxy. Indeed, our 2009 epoch, performed at 0 . ″ $ \overset{\prime \prime }{.} $8 seeing, still shows faint emission consistent with the second epoch brightness, at r′ = 23.6 mag, i′ = 23.3 mag, z′ = 23.7 mag, thereby supporting a host interpretation.

thumbnail Fig. A.13.

GROND image from the co-add of r′i′z′ stacks of GRB 070917 of the first night, showing the afterglow and the improved 4″ X-ray error circle.

A.2.2. GRB 071117

This 5-sec duration GRB was detected with Swift/BAT at 14:50:06 UT, whereas the X-ray localisation was delayed by ∼45 min due to an Earth limb constraint (Ukwatta et al. 2007). GROND observations started ∼9.5 hrs after the GRB, and continued for 5 hrs. Further epochs were observed on the following two nights (November 19 and 20), as well as on November 22 and 24. A final epoch was observed on October 24, 2008. Similar to the finding of Bloom (2007a,b) with Gemini South, we found two optical sources in the initial XRT error circle (at that time not yet UVOT-corrected), with s1 being constant and s2 clearly fading between the first two epochs. Our late-time epoch from 2008 shows extended emission underlying source s2, which we interpret as the host. A spectrum with the ESO VLT/FORS1 instrument revealed a weak emission line at 8688 Å, interpreted as [OII], with an inferred redshift of z = 1.331 (Jakobsson et al. 2007). Due to the relatively bright host, we detected the afterglow only in r′i′z′; already in the first epoch, the host is seen to dominate in the g′-band. In Fig. A.14, we show r′i′ co-adds of a 20 min observation from the first night (left panel), and a 80 min stack of the last epoch. The host galaxy is detected at g′ = 23.5±0.2 mag, r′ = 23.8±0.1 mag, i′ = 23.3±0.1 mag, z′ > 23.8 mag, J> 21.1 mag, H> 20.7 mag, K> 19.5 mag.

thumbnail Fig. A.14.

GROND images from the co-add of r′i′ stacks of the GRB 071117 afterglow (left: red circle) and the host galaxy (right). The latest (retrieved 2021) UVOT-enhanced Swift/XRT position is overplotted and nearly exactly centred on the host galaxy.

A.2.3. GRB 080523

This ∼15 sec duration Swift-detected GRB (Stroh et al. 2008) had an immediate X-ray afterglow detection. Fynbo et al. (2008) reported the ESO/VLT detection of a source inside the X-ray error circle, while Malesani et al. (2008) reported fading by at least 1.5 mag based on a second VLT observation. Since no magnitudes were reported, we list in Table A.2 the g′r′i′z′ magnitudes of the GROND observations on the first night (nearly simultaneous to that of the VLT) and confirm the fading based on a GROND observation on Nov 19, 2014. In the second epoch, no source is detected longward of the i′-band, with upper limits of i′ > 23.8 mag, z′ > 23.4 mag, J> 20.5 mag, H> 19.9 mag, and K> 18.8 mag. However, we clearly detect a source in the optical, with g′ = 24.02±0.29 mag and r′ = 24.69±0.30 mag. The spatial coincidence and the substantially bluer colour with respect to the afterglow suggest that this might be the host galaxy, providing an opportunity to determine the redshift, which had not been possible to deduce from the afterglow spectrum, apart from a z < 3 limit (Malesani et al. 2008).

thumbnail Fig. A.15.

GROND images of the GRB 080523 X-ray afterglow position (black circle), showing the optical afterglow (left, red circle) from the first night’s r′-band observation (as given in Table A.2), the r′-band observation 6 yrs later demonstrating the fading of the afterglow (middle), and the g′-band observation of the same epoch showing the strong blue colour of the emission directly underneath the afterglow position, consistent with a putative host galaxy.

A.2.4. GRB 110818A

GROND observations of this Swift-detected GRB were only possible on the second night. We did detect a source at the ESO/VLT afterglow candidate position (D’Avanzo et al. 2011), at about 2 mag fainter brightness (see Table A.2); thereby we were able to establish fading and confirm the afterglow nature (which, so far, had only been implicit from the redshift measurement via absorption lines (D’Avanzo et al. 2011; Krühler et al. 2015). On the third night, we still detected the source, at similar magnitudes, suggesting that we see the host galaxy at z = 3.3609 (D’Avanzo et al. 2011; Krühler et al. 2015).

A.3. New or updated photometric redshifts

Information on GRBs 130211A and 140619A is given in Sect. A.1.

A.3.1. GRB 080516

This multi-peaked Swift-detected GRB (Holland et al. 2008b) is located in (behind) the Galactic plane, yet an X-ray afterglow was readily discovered. In the beginning, it only faded slowly, before changing to a nominal decay after 4800 s (Page & Holland 2008). GROND observations started 8 min after the GRB trigger and revealed an afterglow candidate (Filgas et al. 2008a) at the corner of the X-ray error circle (Fig. A.16), which had faded below the sensitivity threshold 22 hrs later (Filgas et al. 2008b). The SED of stacked observations of the first night shows a clear Ly drop-out, leading us to report a photometric redshift of 3.2±0.3 (Filgas et al. 2008b). A re-analysis of the data with PS1 (DR2) photometry in the field (also provided in Table A.2) suggests an even higher redshift. Using a foreground Galactic E(B-V) = 0.35 mag (Schlafly & Finkbeiner 2011) and the SMC dust extinction law returns a photometric redshift of 4.1±0.1, with a spectral slope of β = 0.36 ± 0.14 and no additional intrinsic extinction. LMC or MW dust lead to somewhat lower redshift solutions (3.8 and 4.0, respectively), but at an unusually flat β ≈ 0.

thumbnail Fig. A.16.

GROND images from the co-add of r′i′ stacks of the GRB 080516 afterglow (left, red circle) at 28 min post-trigger and the following night (23.5 hrs post-trigger).

thumbnail Fig. A.17.

Spectral energy distribution of the GRB 080516 afterglow at 28 min after the Swift trigger, corrected for the foreground Galactic extinction, providing a best-fit photometric redshift of 4.1±0.1.

A.3.2. GRB 140209A

Swift was not able to slew in response to its BAT trigger due to a Moon-constraint, but despite the arcmin-scale error box and substantial foreground extinction (AV = 2.4 mag), an afterglow was quickly identified, which faded from R = 17.6 to R = 19.4 mag within the first 40 minutes (Perley 2014). GROND observations were performed after 17 hrs (see Table A.2), and 41 hrs, demonstrating a further decline over r′ = 22.7 mag (forced detection) to r′ > 23.4 mag. We derived a position of RA(J2000) = 05h 25m 19.​​s06, Decl.(J2000) = +32°29′ 53 . ″ $ \overset{\prime \prime }{.} $1, ±0 . ″ $ \overset{\prime \prime }{.} $3, consistent with Perley (2014). The SED is clearly red, and the foreground extinction prevents a g′-detection. The noteworthy feature of the SED is a clear dip in the z′-band (Fig. A.18), interpreted as the 2175 Å feature similarly to GRB 070802 (Krühler et al. 2008). It is only the sixth GRB known to show this feature, after GRB 070802 (Krühler et al. 2008; Eliasdóttier et al. 2009), GRB 080607 (Prochaska et al. 2009; Perley et al. 2011), GRBs 080605 and 080805 (Zafar et al. 2012), and GRB 180325A (Zafar et al. 2018). The inferred redshift is z = 3.2 ± 0.3. We note that this is independent of the foreground extinction, but the intrinsic host extinction has substantial uncertainty, both statistically ( A V host $ A_{\mathrm{V}}^{host} $ = 0.8±0.2 mag) and depending on the extinction law (not included in the error estimate).

thumbnail Fig. A.18.

Spectral energy distribution of a stack of GROND images of the first epoch (2014 Feb. 10, 00:40–02:14 UT) of GRB 140209A. The data are corrected for the foreground AV = 2.4 mag.

Appendix B: Notes on individual sources II

This section provides unpublished GROND information on (i) known afterglows or (ii) upper limits in cases of no previous optical/NIR afterglow reports or (iii) if GROND upper limits are deeper than those reported by other groups or (iv) host galaxy candidates in case of no afterglow detection. This includes GRBs with previous GROND GCN if new information is available (such as GRB 110721A or 151023A).

All GROND magnitudes (including JHK) are in the AB system, and not corrected for foreground extinction unless specifically mentioned. Upper limits are at 3σ confidence.

B.1. GRB 071021

This long-duration (T90 = 225 sec) GRB triggered Swift/BAT at 09:41:33 UT, providing an X-ray position shortly after (Sakamoto et al. 2007). Despite early optical observations, no afterglow was found. Deep NIR observations 11.25 hrs post-burst revealed a “faint source” (no magnitudes given) in the HK bands (Castro-Tirado et al. 2007), spurring speculations on a high-redshift GRB. Observations with Subaru/IRCS about 20 hrs post-burst measured K ≈ 21 mag (Terada et al. 2007), confirmed to be in the Vega system (priv. comm. N. Kawai). Several years later, deep imaging detected the host galaxy at this position down to the B band (Perley et al. 2013), and VLT spectroscopy revealed a redshift of z = 2.452 (Krühler et al. 2012b) via host emission lines. Interestingly, Perley et al. (2013) measured K = 20.25 ± 0.23 which means that the Subaru/IRCS detection was already that of the host galaxy, not that of the afterglow emission. Re-analysis and photometric calibration of the TNG H/K data from Castro-Tirado et al. (2007) show that the brightness of the “faint source” is fully compatible with the host measurements of Perley et al. (2013). GROND observations started 14 hrs after the GRB (lasting for 2 hrs), thus between the TNG and Subaru observing times. A second epoch was done on the following night, though it is less deep. In the first epoch, we only detect the ‘faint source’ in the K band which in light of the above details is compatible with the detection of the host galaxy.

B.2. GRB 080405

This GRB was detected with Swift/BAT and Mars Odyssey, but localisation was delayed by ∼22 hrs (Cummings & Hurley 2008). A Swift/XRT observation at ∼43 hrs after the GRB found one X-ray source within the BAT error circle, but no fading could be established. GROND observations started ∼38 hrs after the GRB. There is one bright (r′ ∼ 19 mag), a fainter (r′ ∼ 23 mag) and an even fainter extended object within the 7″ XRT error circle (La Parola et al. 2008), but all three sources were found to not have faded in a GROND observation on Feb. 25, 2009.

B.3. GRB 080414

This INTEGRAL burst with 10 sec duration (Mereghetti et al. 2008) occurred at a galactic latitude of only 0 . ° $ \overset{\circ }{.} $5, behind a correspondingly very large column density of E(B-V) = 7 mag. Follow-up observations with Swift did not reveal an X-ray counterpart. GROND observations started about 6 hrs after the GRB trigger, and did not reveal a counterpart candidate.

thumbnail Fig. B.1.

GROND images from the co-add of g′r′i′ stacks of the afterglow (left) and a late deep image (right). The large circle denotes the Swift/XRT afterglow position, the two small red ones the two components of object ’A’ of Jelinek et al. (2010a).

B.4. GRB 080905B

For this Swift-detected GRB with XRT position, an afterglow was seen both with UVOT (Stroh et al. 2009) as well as the ESO/VLT (Vreeswijk et al. 2009), about 3 . ″ $ \overset{\prime \prime }{.} $8 off the centre of the galaxy 2MASSX J20065732-6233465. The ESO/VLT observations, starting 8.3 hrs after the GRB, reveal a bright source with R∼20.2 mag, and a redshift (based on resonance absorption features) of z = 2.374 (Vreeswijk et al. 2009). GROND observations started 14.8 hrs after the burst, but due to very bad seeing (2 . ″ $ \overset{\prime \prime }{.} $5) the afterglow is not resolved from the galaxy. The limits in Table A.2 refer to the limiting magnitude of the stack of the first four individual 6 min integrations, but not to the brightness of the well-detected unresolved host+afterglow emission!

B.5. GRB 090827

This GRB triggered Swift-BAT, but the onboard software could not identify a position, thus the localisation was distributed after ground analysis (Cummings et al. 2009). Swift/XRT observations clearly identified an X-ray counterpart (Evans et al. 2009b). GROND observations were taken at 4 epochs over 24 days. There are two sources in the revised7 (retrieved: 2020) 4 . ″ $ \overset{\prime \prime }{.} $2 error circle, but both are constant in all our images. Due to one of the four read-out channels of the K-band detector temporarily malfunctioning, there is no K-band coverage of the final XRT-position.

B.6. GRB 100115A

This GRB was found in ground analysis of a preplanned Swift slew manoeuvre (Cummings et al. 2010), but despite the late Swift/XRT follow-up, an X-ray afterglow was identified (Margutti et al. 2010). No UVOT counterpart was detected, but Jelinek et al. (2010a) identified an optical source with the NOT (called object ’A’, which shows two components) which Cucchiara et al. (2010) found to fade based on GMOS data from the first two nights. GROND observations were taken at 3 epochs, one each on the first two nights, the other in 2016. During the first two epochs, we clearly see an object coincident with the south-western component of object ’A’, declining by about 1 mag. The magnitudes of the first epoch are given in Table A.2; the others are: at mid-time 2010-01-17 01:08 UT g′> 22.3 mag, r′ = 23.2±0.2 mag, i′> 23.1 mag, z′> 22.5 mag, and at mid-time 2016-10-28 03:50 UT: g′> 25.3 mag, r′> 25.2 mag, i′> 24.3 mag, z′> 24.1 mag, J> 21.0 mag, H> 20.4 mag, K> 18.7 mag. Thus, the total amplitude in r′ is > 2.​​m9. For the north-eastern component, we measure the following magnitudes: g′ = 24.10±0.12 mag, r′ = 23.43±0.07 mag, i′ = 23.34±0.14 mag, and z′ = 23.42±0.20 mag.

B.7. GRB 100316C

For this Swift-detected GRB a rapid X-ray afterglow position was reported (Stamatikos et al. 2010), but neither Swift/UVOT (starting 81 s after the trigger) nor BOOTES-3 observations (starting 4 s after the trigger) revealed an optical afterglow (Stamatikos et al. 2010; Jelinek et al. 2010b). GROND observations (15.8 hours after the GRB trigger) revealed a candidate at 23rd mag (Afonso et al. 2010b), but a second epoch taken on August 6, 2010, showed this object at the same magnitudes, therefore excluding this object as the afterglow.

B.8. GRB 100816A

The optical afterglow of this short GRB was detected with Swift/UVOT at 17th magnitude (Oates et al. 2010), and then seen to rapidly fade to 20.5 mag after 2.8 hrs (Antonelli et al. 2010). Our GROND imaging, commencing on Aug. 18, 06:33 UT (2.2 days after the GRB), did not reveal the counterpart anymore. We clearly detect the nearby host galaxy at z = 0.80 (Tanvir et al. 2010) in all bands except K.

B.9. GRB 100823A

The NIR afterglow (source C) seen with LIRIS on the William Herschel telescope by Levan et al. (2010) at K(Vega)∼19.5 mag at 8 hrs after the GRB is about 2 mag below our limiting magnitudes, which suffered from particularly bad observing conditions (seeing ∼3″).

B.10. GRB 110721A

Fermi/GBM triggered at 04:47:44 UT (trigger 332916465 / 110721200) (Tierney & von Kienlin 2011) on this FRED-like GRB with a duration of 24 s and a 1-s peak photon flux of 31 ph/cm2/s, among the brightest GRBs seen with Fermi. Combined analysis with Fermi/LAT data suggests a peak energy of 15±2 MeV during the early phase, making it the highest-Epeak GRB (Axelsson et al. 2012). This GRB got even more prominence as being one for which significant polarisation of the prompt emission was detected with IKAROS/GAP (Yonetoku et al. 2012).

Swift/XRT observations of the Fermi/LAT position did reveal one X-ray source, for which GROND observations identified an optical counterpart candidate (Greiner et al. 2011b). Though the 7-channel GROND SED has a red power-law slope (1.2±0.2), consistent with a GRB afterglow, the optical source did not show any fading in the first two nights (Greiner et al. 2011b). Gemini spectroscopy suggested a redshift of z = 0.382 based on CaII H&K (Berger 2011), while an X-shooter spectrum did not provide any further clues (Selsing et al. 2019).

Here, we report a 3rd GROND epoch taken on 2011 July 24 at 7:20–8:50 UT, which confirms the constant flux. Also, an archival NTT/EFOSC R-band image taken on Aug 10, 2010, a year prior to the GRB, shows this source at a similar magnitude. We therefore conclude that the optical source is not the afterglow of GRB 110721A. Instead, the power-law SED and the spectrum suggest a blazar nature. The X-ray detection is consistent with this interpretation, though we note that Chandra et al. (2011) suggested PKS 2211-388 (which is at 9″ distance) as the source of the X-ray emission.

The lack of an optical counterpart is astonishing, given that this is one of the brightest GRBs detected so far.

B.11. GRB 111020A

GRB 111020A triggered Swift/BAT at 06:33:50 UT, and a fading X-ray afterglow was identified (Mangano & Sakamoto 2011), which had also been detected with XMM-Newton (Campana 2011). While the total GRB duration is only 0.5 sec, the substantial positive spectral lag argues against a short-duration classification (Sakamoto et al. 2011). GROND observations started at 23:50, more than 17 hrs after the GRB, and did not detect the optical afterglow. The GROND limits are less constraining than the Gemini observations (Fong et al. 2012b), but we do detect their sources S2 and G3.

B.12. GRB 111026A

This Swift/BAT detected GRB could not be followed up with XRT/UVOT due to a pointing constraint. We obtained only one epoch with GROND of the 3′ BAT error box, and the limits in Table A.2 are those of SkyMapper (g′r′i′z′) and 2MASS (JHK). The GROND images are about 1 mag (g′r′i′z′) and 2 mag (JHK) deeper.

B.13. GRB 111121A

We do not detect the faint emission reported by Fong et al. (2011) for which neither confirmation nor evidence of fading exists. The Swift/XRT afterglow position is within 4″ of a 12th magnitude star, so our upper limits are dominated by the noise of the image subtraction of two stacks about 1 hr apart.

B.14. GRB 111207A

This 3-sec duration Swift-GRB was found in ground-analysis, and only reported 1.5 days post-burst (Cummings et al. 2011). No X-ray counterpart was reported. GROND observations in the following two nights reach deep limits, but image subtraction does not reveal a convincing counterpart candidate within the BAT error circle.

B.15. GRB 120118A

GROND observations started 140 sec after INTEGRAL trigger 6443, but were offset in declination to cover the common error box with the earlier sub-threshold trigger 6441. Unfortunately, the final error box (Götz et al. 2012) is not covered in the g′r′i′z′-channels, and only to about 30% in JHK. The counterpart candidate of Trello et al. (2012) is not covered, even in JHK. The upper limits in Table A.2 refer to the first 4 min of observation. Götz & Bozzo (2012) caution that these two INTEGRAL triggers and the INTEGRAL trigger 6449 could be caused by the high-mass X-ray binary GX 301-4 that was active at this time. While GX 301-4 is covered by our images, it is saturated in all bands.

B.16. GRB 120118B

Observations with the TNG telescope 10 hrs after this Swift-BAT detected GRB did not reveal a new source within the XRT error circle (Osborne et al. 2012a), down to limiting magnitudes of J(Vega) > 20.5 mag and H(Vega) > 20.3 mag D’Avanzo & Palazzi (2012). However, they noted a faint NIR source close to the Swift/XRT afterglow position. Keck observations about one year later identified an extended source with I(Vega)∼23.8 mag at that position, suggestive of the host galaxy, and obtained a redshift of z = 2.943 (Malesani et al. 2013b). GROND observations on the first night were taken about one hour earlier than those with the TNG, but did not find a source, to the limits given in Table A.2. GROND follow-up observations in February 2016 provide the following host magnitudes (all in AB): g′ = 25.57±0.18 mag, r′ = 24.67±0.09 mag, i′ = 24.48±0.15 mag, z′ = 23.92±0.16 mag, J> 21.6 mag, H> 21.1 mag, K> 19.9 mag.

B.17. GRB 120212A

The optical afterglow of this Swift-detected GRB (Sonbas et al. 2012) was detected 14 min after trigger by the 2-m Faulkes Telescope South (Guidorzi et al. 2012a). GROND observations were only possible on the following night because the telescope was closed due to too high humidity. The afterglow is well detected, about 6 magnitudes fainter than at discovery by Guidorzi et al. (2012a), but the photometry is affected by the tail of the bright star to the North-East.

B.18. GRB 120224A

GROND observations of this Swift-detected GRB (Saxton et al. 2012) started 19.58 hrs after the GRB trigger, and revealed a source (Elliott et al. 2012a) at the edge of the revised error circle of the X-ray afterglow (Osborne et al. 2012b). GROND observations on the following night (starting 2012-02-26T00:48) and 6 months later (starting 2012-08-26T07:10) reveal the object at the same magnitude, namely g′ = 25.75±0.28, r′ = 24.00±0.09, i′ = 23.00±0.08, z′ = 22.45±0.06, J = 20.86±0.09, H = 20.95±0.19, K = 19.94±0.12. This SED is well fit with a dusty, star-forming (3±0.2 M⊙/yr) galaxy at redshift 1, consistent with the Balmer-break redshift of 1.1±0.2 as determined with the VLT/X-shooter (Krühler et al. 2015).

B.19. GRB 120324A

For this Swift-detected GRB an X-ray afterglow was rapidly found (Zhang et al. 2012), but due to the low galactic latitude the substantial crowding and foreground absorption complicated the search for an optical/NIR counterpart. The first reported uncatalogued object (Guidorzi & Melandri 2012b) was found to be constant over a few hours and visible on old POSS2 plates (Im et al. 2012), and a fainter source reported with no brightness information, but positioned in the then revised X-ray error circle (Im et al. 2012) was also detected with GROND in several bands about 1.5 hrs earlier (Sudilovsky et al. 2012). However, another GROND epoch taken 17 months later (starting 2013-08-12T02:28) showed this source also at unchanged brightness. Thus, no optical/NIR counterpart was identified for this GRB.

B.20. GRB 120419A

GROND observations of this INTEGRAL burst started about 11 hrs after the GRB, and a second epoch was obtained on 9th July, 2012. The GRB position is in a very crowded field in the Galactic Plane, with large foreground extinction (of order AV∼ 40 mag, Schlegel et al. 1998). The 4 . ″ $ \overset{\prime \prime }{.} $5 Swift/XRT error circle of the X-ray afterglow (Page et al. 2012) contains one bright (13 mag) and 5 fainter (18–19 mag) stars, none of which are variable between the two GROND epochs. Our limits refer to the first epoch on top of the diffuse emission of unresolved sources.

B.21. GRB 120724A

Due to visibility constraints, GROND observations started only 21 hrs after the GRB, under poor seeing conditions. Image subtraction with a later epoch on March 23, 2013 did not reveal the afterglow (Guidorzi et al. 2012c) anymore, but just the host galaxy at redshift 1.48 (Cucchiara et al. 2012): the residuals are completely dominated by the noise of the image subtraction (corresponding to about r′i′z′ > 23 mag), while the formal 3σ upper limits of the images are about 1 mag deeper; thus we refrain from quoting individual limits in the seven GROND filter bands. The host magnitudes are g′ = 22.19±0.04 mag, r′ = 21.02±0.02 mag, i′ = 20.59±0.02 mag, z′ = 20.22±0.02 mag, J = 19.98±0.13 mag, H = 19.61±0.17 mag, K = 19.50±0.30 mag. We note that the brightness of the galaxy is nearly identical to the i′-magnitude given by Guidorzi et al. (2012c) for the fading afterglow.

B.22. GRB 120805A

GROND observations started immediately after evening twilight, but rolling-in clouds severely limited our image depth. The optical afterglow (Gorosabel et al. 2012) is not detected. Another observation on March 9, 2013 reached about 4 mags deeper, and we detect the z = 3.1 (Krühler et al. 2015) host galaxy at the optical afterglow position, at the following magnitudes: g′ = 25.37±0.24 mag, r′ = 24.07±0.10 mag, i′ = 23.32±0.11 mag, z′ = 23.25±0.16 mag, and derive NIR upper limits of J> 21.1 mag, H> 20.4 mag, K> 19.7 mag.

B.23. GRB 120817A

The GROND observations on the first night (after 21.6 hrs) suffered from particularly bad seeing (4″), thus the counterpart candidate (Guidorzi et al. 2012d) is not resolved from the nearby brighter star. In a 1-hr exposure 3 nights later, we clearly detect this object, at a magnitude consistent with Guidorzi et al. (2012d), thereby excluding the afterglow interpretation.

B.24. GRB 121014A

Due to the small Sun distance, no Swift/XRT follow-up could be done, and GROND observations of the 2′ Swift/BAT error circle (Barthelmy et al. 2012) in all 7 channels could only be done for 4 minutes. JHK imaging continued for another 20 min during morning twilight. No new source is found after comparison to PS1 or 2MASS, where the JHK upper limits in Table A.2 are those of 2MASS, while the actually reached sensitivities are J> 19.2 mag, H> 18.6 mag, K> 17.8 mag.

thumbnail Fig. B.2.

GROND image from the co-add of g′r′i′ stacks of the host candidate of GRB 121209A within the Swift/XRT position of the afterglow (red circle).

B.25. GRB 121102A

GROND observations did not reveal an afterglow. The 1 . ″ $ \overset{\prime \prime }{.} $5 Swift/XRT error circle (Osborne et al. 2012c) does contain an object, but correcting for a fraction of the foreground extinction AV = 5.1 mag, the colours suggest a B star. Follow-up observations on the subsequent two nights show no fading of this object.

B.26. GRB 121209A

Based on observations on the first night, a detection of a faint source in r′i′z′ within the Swift/XRT error circle was reported (Krühler et al. 2012a). Another GROND observation on 2016 Oct. 28 under good conditions returns similar magnitudes of this object: g′ = 24.79±0.14 mag, r′ = 24.04±0.10 mag, i′ = 24.10±0.24 mag, z′ = 23.63±0.24 mag. The PSF of this object is about 50% larger of that of stars of similar brightness. This makes it a possible host galaxy candidate, rather than a chance coincidence with a galactic stellar object.

B.27. GRB 121226A

GROND observations (under poor seeing conditions) started about 10 hrs after the GRB. In stacked images of 150 min g′r′i′z′ and 120 min JHK exposure, centred at Decl. 27 07:05 UT, we detect the object first reported by Castro-Tirado et al. (2012) within the Swift/XRT error circle, at g′ = 24.68±0.34 mag, r′ = 24.05±0.13 mag, i′ = 24.30±0.22 mag, z′ = 24.00±0.23 mag, J > 22.6 mag, H > 22.0 mag, K > 21.0 mag. In a later GROND observation on Feb. 15, 2013 we obtain very similar magnitudes, supporting the conclusion of Malesani et al. (2013a) of host emission, based on the positional coincidence with the XRT position and of a radio source (Fong et al. 2012a) (though also no radio variability has been reported). Since our early detection is close to the limit of the observations, and thus the residuals are completely dominated by the noise of the image subtraction, we refrain from providing upper limits for the afterglow.

B.28. GRB 130327B

The X-ray source (Krimm & Troja 2013) found with Swift/XRT within the error circle of this Fermi/LAT-localised GRB (Ohno et al. 2013) remained constant over about 10 days and is not considered to be the X-ray afterglow. In GROND observations of this X-ray source, starting 3 days after the GRB (and because of this delay not included in Table A.2), we detect 2 objects within and another 2 at the border of the X-ray source, all of which remained constant over 4 days of GROND observations.

B.29. GRB 130603B

GROND observations of this short Swift-detected GRB (Melandri et al. 2013) were possible only at 31 hrs after the trigger. The optical afterglow (Levan et al. 2013) is not detected on top of the bright (about 21 mag in r′i′z′) host galaxy. The residuals are completely dominated by the noise of the image subtraction, thus we refrain from giving individual limits in the seven filter bands in Table A.2.

B.30. GRB 130612A

Due to withdrawn override permission, GROND observations started only 24 hr after the GRB, but still detected the rapidly fading afterglow of this Swift-detected GRB with UVOT afterglow detection (Racusin et al. 2013). Another GROND observation 3 years later (May 4, 2016) did not reveal any source down to r′> 25.5 mag, implying another > 1 mag fading.

B.31. GRB 130807A

Within the Swift/XRT error circle, we do not detect any new object besides a source at RA(J2000) = 17h 59m 12.​​s0, Decl.(J2000) = -27°36′ 59″, which is known from the VVV survey (observations taken 2011). Given the low galactic latitude (1 . ° $ \overset{\circ }{.} $9), the field is very dense, and the upper limits in Table A.2 refer to rare not-populated regions.

B.32. GRB 131014A

For this Fermi/LAT-detected GRB, a rapid Swift observation at 12 hrs after the GRB trigger revealed an X-ray afterglow (Kennea & Amaral-Rogers 2013). Three different faint objects have been found within or at the edge of the X-ray error circle, and for one of these a constant brightness over the first 2 days was established (Kann et al. 2013). A further GROND observation was executed starting 2013-10-18T07:21, which showed that also sources #2 and #3 have not faded relative to the observations on Oct. 16th, and therefore none of the three sources is the optical afterglow of GRB 131014A.

B.33. GRB 131209A

The LAT localisation of this Fermi-detected GRB came with a 0 . ° $ \overset{\circ }{.} $9 error (Vianello & Omodei 2013), and was subsequently improved by triangulation with Konus-MESSENGER to a 1 . ° $ \overset{\circ }{.} $5×0 . ° $ \overset{\circ }{.} $2 annulus (Hurley et al. 2013), in which Swift/XRT follow-up observations found two X-ray sources, one of which was uncatalogued and showed marginal fading (Grupe & Breeveld 2013). Further Swift/XRT observations identified this source as variable, but unrelated to GRB 131209A (Grupe 2013), and due to the observed rapid X-ray variability a Narrow Line Seyfert 1 galaxy or a blazar nature was suggested. GROND observations targeted this X-ray source on Dec. 11th in morning twilight (in NIR-only mode), and again on Dec. 14th. At the border of the X-ray error circle we identify an optical source (at RA(J2000) = 09h 07m 18.​​s8, Decl.(J2000) = -33°51′ 22″, ±0 . ″ $ \overset{\prime \prime }{.} $5) which has an optical/NIR SED consistent with a power law: g′ = 23.39±0.30 mag, r′ = 21.42±0.05 mag, i′ = 20.73±0.05 mag, z′ = 20.36±0.06 mag, J = 19.43±0.04 mag, H = 19.15±0.04 mag, K = 18.72±0.07 mag. We derive a power-law slope of 1.33±0.07, and given the large g′-dropout, a Ly-absorption fit results in a redshift 3.84±0.17. We propose this source J090718.8-335122 as the counterpart of the variable X-ray source of Grupe (2013), and suggest a high-redshift blazar interpretation. The upper limit for the GRB afterglow in JHK is given in Table A.2, but we note that the GROND observation only covered a tiny fraction (∼10%) of the combined Fermi/LAT circle and triangulation annulus.

thumbnail Fig. B.3.

A LePhare fit of the spectral energy distribution of the extended object within the Swift/XRT error circle of GRB 140331A, based on a stack of several GROND images with a total exposure of 1 hr. The data have been corrected for the foreground AV = 0.15 mag.

B.34. GRB 140129A

After the GROND g′r′i′z′-detection on the first night (see Table A.2) of the UVOT-detected afterglow of this Swift/BAT-triggered GRB, a second epoch was performed two nights later, for which we measure g′ = 24.30±0.24 mag, r′ = 23.79±0.17 mag, i′ > 23.9 mag, z′ > 23.2 mag. A final observation on July 21st, 2014, reveals no sign of host emission within 5″ radius, to upper limits of g′ > 24.5 mag, r′ > 24.3 mag, i′ > 23.6 mag, z′ > 22.9 mag, J> 20.8 mag, H> 20.2 mag, and K> 18.7 mag.

B.35. GRB 140331A

GROND observations of this Swift GRB started about 21.5 hours after the GRB trigger, and a second epoch was done on April 18, 2014. The source, identified by Littlejohns et al. (2014) within the enhanced Swift/XRT error circle (Beardmore et al. 2014), is constant between the two epochs, and our magnitudes are consistent with those of Littlejohns et al. (2014) taken about 1 hr after the GRB, as well as the SDSS measurements (SDSS J085927.51+024304.0). The SED is very steep in the blue, and Littlejohns et al. (2014) suggested a photometric redshift of 4.65 (+0.34,-2.80). However, the source is clearly extended, and a fit using LePhare (Arnouts et al. 1999; Ilbert et al. 2006) (instead of a power law) provides a decent fit for a galaxy at redshift 0.65±0.10, and E(B-V) = 0.4 mag (Fig. B.3). The SDSS DR16 query returns a photometric redshift of 0.676±0.100. Whether or not this is the host galaxy remains open. The upper limits in Table A.2 are for the region outside this galaxy within the XRT error circle.

B.36. GRB 140719A

For this Swift-detected GRB, the X-ray afterglow was rapidly localised (Starling et al. 2014). GROND observations started 16.8 hrs after the GRB trigger and revealed a faint source in two filter bands (Bolmer et al. 2014), at coordinates RA(J2000.0) = 11:26:24.3, Decl.(J2000) = -50:08:04.8 (±0 . ″ $ \overset{\prime \prime }{.} $2), 1 . ″ $ \overset{\prime \prime }{.} $7 from the X-ray position which itself has a 1 . ″ $ \overset{\prime \prime }{.} $6 error (enhanced position, (Evans et al. 2009a)). This is close to the bright (8th mag) star HD 99481, which hampered detection in other bands. No detections from other groups were reported. A second epoch GROND observation performed five nights later (mid-time 2014-07-24T23:34), finds r′ = 24.33±0.27, i′ > 23.8 and z′ = 23.01±0.32 (forced detection). The marginal fading prevents a secure statement about an optical afterglow detection, but the second epoch magnitudes could likely resemble host emission.

B.37. GRB 140815A

For this INTEGRAL-IBIS detected burst with a positional accuracy of 2.​​′5 (Mereghetti et al. 2014), a Swift follow-up observation at 19.2–31.8 ks after the GRB identified an X-ray source (Mangano & Vargas 2014) within the INTEGRAL error circle. No further Swift observation was done, so the afterglow nature through fading could not be established. GROND observations revealed an optical source within the X-ray error circle (Graham et al. 2014). Additional GROND observations two months later (midtime 2014-10-17T09:18 and 2014-10-24T06:01) show this source at unchanged brightness. Thus, this object is not the optical afterglow of GRB 140815A, but could be the counterpart of the X-ray source in case it is not the X-ray afterglow.

B.38. GRB 141022A

For this Swift-detected GRB, a rapid X-ray afterglow was reported, but no UVOT detection (Vargas et al. 2014). GROND observations started 11 min. after the GRB trigger and revealed a single source inside the X-ray error circle (Kann et al. 2014) which did not vary substantially over the next hour. Another GROND observation on the following night was too shallow for a decisive answer, but a third GROND observing run half a year later (mid-time 2015-04-24T08:14) showed this source still at the same brightness, thus excluding its afterglow nature.

B.39. GRB 150428A

For this Swift-detected GRB, a rapid X-ray afterglow was reported, but no UVOT detection (Page et al. 2015). The originally reported source from GROND observations (Knust et al. 2015a) was not consistent with the enhanced XRT position (Osborne et al. 2015), and also shown with several later observations to remain constant in time. Andersen et al. (2015) reported a marginally detected source with R = 22.5±0.43 mag at a median time of 0.887 hrs after the GRB trigger. Checking our early GROND images, we do not detect this source (or similarly at the slightly offset radio source position Leung et al. (2021)), with 3σ upper limits of r′ > 23.8 (mid-time 1.05 hrs post-burst) and r′ > 24.3 (mid-time 1.41 hrs post-burst). If the above marginal detection were true, this would imply a decay slope of −7 between their and our GROND epoch. We therefore assume that an optical/near-infrared afterglow of this GRB has not been detected.

B.40. GRB 150915A

For this Swift-detected GRB a bright, fading X-ray afterglow was readily detected (D’Elia et al. 2015a), but no UVOT counterpart was identified. With GROND-observations starting 2.2 hrs post-burst, a candidate afterglow was found at the border of the X-ray error circle (Yates et al. 2015). Spectroscopy with X-Shooter on the ESO/VLT derived a redshift of z = 1.968 of this source based on both, metal line absorption features as well as emission lines from the putative host galaxy D’Elia et al. (2015b). Contemporaneous NOT observations found the object to brighten during the first three hours, but like for the initial GROND observation did not find a fading behaviour (Xu et al. 2015b). We obtained two further epochs two nights later (mid-time Sep. 18, 01:47 UT) and about one year later (mid-time 2016-10-29T01:03) which clearly identifies the fading nature, but also reveals the host galaxy underneath the afterglow (Fig. B.4), at the following brightness: g′ = 23.24±0.05 mag, r′ = 23.45±0.05 mag, i′ = 23.52±0.11 mag, z′ = 23.89±0.21 mag, J = 22.31±0.41 mag.

thumbnail Fig. B.4.

GROND images of two epochs of the GRB 150915A afterglow on top of the spectroscopically identified host galaxy (right panel), with clear fading by up to 1.3 mag in the z′-band.

B.41. GRB 151023A

GROND observations of this Swift/BAT-detected burst started about 10 hrs after the BAT trigger, and a new source had been quickly reported in Knust et al. (2015b). However, further GROND imaging did not reveal fading, and also Butler et al. (2015) detected this source at similar brightness 2–3 hrs later. Also, after correction for the Galactic foreground (EB − V = 1.38 mag) the SED of the reported source resembles the stellar spectrum of a cool star. Thus, the object reported in Knust et al. (2015b) is unrelated to GRB 151023A.

B.42. GRB 151212A

Swift follow-up observations of this MAXI-detected GRB at 8.4–8.6 ks after the trigger revealed a likely X-ray afterglow (Roegiers et al. 2015). GROND observations at ≈14.1 hrs after the GRB trigger identified an optical source with a power law-like SED, suggestive of the afterglow (Wiseman et al. 2015). Follow-up GROND observations on the second and third night after the GRB trigger found this object to fade by > 1.3 mag in the bluest filter bands, thus confirming the afterglow identification.

thumbnail Fig. B.5.

GROND images of two epochs of the GRB 160804A afterglow on top of a relatively bright SDSS galaxy (middle), with the afterglow emission clearly showing up in the image subtraction (right panel).

B.43. GRB 160804A

For this Swift-detected GRB, a fading optical source was identified in the first UVOT exposures (Breeveld & Marshall 2016), at a position consistent with the X-ray afterglow (Osborne et al. 2016). The position of this optical source was readily noticed to be coincident with that of a SDSS galaxy (Tyurina et al. 2016). All ground-based observations started a couple hours (or later) after the BAT trigger, returning brightness estimates consistent at first glance with the SDSS catalogue magnitudes (Tyurina et al. 2016; Malesani et al. 2016; Moskvitin et al. 2016; Mazaeva et al. 2016; Watson et al. 2016), including the first GROND epoch (mid-time Aug. 5, 00:15 UT) (Bolmer & Greiner 2016). A second GROND epoch was taken 6 months later (mid-time 2017-02-05T08:17), providing the following estimates (numbers in brackets are from the first epoch):

g′ = (21.50±0.02) 22.47±0.04 mag,

r′ = (21.21±0.02) 22.19±0.03 mag,

i′ = (20.85±0.02) 21.28±0.03 mag,

z′ = (20.66±0.02) 21.02±0.04 mag,

J = (20.20±0.03) 20.67±0.18 mag,

H = (19.88±0.04) 20.10±0.13 mag,

K = (19.69±0.08) > 20.0 mag,

which are consistent with the SDSS-catalogue PSF-magnitudes. This demonstrates clear fading in the blue bands (see Fig. B.5), suggesting that most of the first-day observations indeed detected afterglow light. The implied shallow brightness decline is matching that of the X-ray emission observed with Swift/XRT. A power-law fit to the Galactic foreground (E(B-V) = 0.03 mag) corrected magnitudes gives a slope of 1.12±0.05; when omitting the JHK bands due to their larger fraction of host light, the slope remains unchanged, but the error towards flatter slopes increases to −0.20.

Table B.1.

Layout of the table (available in full at CDS) of all 517(514 plus the 3 with large error boxes; see Sect. 3) GROND-observed GRBs.

All Tables

Table 1.

Eight GRBs purposely ignored for GROND observations.

In the text
Table 2.

GROND afterglow detection fraction, separately for the 465 long- and 49 short-duration GRBs, as a function of time delay of the start of the observation after the GRB trigger, based on a total of 514 bursts.

In the text
Table A.1.

Summary of the new findings in this study.

In the text
Table A.2.

Measurements of either unpublished GROND-observed GRBs or those with updates relative to published GCNs. Only GRBs with particular importance (and described in the appendix in detail) are listed here; the full table is available at CDS.

In the text
Table B.1.

Layout of the table (available in full at CDS) of all 517(514 plus the 3 with large error boxes; see Sect. 3) GROND-observed GRBs.

In the text

All Figures

thumbnail Fig. 1.

Sky distribution in equatorial coordinates of the GROND GRB sample. We note the lack of extinction bias towards the Galactic plane, made evident based on the ratio of detected and non-detected afterglows. The apparent ‘concentration’ of observed GRBs towards the South Pole is due to a 2σ lack of triggers in the −40° < Dec < −50° range: 32 versus a mean of 50 (between −70° and +20°). These results are updated from Greiner (2019).
In the text
thumbnail Fig. 2.

Histogram of the time between GRB alert and start of the GROND observation in 1 hr bins, showing all 514 GRBs (black), and those with optical afterglow detection (red). The inset shows a zoom of the distribution during the first hour, in 5 min bins. The fastest reaction time was 78 s (for GRB 130514A).
In the text
thumbnail Fig. 3.

Histogram of the 15–150 keV GRB fluences as measured with Swift/BAT for the detected (black) and non-detected (red) afterglows. For each category, we show the long- (dashed lines) and short-duration (solid lines, hashed) GRBs separately. There is only a weak tendency for bright GRBs to be better detectable than faint GRBs.
In the text
thumbnail Fig. 4.

Pie diagram of the satellite-origin of the 514 GROND-observed GRBs: the outer circle represents the overall ratio of the missions providing the trigger, the inner circle depicts the ratio of detected (darker colour) vs. non-detected (lighter colours) optical/NIR afterglows. The thin slice above MAXI is the optically identified iPTF14yb transient, which later was related to GRB 140226A based on a Mars/Odyssey, Konus-Wind and INTEGRAL data. The vast majority of GROND-observed are from Swift and thus have XRT-positions and spectra.
In the text
thumbnail Fig. 5.

Venn diagram of the detected afterglows out of the sample of 514 GROND-observed GRBs visible from Chile between 21 May 2007 and 1 Oct 2016, i.e. 458 of the 514 GROND-observed GRBs have an immediate Swift/XRT detection, 272 a GROND detection, 268 a combined GROND and Swift/XRT detection, and so on. Radio detections are taken from https://www.mpe.mpg.de/~jcg/grbgen.html.
In the text
thumbnail Fig. 6.

Sample of 122 long GRB afterglows observed with GROND within the first 4 hr, coloured according to redshift. For bright afterglows, these are the first 4-min exposures; for fainter afterglows, stacking of multiple exposures has been applied (as indicated by horizontal ‘error’ bars) and a compromise between amount of stacking and time after the GRB has been adopted. The sensitivity limit of GROND@2.2m is given as dashed line, assuming a start of the exposure at 150 s and an exposure of 60 s, thereafter improving as time progresses: at early times, this is close to the typical t−1 afterglow fading rate. For those afterglows with J-band upper limits but z′-detections, we plotted an open square at the expected J-band value (using a mean colour of z − J = 0.15 mag), connecting it with the downwards arrow with a dotted line (GRB 120328A at 6534 s has z′ = 23.79±0.35, outside of the plot). The sky background limits our sensitivity to J(AB) ≲ 22 mag, irrespective of exposure duration.
In the text
thumbnail Fig. A.1.

GROND z′-band image of GRB 081127 from the 55 min co-add of the first night (left), showing the optical afterglow within the 1 . ″ $ \overset{\prime \prime }{.} $8 XRT error circle (drawn here with a 3″ radius for better visibility), and the second epoch from 26 Aug. 2016 (right).
In the text
thumbnail Fig. A.2.

GROND images from the co-add of g′r′i′ stacks of the afterglow (left, red circle) and the host galaxy (right) of GRB 101017A. The circle denotes the GROND position of the afterglow, slightly north but consistent with the UVOT position.
In the text
thumbnail Fig. A.3.

GROND r′i′-band images from 2 days after GRB 120215A (left), and 7 months later (right) with the XRT error circle (black) and the GROND afterglow (red).
In the text
thumbnail Fig. A.4.

GROND r′-band images from 2 days after GRB 120302A (left) and 3 weeks later (right) The black circle denotes the Swift/XRT position of the afterglow, while the red circle shows the afterglow and host.
In the text
thumbnail Fig. A.5.

GROND image from the co-add of r′i′z′ stacks of the afterglow candidate (red circle) of GRB 120328A. The black circle denotes the Swift/XRT position of the X-ray afterglow.
In the text
thumbnail Fig. A.6.

GROND image from the 94 min i′ stack (left) and a later epoch (right), demonstrating the fading of the afterglow. The large circle denotes the Swift/XRT afterglow error circle, the small red one encircles the afterglow position.
In the text
thumbnail Fig. A.7.

Fit to the GRB 130211A afterglow SED, using the foreground extinction-corrected GROND data with SMC-type host-intrinsic extinction.
In the text
thumbnail Fig. A.8.

GROND image from the co-add of g′r′ stacks of the two OBs. The circle denotes the Swift/XRT afterglow position.
In the text
thumbnail Fig. A.9.

GROND images from the co-add of 25 min from the first night (left, afterglow) and 100 min of the third night (right, host). The black circle denotes the Swift/XRT position of the X-ray afterglow of GRB 140412A, and the red circle marks the position of the afterglow and host.
In the text
thumbnail Fig. A.10.

Swift/UVOT white (left) and GROND g′ image (right) of GRB 140619A. The ≈3 mag fading confirms the afterglow nature of the UVOT detection, and the GROND detection two months later is likely the GRB host galaxy. The circle (radius 1 . ″ $ \overset{\prime \prime }{.} $4) is to guide the eye; the Swift/XRT error circle (1 . ″ $ \overset{\prime \prime }{.} $7) is omitted for better legibility.
In the text
thumbnail Fig. A.11.

GROND z′-band image from the first 10 min OB from the first night (top left; mid-time 2014-07-10T23:09), the stack of two 30 min OBs from the next night (top right’ mid-time 2014-07-12T01:13), and the GROND light curve with a best-fit slope of 1.2±0.1 (bottom). The 1.​​′7 INTEGRAL error circle is outside this figure, but the combined evidence of clear fading and typical power-law SED identifies this source without doubt as the afterglow of GRB 140710B.
In the text
thumbnail Fig. A.12.

GROND J-band images from the co-add of two 20 min OBs each from the May 20th (left), and May 22nd (right), respectively, of GRB 150518A. The red circle is the 2 . ″ $ \overset{\prime \prime }{.} $6 XRT error circle. We also note the slight shift of the centroid of the emission.
In the text
thumbnail Fig. A.13.

GROND image from the co-add of r′i′z′ stacks of GRB 070917 of the first night, showing the afterglow and the improved 4″ X-ray error circle.
In the text
thumbnail Fig. A.14.

GROND images from the co-add of r′i′ stacks of the GRB 071117 afterglow (left: red circle) and the host galaxy (right). The latest (retrieved 2021) UVOT-enhanced Swift/XRT position is overplotted and nearly exactly centred on the host galaxy.
In the text
thumbnail Fig. A.15.

GROND images of the GRB 080523 X-ray afterglow position (black circle), showing the optical afterglow (left, red circle) from the first night’s r′-band observation (as given in Table A.2), the r′-band observation 6 yrs later demonstrating the fading of the afterglow (middle), and the g′-band observation of the same epoch showing the strong blue colour of the emission directly underneath the afterglow position, consistent with a putative host galaxy.
In the text
thumbnail Fig. A.16.

GROND images from the co-add of r′i′ stacks of the GRB 080516 afterglow (left, red circle) at 28 min post-trigger and the following night (23.5 hrs post-trigger).
In the text
thumbnail Fig. A.17.

Spectral energy distribution of the GRB 080516 afterglow at 28 min after the Swift trigger, corrected for the foreground Galactic extinction, providing a best-fit photometric redshift of 4.1±0.1.
In the text
thumbnail Fig. A.18.

Spectral energy distribution of a stack of GROND images of the first epoch (2014 Feb. 10, 00:40–02:14 UT) of GRB 140209A. The data are corrected for the foreground AV = 2.4 mag.
In the text
thumbnail Fig. B.1.

GROND images from the co-add of g′r′i′ stacks of the afterglow (left) and a late deep image (right). The large circle denotes the Swift/XRT afterglow position, the two small red ones the two components of object ’A’ of Jelinek et al. (2010a).
In the text
thumbnail Fig. B.2.

GROND image from the co-add of g′r′i′ stacks of the host candidate of GRB 121209A within the Swift/XRT position of the afterglow (red circle).
In the text
thumbnail Fig. B.3.

A LePhare fit of the spectral energy distribution of the extended object within the Swift/XRT error circle of GRB 140331A, based on a stack of several GROND images with a total exposure of 1 hr. The data have been corrected for the foreground AV = 0.15 mag.
In the text
thumbnail Fig. B.4.

GROND images of two epochs of the GRB 150915A afterglow on top of the spectroscopically identified host galaxy (right panel), with clear fading by up to 1.3 mag in the z′-band.
In the text
thumbnail Fig. B.5.

GROND images of two epochs of the GRB 160804A afterglow on top of a relatively bright SDSS galaxy (middle), with the afterglow emission clearly showing up in the image subtraction (right panel).
In the text

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