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Appendix A: Data tables

Appendix B: GRBs without afterglow detection by GROND

B.1. GRB 070729

The original 90% c.l. XRT error circle radius was 57 (Guidorzi et al. 2007a), which was refined to 45 some hours later (Guidorzi et al. 2007b). A host galaxy candidate was soon reported (Berger & Kaplan 2007). However, the final XRT position lies about 9′′ northeast and does not overlap with the previous XRT error circle (Evans 2011a,b).

GRB 070729 was the first short GRB observed with GROND after its commissioning in mid-2007. GROND observations started 6 h after the burst and continued for 4.5 h until sunrise. A second-epoch observation was performed the following night for 1 h. No transient object between the two epochs was detected in any band (Küpcü Yoldaş et al. 2008).

B.2. GRB 071227

GROND started observing the field 4 h after the GRB trigger. At that time, the weather conditions were not good. GROND could not detect the afterglow in any band (Table 4). Second-epoch observations were performed the following night. GROND was on target 29 h after the burst and observed for one hour. At that time, the host galaxy had already been discovered by Swift/UVOT (Sakamoto et al. 2007a; Cucchiara & Sakamoto 2007), and its redshift was measured to be z = 0.381 ± 0.001 (D’Avanzo et al. 2007, 2009; Berger et al. 2007b). VLT observations revealed an optical afterglow situated 31 away from the center of its host, an edge-on galaxy (D’Avanzo et al. 2008, 2009). GROND could not detect the afterglow anymore, only deep limiting magnitudes can be provided: g′r′i′z′JH = 25.5, 25.0, 24.2, 24.4, 21.5, 20.5 at 29 h after the burst. The r′-band upper limit is in agreement with the expectations based on the VLT R-band detection at 0.3 d after the burst if the optical afterglow was fading analogeous to its X-ray counterpart with a decay slope of α ~ 1 (see Fig. 7 in D’Avanzo et al. 2009). The GRB host galaxy is discussed in detail by D’Avanzo et al. (2009).

B.3. GRB 080905A

GROND started observing the field of GRB 080905A about 17.5 h after the burst. Observations continued for only 11 min at a seeing of 22. The combined g′r′i′z′-band image as well as the combined JHK_s-band image do not show the afterglow and faint host galaxy discovered with the ESO/VLT (Rowlinson et al. 2010). Our non-detection is in agreement with these authors, according to whom at the time of our observations the magnitude of the afterglow was around R_C = 24, about 1 mag below our detection limit. Although the field is very crowded with stars, the afterglow was situated in a region free of stars. In addition, it was well separated from the center of its suspected anonymous host galaxy. Therefore, the upper limits we can provide (Table 4) are not affected by the light of the host galaxy. We refer to Rowlinson et al. (2010) for a detailed study of this burst and its host galaxy.

B.4. GRB 091109B

GROND observed GRB 091109B six hrs after the trigger. The weather conditions over La Silla observatory were not good at that time. Although GROND was on target for one hour, observations were not deep enough because of clouds. Inside the 28 90% c.l. XRT error circle, no source can be detected in the GROND images (Table 4).

A faint optical transient was discovered by VLT/FORS in the R_C band at the time when GROND was observing (Levan et al. 2009b; Malesani et al. 2009), but it was not detected in the NIR (VLT/High Acuity Wide field K-band Imager (HAWK-I)). The non-detection of the afterglow by GROND is in agreement with the magnitude reported by Levan et al. (2009b), R_C ~ 25, which is deeper than our limiting r′-band magnitude (23.3; Table 4). Re-analysing the VLT/FORS data, we find that from 20 ks to 40 ks the light curve of the afterglow can be fitted with a single power-law with a slope of α = 0.80 ± 0.04. For this time period, there are also simultaneous Swift/XRT observations which, within errors, can be fitted with the same decay slope (α_X = 1.08 ± 0.36).

B.5. GRB 100206A

GROND started observing the field 11 h after the trigger. Observations were performed at high airmass and under poor seeing conditions. No evidence for an afterglow candidate was found in any band (Nicuesa Guelbenzu et al. 2010). Perley et al. (2011) published a detailed investigation of the GRB host galaxy.

B.6. GRB 100625A

GROND visited the field of GRB 100625A several times. First-epoch observations started 11.7 h after the GRB trigger and lasted for about 1 h. Second-epoch observations were done on June 27, about 39 h after the trigger, and a third run was performed on July 1 (about 5.5 d after the trigger). Further data of the field were collected in 2010.

Within the 90% c.l. XRT error circle (Goad et al. 2010b), an object is detected in all GROND epochs, the potential GRB host galaxy (Berger et al. 2010a). No evidence was found in the GROND data for a decaying afterglow superimposed upon this galaxy (Table 4; note that these upper limits refer to an isolated afterglow).

B.7. GRB 100628A

GROND started observing the field about 17 h after the GRB trigger and remained on target for 1.5 h. At that time, two extended objects were already detected inside the final 90% c.l. XRT error circle (Berger et al. 2010e,d). No optical afterglow was detected.

B.8. GRB 101219A

Observations with GROND started about 80 min after the GRB trigger and continued for about two hrs. Although they were performed under good weather conditions (seeing 08, airmass 1.1), the proximity of the Moon affected the depth of the observations. No optical transient was detected by GROND in any band down to deep flux limits (Table 4).

B.9. GRBs with arcmin-sized error circles

This sample contains four bursts where only a Swift/BAT or, in one case, an INTEGRAL/IBIS error circle is known. They are typically 3 arcmin in radius. These events are GRBs 071112B, 081226B, 091117A, and 101129A. Because of visibility constraints by GROND or Swift/XRT in these cases, GROND was on target not earlier than between 6 and 31 h after the corresponding GRB trigger. Given that, on average, short GRB afterglows are intrinsically substantially fainter than those of long GRBs (see Kann et al. 2010, 2011), it was not very likely that in these cases GROND could image the afterglow in any band. Indeed, only upper limits can be provided (Table 4).

Appendix C: Additional observations reported in the literature

C.1. GRB 070729

Swift/BAT triggered on GRB 070729 at 00:25:53 UT (Guidorzi et al. 2007a) and had a duration of T₉₀(15–350 keV) = 0.9 ± 0.1 s (Guidorzi et al. 2007c). The burst was also seen by Konus A (Golenetskii et al. 2007b). An uncatalogued X-ray source was found by Swift/XRT but no optical afterglow by Swift/UVOT (Guidorzi et al. 2007a). Inside the initial XRT error circle, Berger & Kaplan (2007) reported the detection of an extended object visible in the K band. A refined XRT error circle with a radius of was later reported by Guidorzi et al. (2007b). This error circle lies away from the initial XRT position. Optical follow-up observations were performed in the R band with the Swope 40-inch telescope at Las Campanas Observatory, but no sources were detected inside the XRT error circles, implying that the aforementioned galaxy is a red object (Berger & Murphy 2007). No afterglow was detected in the radio band (Chandra & Frail 2007). The position of the XRT afterglow was later refined and shifted by about 5′′ in NE direction while it shrunk to (Evans 2011a,b).

C.2. GRB 071227

This was a bright and multi-peaked GRB with T₉₀(15–350 keV) = 1.8 ± 0.4 s that triggered Swift/BAT at 20:13:47 UT (Sakamoto et al. 2007a; Sato et al. 2007). It was also detected by Konus-Wind (Golenetskii et al. 2007a) and Suzaku/Wide-Band All-Sky Monitor (WAM; Onda et al. 2008). Swift localized a bright X-ray afterglow (Beardmore et al. 2007). UVOT observations (Sakamoto et al. 2007a; Cucchiara & Sakamoto 2007) revealed a single faint source near the XRT error circle, which was identified as a galaxy also visible in the DSS (Berger et al. 2007a). VLT (D’Avanzo et al. 2007, 2009) and Magellan (Berger et al. 2007b) spectroscopy revealed a redshift of this galaxy of z = 0.381 ± 0.001, and further VLT follow-up detected the optical afterglow (D’Avanzo et al. 2008, 2009) at the tip of this edge-on spiral galaxy.

C.3. GRB 080905A

Swift/BAT and Fermi/GBM triggered on GRB 080905 at 11:58:55 UT (Pagani et al. 2008a; Bissaldi et al. 2008). The BAT light curve shows three peaks with a total duration of about 2 s (Pagani et al. 2008a). Its duration was T₉₀ (15–350 keV) = 1.1 ± 0.1s (Cummings et al. 2008). A fading X-ray afterglow was found, but no optical afterglow was detected with UVOT (Pagani et al. 2008a). A faint afterglow candidate was then discovered with the VLT (Malesani et al. 2008), and a host galaxy was also seen (de Ugarte Postigo et al. 2008). The revised XRT error circle is in agreement with this afterglow position (Evans et al. 2008). The afterglow is located in an outer arm of a star-forming spiral galaxy at z = 0.1218 ± 0.0003, making it the closest short GRB known so far. This event has been analyzed in detail by Rowlinson et al. (2010).

C.4. GRB 080919

GRB 080919 triggered Swift/BAT at 00:05:13 UT. The burst consists of a single spike and had a total duration of T₉₀ (15–350 keV) = 0.6 ± 0.1 s. Swift/XRT began observing about 71 s after the BAT trigger. The detected X-ray afterglow could be localized with high precision (; Preger et al. 2008a), but remained undetected from the second orbit on. Swift/UVOT started observing about 11 s after XRT, but no afterglow candidate could be found in the white filter down to m = 18 (Preger et al. 2008b; Baumgartner et al. 2008; Immler & Holland 2008). The size of the X-ray error circle could finally be improved to (Preger et al. 2008c). Ground-based observations with the robotic REM telescope on La Silla, Chile, started already 74 s after the BAT trigger and revealed a bright NIR source in the XRT error circle (H = 13.73 ± 0.03). This source, however, is also listed in the 2MASS catalogue and might therefore be an unrelated Galactic foreground object (Covino et al. 2008). No further follow-up observations are reported in the literature. The position of the XRT afterglow was slightly refined three years after the event (Evans 2011a,b).

C.5. GRB 081226A

GRB 081226A triggered Fermi/Gamma-ray Burst Monitor (GBM) and Swift/BAT at 01:03:37 UT (Godet et al. 2008; Kouveliotou & Connaughton 2009). Its duration was T₉₀(15–350 keV) = 0.4 ± 0.1 s (Krimm et al. 2008). Swift/XRT started observing the field 94.5 s after the BAT trigger, and an afterglow was found (Godet 2008). UVOT started observing 156 s after the trigger, but no optical afterglow was identified (Hoversten & Godet 2008). Optical observations by Robotic Optical Transient Search Experiment (ROTSE)-IIIc starting 25 s after the GRB could only reveal upper limits on any optical afterglow (Schaefer et al. 2008). GROND detected an afterglow candidate (Afonso et al. 2008), but observations with Gemini-S did not reveal a fading behavior, neither of this source nor of a second one found in the XRT error circle (Berger et al. 2008a,b). No radio counterpart of the optical afterglow candidate(s) could be found with the Australia Telescope Compact Array (ATCA; Moin et al. 2009b). The position of the XRT afterglow was slightly refined three years after the event (Evans 2011a,b).

C.6. GRB 090305

The burst triggered Swift/BAT at 05:19:51 UT. The BAT light curve shows a single short spike with a duration of T₉₀(15–350 keV) = 0.4 ± 0.1 s. XRT began observing the field 93 s after the trigger, but no X-ray afterglow was initially detected. UVOT started observing 96 seconds after the trigger, but no optical afterglow candidate was discovered either (Beardmore et al. 2009a; Krimm et al. 2009). Despite the lack of an XRT position, rapid follow-up of the BAT error circle with Gemini-S/GMOS and Magellan/Baade led to the discovery of the optical afterglow (Cenko et al. 2009; Berger & Kelson 2009). In addition, a re-analysis with relaxed constraints allowed the detection of an extremely faint X-ray afterglow at the position of the optical counterpart (Beardmore et al. 2009c). No host galaxy was detected down to deep limits right under the optical afterglow position (Berger 2010b).

C.7. GRB 090927

The burst was detected by Swift/BAT at 10:07:16 UT (Grupe et al. 2009a). It had a Fast Rise Exponential Decay (FRED)-like shape with some substructure and a duration of T₉₀(15–350 keV) = 2.2 ± 0.4 s (Stamatikos et al. 2009). It was also detected by Fermi/GBM (Gruber et al. 2009). The final classification of the burst is not totally clear. It is more likely a long GRB because it shows significant spectral lag and was relatively soft (Grupe et al. 2009b). After its BAT trigger, Swift could not immediately slew to the field due to an Earth-limb constraint. When Swift/UVOT began observing the field 2121 s after the trigger, it immediately discovered an optical afterglow candidate (Gronwall & Grupe 2009; Kuin & Grupe 2009). Only thereafter was the detection of the X-ray afterglow announced, a quite unusual situation (Evans et al. 2009). The afterglow was observed with the 1-m f/4 Zadko telescope in Western Australia (Klotz et al. 2009), which started observations 50 min after the trigger (with the first magnitude value for t ~ 2 h). It was also observed with the Faulkes Telescope South in Australia (Cano et al. 2009), which observed 4.2 h after the onset of the GRB, and with VLT/FORS2 on ESO Paranal (Levan et al. 2009a), which observed 16.5 h after the burst trigger. The VLT observations allowed for a measurement of the afterglow redshift (z = 1.37; Levan et al. 2009a). Radio observations with the Australian ATCA array did not reveal the afterglow (Moin et al. 2009a).

C.8. GRB 091109B

Swift/BAT triggered on GRB 091109B at 21:49:03 UT (T₉₀(15–350 keV) = 0.30 ± 0.03 s; Oates et al. 2009b). An X-ray afterglow was immediately detected, but no optical afterglow was found (Oates et al. 2009a). The burst was a symmetrical spike with no sign of extended emission (Oates et al. 2009b). A faint, rapidly decaying afterglow was discovered with the VLT at coordinates RA, Dec(J2000) = 07:30:56.61, −54:05:22.85 (Levan et al. 2009b; Malesani et al. 2009).

C.9. GRB 100117A

The burst triggered Swift/BAT (de Pasquale et al. 2010a) and Fermi/GBM (Paciesas 2010) at 21:06:19 UT. It had a duration of T₉₀(15–350 keV) = 0.3 ± 0.05 s (Markwardt et al. 2010). Swift/XRT began observing the field 80 s after the BAT trigger and found a bright X-ray afterglow, which could be localized with an uncertainty of (radius) that could later be refined to (Sbarufatti et al. 2010). UVOT started observing about 1 min later but could not find an optical counterpart (de Pasquale et al. 2010a,b). The optical afterglow was detected by Gemini-North 8.3 h after the burst with r_AB = 25.46 ± 0.20 (Levan et al. 2010a). The burst is in detail discussed in Fong et al. (2011).

C.10. GRB 100206A

The burst triggered Swift/BAT at 13:30:05 UT (Krimm et al. 2010b) and had a duration of T₉₀ = 0.12 ± 0.03 s (Sakamoto et al. 2010). XRT started observing the field 75 s after the trigger and found an uncatalogued X-ray source (Krimm et al. 2010b), whose coordinates were later refined to RA, Dec(J2000) = 03:08:38.94, 13:09:25.5, with an error radius of (Goad et al. 2010a). The burst was also seen by Fermi/GBM with a spectral peak at  keV, assuming a Band function (von Kienlin 2010). No optical counterpart was detected by Swift/UVOT (Krimm et al. 2010b; Marshall & Krimm 2010) and other ground-based observatories (Bhattacharya et al. 2010; Guziy et al. 2010; Noda et al. 2010; Leloudas et al. 2010; Yurkov et al. 2010; Mao et al. 2010; Andreev et al. 2010; Rumyantsev et al. 2010). Evidence for a galaxy close to the XRT error circle was soon reported based on archival images of the field (Miller et al. 2010), whose redshift was later determined to be z = 0.41 (Cenko et al. 2010a). Morgan et al. (2010) found that at this redshift this galaxy is very bright in JHK, suggesting that this is a luminous infrared galaxy. Levan et al. (2010c) speculated about the discovery of the faint optical afterglow of GRB 100206A based on William Herschel Telescope (WHT) observations starting 7 h after the burst. However, no fading of this source was seen on Gemini images taken 7 and 11.5 h after the event, suggesting that in fact this source could be the true GRB host galaxy (Berger et al. 2010b; Berger & Chornock 2010). This placed a limit of i > 24.7 on the brightness of the optical afterglow at 15.7 h after the burst (Berger & Chornock 2010). The position of the XRT afterglow was slightly refined three years after the event (Evans 2011a,b). Perley et al. (2011) dispute that another faint source very close to the z = 0.41 galaxy might instead be the host.

C.11. GRB 100625A

Swift/BAT triggered and located GRB 100625A at 18:32:28 UT (Holland et al. 2010a). The BAT light curve showed a single spike with a substructure and a duration of about 0.33 s. XRT started observing the field 48 s after the trigger and found an uncatalogued X-ray source, whose coordinates were later refined to RA, Dec(J2000) = 01:03:10.98, −39:05:18.3, with an error radius of . No optical counterpart was detected by Swift/UVOT. The burst was also seen by Konus-Wind and Fermi. The Fermi/GBM light curve shows two closely spaced narrow pulses with a duration (T₉₀) of about 0.32 s (50–300 keV; Holland et al. 2010b). Inside the XRT error circle, an object was reported in the optical bands by ground-based observatories (Levan & Tanvir 2010; Berger et al. 2010a; Tanvir & Levan 2010). However, the non-variation of the object and its extended shape pointed to it being a host galaxy candidate. The position of the XRT afterglow was slightly refined three years after the event (Evans 2011a,b).

C.12. GRB 100628A

Swift/BAT triggered and located GRB 100628A at 08:16:40 UT as did INTEGRAL (Beckmann et al. 2010). In the BAT window, it had a duration of T₉₀(15–350 keV) = 0.036 ± 0.009 s (Immler et al. 2010). Swift/XRT began observing the field 86 s later and located an X-ray afterglow at coordinates RA, Dec(J2000) = 15:03:52.95, −31:39:41.7, with an error radius of (Starling & Immler 2010). No optical afterglow was found, neither by UVOT nor by ground-based observatories

(Immler 2010; Burenin et al. 2010; Berger et al. 2010e,d; Suzuki et al. 2010; Levan et al. 2010b). Based on Magellan observations, Berger et al. (2010e) noticed, however, the presence of several galaxies close to and inside the XRT error circle (see also Berger 2010c). The position of the X-ray afterglow was later rejected when another faint X-ray source was found that had faded away. This source at coordinates RA, Dec(J2000) = 15:03:52.41, −31:39:30.2 (error radius 7′′) is now considered as the most likely X-ray afterglow (Starling et al. 2010). Inside this error circle Berger (2010d) reports the presence of two galaxies; they, however, did not show any evidence of a superposed optical afterglow. For one of these galaxies, Cenko et al. (2010b) measured a redshift of z = 0.102.

C.13. GRB 100702A

The burst triggered Swift/BAT at 01:03:47 UT (Siegel et al. 2010a). It was a FRED-like single-peaked burst with a duration of T₉₀(15–350 keV) = 0.16 ± 0.03 s (Baumgartner et al. 2010). Swift slewed immediately to the burst and found a bright X-ray afterglow, which faded rapidly after an early plateau phase and was already undetected after the first orbit (Grupe & Siegel 2010; Siegel et al. 2010b). No optical/NIR afterglow candidate was found, neither in rapid response observations by ROTSE-IIIc located at Mt. Gamsberg, Namibia (Flewelling et al. 2010) nor by optical observations with the acquisition camera of VLT/X-shooter (Malesani et al. 2010) and NIR observations using Persson’s Auxilliary Nasmyth Infrared Camera (PANIC) at the Magellan/Baade telescope (Fong et al. 2010; Berger et al. 2010c).

C.14. GRB 101219A

GRB 101219A was a short-hard burst localized by Swift/BAT at 02:31:29 UT (Gelbord et al. 2010), which was also detected by Konus-Wind (Golenetskii et al. 2010). The BAT light curve consists of a single spike with a duration of T₉₀(15–350 keV) = 0.6 ± 0.2 s (Krimm et al. 2010a). A fading X-ray afterglow was found 60 s after the trigger, but no optical afterglow was detected (Gelbord et al. 2010). Inside the XRT error circle, a faint extended object was observed in the i and r band with the Gemini South telescope 43 min after the burst (Perley et al. 2010). In addition, the same faint source was detected in the J band with the 6.5-m Magellan Baade telescope 1.5 h after the trigger (Chornock et al. 2010). Second-epoch observations showed no variable source inside the XRT error circle. Spectroscopic observations performed on the host galaxy candidate with GMOS mounted at the Gemini-North telescope derived a redshift of z = 0.718 (Chornock & Berger 2011). The position of the XRT afterglow was slightly revised two years after the event (Evans 2011a,b).

© ESO, 2012