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A&A
Volume 614, June 2018
Article Number A29
Number of page(s) 8
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/201731755
Published online 08 June 2018

© ESO 2018

1 Introduction

It is widely accepted that long-duration gamma-ray bursts (GRBs) are related to the death of massive stars (Hjorth et al. 2003; Stanek et al. 2003). Due to their large γ-ray luminosity, they can be detected to very high redshift, and thus can provide a unique probe into the early Universe. How the afterglow emission evolves both in frequency space and with time depends on the properties of the burst environment (e.g., gas density profile, dust) and on the progenitor itself (e.g., temporal energy injection profile, as well as mass, rotation, and binarity, all of which influence the density and structure of the circumburst medium, e.g., Yoon et al. 2012).

When the relativistically expanding blast wave interacts with the circumburst medium, an external shock is formed, the macroscopic properties of which are well understood. Under the implicit assumptions that the electrons undergo Fermi acceleration at the relativistic shock to a power-law distribution, their dynamics can be expressed in terms of four main parameters: (1) the total internal energy in the shocked region as released in the explosion, (2) the electron density n and radial profile of the surrounding medium, (3) the fraction of the shock energy that goes into electrons ϵe, and (4) the ratio of the magnetic field energy density to the total energy ϵB. Measuring the energetics or the energy partition (ϵe/ϵB) has been challenging, and observations at multiple passbands have thus far only been possible for a dozen of the more than 1000 GRB afterglows detected so far.

The observational difficulty of establishing whether the observed synchrotron spectrum is in the fast or slow cooling stage introduces adegeneracy when attempting to explain the spectrum in terms of the physical model parameters. The minimal and simplest afterglow model has five parameters (not counting the distance/redshift). The degeneracy between many of these parameters makes it even more difficult to draw firm conclusions. Thus, it is not surprising that many previous attempts had to compromise whenever assumptions were made about individual parameters (e.g., Panaitescu & Kumar 2002; Yost et al. 2003; Chandra et al. 2008; Cenko et al. 2010; Greiner et al. 2013; Laskar et al. 2014; Varela et al. 2016), but contradictions between analyses with different assumptions surfaced only in the rare cases where the same GRB afterglows were analyzed based on different data sets (e.g., McBreen et al. 2010; Cenko et al. 2011).

Here, we report our multi-epoch, multifrequency observations of GRB 151027B, in an attempt to collect an exhaustive data set that would allow us to determine all these parameters.

2 GRB 151027B detection and afterglow observations

2.1 GRBprompt and afterglow detection

GRB 151027B was detected by the Swift (Gehrels et al. 2004) Burst Alert Telescope (BAT, Barthelmy et al. 2005) on 2015 October 27 at T0 = 22:40:40 UT (MJD = 57322.944907) as the 1000th Swift burst (Ukwatta et al. 2015). The prompt light curve shows a complex structure with several overlapping peaks that starts at ~T0 and extends for about 100 s, leading to a formal duration T90 (15–350 keV) of 80±36 s (Sakamoto et al. 2015). Swift slewed immediately to the BAT-derived position, allowing the X-ray afterglow to be discovered readily with the Swift X-ray telescope (XRT, Burrows et al. 2005) with a 4′′ accurate position (later refined to 1.′′8). This in turn allowed the discovery of the optical afterglow one hour later by the Nordic Optical Telescope (Malesani et al. 2015), and a redshift determination of z = 4.063 with VLT/X-shooter four hours after that (Xu et al. 2015). In addition to our GROND observations (see below), detections of the optical afterglow were also reported by MASTER (Buckley et al. 2015), RATIR (Watson et al. 2015) and the 2m Faulkes Telescope North in Hawaii (Dichiara et al. 2015). Swift/UVOT did not detect the afterglow, consistent with the redshift and galactic foreground extinction (Breeveld et al. 2015).

2.2 GROND observations

Observations with GROND (Greiner et al. 2008) started on 2015 October 28 at 06:26 UT, about 8 hr after the trigger, at a Moon distance of only 37°. Simultaneous imaging in g′r′i′z′JHKs continued for several further epochs (see the observation log in Table 1) until 2015 November 18, when the afterglow could no longer be detected. During the night of November 5–6, a field with SDSS coverage (RA(2000.0) = 03h 45m, Dec(2000.0) = –06°15′) was observed immediately after the GRB field under photometric conditions.

GROND data have been reduced in the standard manner (Krühler et al. 2008) using pyraf/IRAF (Tody 1993; Küpcü Yoldaş et al. 2008). The optical–near-IR (NIR) imaging was calibrated against the primary SDSS1 standard star network, or catalogued magnitudes of field stars from the SDSS in the case of g′ r′ i′z′ observations, or the 2MASS catalog for JHKs imaging. This results in typical absolute accuracies of ±0.03 mag in g′r′i′z′ and ±0.05 mag in JHKs. Comparison stars covered by the finding chart of GRB 151027B (Fig. 1) are given in Table A.1.

Despite its high redshift, the afterglow was detected in all seven bands (Table 1) at a common position of RA(2000.0), Dec(2000.0) = 76.°21955, –6.°45029, or 05:04:52.69 –06:27:01.1, with a 1σ error of ±0.′′25. This is fully consistent with both the UVOT-corrected Swift/XRT position (Osborne et al. 2015) and the NOT-derived position (Malesani et al. 2015).

Table 1

GROND observations; all in the AB system, not corrected for Galactic foreground extinction corresponding to E(B–V) = 0.18 mag (AV = 0.55 mag) (Schlafly & Finkbeiner 2011).

thumbnail Fig. 1

Finding chart of the afterglow of GRB 151027B based on a GROND z′ -band image, with secondary standard stars of Table A.1 encircled. North is up and east to the left.

2.3 ATCA observations

We observed the field of GRB 151027B under program C2955 (PI: Greiner) simultaneously at 5.5 and 9 GHz with the Australia Telescope Compact Array (ATCA), beginning at October 30.54 UT for 3.3 hr; the corresponding detection at 9 GHz was reported earlier (Greiner et al. 2015b). Over the following three months, we observed the GRB 151027B position at another seven epochs. A summary of the observing log, including the telescope configuration, is given in Table 2. The observations were mostly performed with the CFB 1M–0.5K mode, providing 2048 channels per 2048 MHz continuum intermediate frequency (IF; 1 MHz resolution) and 2048 channels per 1 MHz zoom band (0.5 kHz resolution). Data analysis was done using the standard software package MIRIAD (Sault et al. 1995), applying appropriate bandpass, phase, and flux calibrations. The quasar 0458–020 was used as phase and 1934–638 as flux calibrator. Multifrequency synthesis images were constructed using robust weighting (robust = 0) and the full bandwidth between its flagged edges. The noise was determined by estimating the root mean square (rms) in emission-free parts of the cleaned map.

Given the largely varying flux levels between different observations and the large flux differences between the two frequencies, we note that the signal-to-noise ratio in most detections is so high that it is unlikely our measurements are wrong. We employed two further tests. First, for the November 14 observation, we made separate images for the top and bottom of the 9 GHz band, resulting in flux measurements of 96 μJy (8–9 GHz) and 106 μJy (9–10 GHz), thus providing an internally consistent result. Second, we checked for other sources in the field for evidence of this variation, but did not find any (see Fig. 2).

Table 2

ATCA observing details.

thumbnail Fig. 2

Radio light curve of the afterglow of GRB 151027B at 5.5 and 9 GHz, with the ALMA 97.5 GHz 2σ upper limitoverplotted (lower panel). The upper panel shows the measured fluxes of selected brighter (130–500 μJy) sources. While their nature or intrinsic variability is not known, their <20% flux variation demonstrates that the strong fluctuations seen for the GRB 151027B afterglow (which would correspond to an amplitude between 0.2 and 2 in this graph) is not an instrumental artifact.

2.4 ALMA observations

ALMA observations were triggered under proposal-ID 2015.1.01558.T (PI: S. Schulze). A band 7 (343.495 GHz) observation was performed starting on 2015 November 2 at 05:22 UT under a precipitable water vapor (PWV) of 0.71 mm, and a band 3 (97.495 GHz) observation started on 2015 November 4 at 07:45 UT under a PWV of 0.31 mm. The data analysis was performed using the standard ALMA data analysis package CASA (McMullin et al. 2007; Petry et al. 2012), following the default calibration path also used in ALMA Quality Assurance. The final images are shown in Fig. 3.

Within the GROND error circle of 0.′′25 in band 3, we find a peak with a flux of 0.0619 mJy; given the rms noise of 0.0210 mJy, this corresponds to nearly 3σ. However, the area of the error circle contains 1914 spatial resolution pixels, so we expect 1914 pixels × 0.0016 = 3 pixels to be above a 3σ flux level. Thus, the presence of the source-like point in the error circle is compatible with a random occurrence, likely thermal noise. In fact, there are similar peaks outside the error circle.

In band 7, we find a peak within the error circle of 0.177 mJy, which (given the rms noise of 0.0496 mJy) corresponds to 3.5σ. However, the area of the error circle contains 7793 spatial resolution pixels, so we expect 7793 pixels × 0.0002 = 1.6 pixels to be above a 3.5σ flux level.

Summarizing, no source is detected in either observation, with 2σ upper limits of 42 μJy in band 3 (97.495 GHz, integrated over a bandwidth of 7 GHz, and taking into account that only about 87% of each of the four spectral windows was used; edge channels are not good) at 7.378 days after the GRB, and 100 μJy in band 7 (343.495 GHz, also with a bandwidth of 7 GHz) at 5.279 days after the GRB. These values include the primary beam correction, which is >0.99 because it isclose to the center of the field of view.

thumbnail Fig. 3

ALMA images of the GRB 151027B location at band 3 (97.5 GHz; left) and band 7 (343.5 GHz; right) in the International Celestial Reference System (ICRS). The concentric circles around our best-fit GROND position are the 1σ, 2σ, and 3σ error circles. The contour in the lower left of each figure gives the size of the synthesized beam of the observation. The smaller beam size makes the band 7 image look smoother than that of band 3; the rms noise is actually worse (see text).

3 Results

Here we analyze our data in the context of the GRB fireball model (Meszaros & Rees 1997; Granot & Sari 2002). Throughout this paper, we use the definition Fν ∝ t−αν−β, where α is the temporal decay index and β is the spectral slope.

3.1 Radio scintillation

The large-amplitude radio variability observed in this GRB is very unusual. In the context of the canonical fireball scenario a smoothly varying afterglow would be expected, perhaps with a rapid rise and decay due to reverse shock emission, none of which is akin to our data. Moreover, we observe large variations between the simultaneously covered 5.5 and 9 GHz bands, i.e., the inferred spectral slope changes between <–2.3 and > 2.9 within days, while temporal slopes in the range <−15 and >9 over 2–3 days are implied. We are not aware of any physical process(es) in GRB jets or shocks capable of producing emission with such properties, and thus consider the afterglow radio emission to be strongly influenced by scintillation.

Interstellar scintillation effects have been observed in GRB radio light curves, and have been used to obtain indirect measures of the source size (for a recent review, see Granot & van der Horst 2014). This method relies on the fact that propagation effects in the interstellar medium cause modulations of the flux of a compact source, while a source larger than a certain angular size will not vary (Rickett 1990). In the case of GRBs, the source is the evolving shock front of the jet, which is very compact at first but expands over time. This can result in strong modulations at early times, which get quenched at later times (Frail et al. 1997; Goodman 1997; Frail et al. 2000). These variations can be found between observations on different days, but intraday variability has also been observed in GRBs (e.g., Chandra et al. 2008; van der Horst et al. 2014). The typical procedure for relating the source size to the scintillation effects is to estimate the scintillation strength and timescale using the methods of Walker (1998) combined with the NE2001 model of the free electrons in our galaxy (Cordes & Lazio 2002). In the strong scattering regime, there are two possible types of scintillation, refractive and diffractive. In both cases the modulation strength depends on the source size compared to the angular scale for scintillation, which ranges from a few to a few tens of microarcseconds. Diffractive scintillation gives stronger flux modulations than refractive scintillation, but the angular scale for diffractive scintillation is smaller than that for refractive scintillation. Furthermore, the former is a narrowband phenomenon while the latter is broadband, but they could both be at play in GRB afterglow observations.

The redshift of GRB 151027B is 4.063, which means that 1 arcsecond on the sky corresponds to a distance of 7.05 kpc, so 1 microarcsecond corresponds to 2.2 × 1016 cm. A size of 1016 –1017 cm is typical for the jet size, so strong scintillation effects are expected for this GRB, also because the high redshift of the GRB means that 40 days in the observer frame corresponds to 8 days in the source rest frame. The scintillation timescale of several hours to days that we observe for GRB 151027B is plausible, but the observed modulation seems to be too large to be accommodated within this framework. The maximum modulation index for diffractive scintillation is 1, i.e., the flux can increase or decrease by a factor of 2 due to scintillation, and the modulation index for refractive scintillation is always smaller than 1. Both of these changes are significantly smaller than the jumps in flux that we have observed for GRB 151027B, which are more than a factor of 5 between some observations (at the 2σ level). For instance, at 9 GHz the flux changes from <15μJy at 14.7 days, to 100 ± 10μJy at 17.7 days, and then to <15μJy at 19.7 days; flux changes of more than a factor of 5, both up and down.

Given that these strong flux modulations cannot be explained by physical processes in the source itself, scintillation does seem to be the most natural way to explain the observations, as has been done for other GRBs with radio flux modulations. However, in this particular case, we have to deviate from the typical methodology applied in the modeling of scintillation effects on GRB radio light curves, due to the very large and fast modulations. One of the underlying assumptions of the usual methodology is that the scattering happens at one location, the scattering screen, which resides at a typical distance (usually 1 kpc from the observer). However, many studies of interstellar scintillation with pulsars and active galactic nuclei have shown that the distance of the scattering screen is quite uncertain. Varying this distance can have strong effects on both the modulation strength and timescale. For example, some quasars have shown extreme intraday variability, indicating that their scattering screen is significantly closer than is usually assumed (Dennett-Thorpe & de Bruyn 2002; Bignall et al. 2006; Macquart & de Bruyn 2007; de Bruyn & Macquart 2015). Furthermore, extragalactic sources may be shining through multiple scattering screens inside our galaxy, complicating the scintillation behavior even further. Every scattering screen will impose its own modulation strength and timescale, possibly leading to enhanced and complex scintillation behavior.

The bottom line is that the observed fluctuations in GRB 151027B can be explained by scintillation, but the large modulation amplitude and rapid variations suggests that the scattering screen is at a smaller distance, that there are multiple screens, or a combination of the two. Many more detailed studies of various radio sources, including GRB afterglows, are needed to fully probe the scintillation behavior of the interstellar medium in our local environment.

3.2 Constraints on the fireball model

Both the X-ray and the optical light curves can be modeled with a smoothly broken power law (Fig. 4) with α1 = 0.44 ± 0.19, α2 = 1.44 ± 0.14, and tb ~ 22.5 ks, consistent with the magnitudes observed by NOT and RATIR (Malesani et al. 2015; Watson et al. 2015). These temporal slopes were used to rescale an XRT spectrum from data taken between T0 + 15 ks and T0 + 32 ks to the stacked GROND data taken between T0 + 32 ks and T0 + 34 ks (shaded gray intervals in Fig. 4). The resulting broadband spectral energy distribution (SED) is best fit with a single power law of slope β =0.81 ± 0.01, with a negligible amount of dust (AV = 0.01 ± 0.01 mag), independent of the extinction model (see Bolmer et al. 2018 for more details on the extinction determination, where a broken power-law model has been preferred in order to derive a conservative extinction value). In these fits, the g′ r′ filters were ignored owing to additional uncertainty from absorption from the Lyα forest. The spectral slope in the X-ray to optical–NIR does not change with time within errors (β1 = 0.81 ± 0.01, β2 = 0.83 ± 0.03, and β3 = 0.89 ± 0.07) as evidenced by the other broadband SEDs at later times (see Fig. 5), nor do the data require a spectral break at later times. The above post-break parameters are fully consistent (within 2σ) with an afterglow with an electron power-law distribution with p = 2.62 ± 0.02, evolving via slow cooling into an ISM environment where the cooling break is above the Swift/XRT upper energy boundary: measured α2 = 1.44 ± 0.14 versus predicted α2 = 1.22 ± 0.02. A cooling break below the GROND bands would imply p = 1.62 ± 0.02 and α2 = 0.72, inconsistent with our observed light curve. In the preferred scenario, the cooling break νc would move to lower frequencies proportional to t−1∕2. Since we also do not see any signature of a spectral break in the X-ray band up to 2 × 105 s after the GRB, after back-extrapolation this implies that νc (31 ks) >20 keV. We finally note that the pre-break phase is consistent with the plateaus seen in many Swift-detected GRBs (e.g., Dainotti et al. 2017, and references therein), with the optical data (primarily NOT) fitting the picture, and with being in the same synchrotron spectral regime.

The remaining question then is the relative ordering of the peak frequency νm and the self-absorption frequency νsa. Given the multiple radio detections with ATCA implies that the self-absorption frequency should be below 5 GHz already at 2.8 days after the GRB in order for the scintillation amplitude not to exceed a factor of 10. The ALMA limits then require νm to be above the self-absorption frequency. Considering the canonical decrease in νm according to t−3∕2, our following two observational constraints fix the value of νm(t) to better than 20%: (i) at the time of the first GROND observation, νm (31 ks) < 1.3 × 1014 Hz, and (ii) the ALMA band 7 limit together with the interpolated optical–NIR fluxes at this epoch imply νm (5.279 d) > 1.8 × 1012 Hz. Back-extrapolating the latter limit to the first GROND observation (by a factor of (31 ks/456.1 ks)−3∕2 = 56.4) implies an inferred νm(31 ks) = (1.15 ± 0.15) × 1014 Hz.

With these observational constraints it is possible to determine the fireball parameters. We observe the following set of relations, allat 31 ks after the GRB:

Within the canonical fireball scenario (Granot & Sari 2002) in the slow cooling case with the ordering νsa < νm < νc and ISM densityprofile, the self-absorption frequency remains constant. Because it is always below 100 MHz, i.e., below our observed frequencies, for the entire allowed parameter range (see below), it does not provide any additional constraints.

These constraints on the observed frequencies and fluxes lead to bounds on the fireball parameters (see Fig. 6). While the observations do not uniquely constrain all parameters, we can use an efficiency argument to derive a likely parameter range. In the standard picture, a fraction ϵγ of the explosion energy is radiated in the prompt radiation (observable as Eγ,iso), and the remaining fraction ending up askinetic energy Ekin of the swept up ambient gas. Early observations suggested near equipartition between these two channels, though later considerations including proper error estimates suggest ϵγ in the range of ~ 0.1–0.5 (Granot et al. 2006). Assuming ϵγ = 0.2 and using Eγ,iso (15–10 000 keV) = (5 ± 1) × 1052 erg (based on a best-fit cutoff power-law ofthe Swift/BAT data2 giving an energy fluence of (14.7 ± 2.6) × 10−7 erg cm−2), the GRB 151027B fireball parameters are constrained as follows: external density n =0.03 − 5 cm−3, ϵe = 0.3–1.0, and ϵB = 4 × 10−4–2 × 10−6 (see Fig. 6). We note that our derived ϵe is higher than the majority of published afterglows, though still in the allowed range.

For the solution with the lowest kinetic energy, Ekin = 11.2 × 1052 erg (remaining parameters see caption of Fig. 6), we then compute the X-ray, optical I-band, and 7 GHz radio (average of 5.5 and 9 GHz) light curves, which are shown in Fig. 7. We note that the model was derived without using constraints from the radio bands, so it is interesting that the “predicted” radio light curve in Fig. 7 corresponds roughly to the mean of the radio detections and upper limits. This implies that the scintillation interpretation of the measured radio fluxes is reasonable and that the model is reasonably good. Thus, the observed scintillation corresponds to about a factor of ±3 variation in either direction, but it is not a one-way excursion.

thumbnail Fig. 4

Light curve of the afterglow of GRB 151027B at X-rays as observed with Swift/XRT (top), and in the optical–NIR as observed with GROND (no extinction-correction applied), complemented with two measurements by NOT and RATIR (Malesani et al. 2015; Watson et al. 2015). Error bars are plotted, but are mostly smaller than the symbol size. The vertical gray bands mark the time intervals for which the spectral energy distributions have been established (see text for more details, and Fig. 5).
thumbnail Fig. 5

Observer-frame optical–NIR to X-ray spectral energy distribution of the afterglow of GRB 151027B at the three epochs marked in Fig. 4 with the gray shading. Error bars are plotted, but are mostly smaller than the symbol size.
thumbnail Fig. 6

Constraints on the fireball parameters for the afterglow of GRB 151027B. Small black dots delineate the allowed phase space by the four constraints at 31 ks, and colored crosses indicate different possible solutions for seven different values of the kinetic energy. The solution with the lowest kinetic energy is marked with the red-filled octagon: Ekin =11.2 × 1052 erg, external density n = 5 cm−3, ϵe = 0.9, and ϵB = 1.5 × 10−5. The thick-lined triangles enclose the allowed parameter range if Ekin < 5 × Eγ,iso.
thumbnail Fig. 7

Model light curves for the afterglow of GRB 151027B at X-rays (red), optical (green, extinction-corrected), and radio (blue; open and filled symbols are 5.5 and 9 GHz, respectively; circles = detection, triangles = upper limits) for the lowest-Ekin parameter set. Data are drawn with error bars, which are mostly smaller than the symbol size. The vertical line denotes the break time of 22.5 ks (see Sect. 3.2). Radio data have not been used in deriving the model, so the blue curve is actually a “predicted” light curve.

4 Conclusions

Our data set of the GRB 151027B afterglow can be explained with the simplest version of the standard fireball scenario, displaying a single synchrotron spectrum evolving according to standard dynamics.

We find that the blast wave moves into a constant-density environment, in the slow cooling regime. While we do not see the characteristic movement of the cooling frequency in or through the X-ray band, the constancy of the peak flux over the fullobserving epoch as evidenced by our data requires a constant density profile. The derived fireball parameters are all within the range expected and discussed in the literature. For the smallest allowed kinetic energy, ϵe is pushed towards the upper limit of 1.

After GRBs 000131, 050904, 090423, 111008A, 120521C, 130606A, 140304A, 140311A, 140515A the afterglow of GRB 151027B is the tenth above a redshift of 4 that has been detected in the radio band (see the online summary table).3 Its peak spectral radio luminosity (2 × 1031 erg/s/Hz) isamong the top one-quarter of radio afterglows (Chandra & Frail 2012), but certainly not exceptional. However, the large-amplitude and rapid flux fluctuations up to 9 GHz are exceptional, and imply that scintillation plays a major role, even at 45 days (9 days rest-frame) post-burst.

We finally mention that the ALMA flux limits are close to the prediction of our model, so we cannot completely rule out that the 3.5σ blob in the band 7 image is not actually the afterglow.

With the above caveats it is worth noting that this is one of the few afterglows of long-duration GRBs for which the simplest version of the afterglow scenario describes a rather extensive multi-epoch and multifrequency data set. In many cases, more data also means a need for a more complicated afterglow scenario. This is independent of the publication bias that afterglows with exciting irregular behavior, such as GRBs 071031 (Krühler et al. 2009), 080129 (Greiner et al. 2009), 081029 (Nardini et al. 2011), 100621A (Greiner et al. 2013), 100814A (Nardini et al. 2014), 111209A (Greiner et al. 2015a; Kann et al. 2018), get more easily published than standard GRB afterglows. It remains to be investigated whether some of the standard afterglows can be fitted with the next-simplest version of afterglow models. The hydrodynamical simulations including the incorporation of the off-axis angle view (van Eerten 2015) are one way, and analytical jet spreading models are another.

While there is a wealth of published papers dealing with the fireball modeling of individual GRB afterglows, the vast majority only allow consistency checks, since the data are not sufficient to derive all five model parameters (plus redshift). The three historical exceptions for which all parameters could be determined are GRBs 980703 (Frail et al. 2003), 000926 (Harrison et al. 2001), and 090323 (Cenko et al. 2011). More recently, our group managed to add another four GRBs to this sample: 100418A, 110715A, 130418A (Varela 2017), and 121024A (see Varela et al. 2016 for details). This small sample, out of a total of >700 known X-ray/optical afterglows, shows the challenge of testing the afterglow model(s). And even these seven GRB afterglows are not uniquely described by a single set of parameters or the simplest fireball version: one GRB is equally well described by either wind or ISM density profile, two other GRBs show substantial flaring activity implying additional energy injection, and another two GRBs show strong evidence of an inverse Compton component. There are indications in this sample for a preference of a wind-like GRB environment, contrary to the results of many early analyses. However, this topic–and other issues related to afterglows like a reliable distribution of microphysical parameters–require a substantially larger sample size.

From an observer’s point of view, it is not obvious how best to reach a larger sample size, i.e., what a guaranteed-success strategy would look like. Radio observations are only meaningful at late times when scintillation has ceased, but then the X-ray and optical–NIR instrumentation typically is not sensitive enough to detect the afterglow any longer. However, radio observations provide crucial constraints for the afterglow modeling. Alternatively, dedicated multi-band, multi-epoch ALMA monitoring seems promising for two reasons: First, it is sensitive enough to cover a larger time interval of the afterglow emission (2–3 weeks). Second, the νm -crossing is faster than that of νc, allowing (in combination with the decay slope) a potentially better distinction between wind and ISM environment. While rapid (within a day) target-of-opportunity (ToO) observations are allowed with ALMA, the general acceptance level of GRB-related ToO proposals is going down after more than a decade of Swift-driven afterglow studies, and the need for proposals to be accepted at several observatories during the same semester does not make things easier (see, e.g., Middleton et al. 2017 for a description of the problem and suggested solutions). Instead of attempting full multiwavelength coverage over a long time interval, a graded approach with dense X-ray/optical/sub-mm coverage during the first days and sub-mm/radio at later stages might be a better approach. This is particularly motivated by the potential of trans-relativistic dynamical models and models including jet dynamics that improve upon earlierclosure relations. The number of open questions and the impact that a proper knowledge of the GRB afterglow emission process would have on a variety of other astrophysical areas certainly justifies a concerted approach.

Acknowledgements

During the writing of this paper, the radio astronomy community lost a great scientist, and we lost a dear colleague and collaborator, Ger de Bruyn. His insights in radio scintillation were crucial in the work presented in this paper, and he will be missed by many. J. G. is particularly grateful to Phil Edwards for scheduling the many ATCA ToO observations. S. K., A. N. G., S. S., and D. A. K. acknowledge support by DFG grant Kl 766/16–1. D. A. K. acknowledges financial support from the Spanish research project AYA 2014-58381-P, and from Juan de la Cierva Incorporación fellowships IJCI-2015-26153 and IJCI-2014-21669. J. F. G., T. K., and P. W. acknowledge support through the Sofja Kovalevskaja award to P. Schady from the A. von Humboldt foundation of Germany. Part of the funding for GROND (both hardware and personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). The Australia Telescope Compact Array is part of the Australia Telescope National Facility, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. Facilities: Max Planck:2.2 m (GROND), ATCA, ALMA, Swift

Appendix A Secondary standard stars

Table A.1

Secondary standard stars used to calibrate the optical–NIR afterglow measurements.

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All Tables

Table 1

GROND observations; all in the AB system, not corrected for Galactic foreground extinction corresponding to E(B–V) = 0.18 mag (AV = 0.55 mag) (Schlafly & Finkbeiner 2011).

In the text
Table 2

ATCA observing details.

In the text
Table A.1

Secondary standard stars used to calibrate the optical–NIR afterglow measurements.

In the text

All Figures

thumbnail Fig. 1

Finding chart of the afterglow of GRB 151027B based on a GROND z′ -band image, with secondary standard stars of Table A.1 encircled. North is up and east to the left.
In the text
thumbnail Fig. 2

Radio light curve of the afterglow of GRB 151027B at 5.5 and 9 GHz, with the ALMA 97.5 GHz 2σ upper limitoverplotted (lower panel). The upper panel shows the measured fluxes of selected brighter (130–500 μJy) sources. While their nature or intrinsic variability is not known, their <20% flux variation demonstrates that the strong fluctuations seen for the GRB 151027B afterglow (which would correspond to an amplitude between 0.2 and 2 in this graph) is not an instrumental artifact.
In the text
thumbnail Fig. 3

ALMA images of the GRB 151027B location at band 3 (97.5 GHz; left) and band 7 (343.5 GHz; right) in the International Celestial Reference System (ICRS). The concentric circles around our best-fit GROND position are the 1σ, 2σ, and 3σ error circles. The contour in the lower left of each figure gives the size of the synthesized beam of the observation. The smaller beam size makes the band 7 image look smoother than that of band 3; the rms noise is actually worse (see text).
In the text
thumbnail Fig. 4

Light curve of the afterglow of GRB 151027B at X-rays as observed with Swift/XRT (top), and in the optical–NIR as observed with GROND (no extinction-correction applied), complemented with two measurements by NOT and RATIR (Malesani et al. 2015; Watson et al. 2015). Error bars are plotted, but are mostly smaller than the symbol size. The vertical gray bands mark the time intervals for which the spectral energy distributions have been established (see text for more details, and Fig. 5).
In the text
thumbnail Fig. 5

Observer-frame optical–NIR to X-ray spectral energy distribution of the afterglow of GRB 151027B at the three epochs marked in Fig. 4 with the gray shading. Error bars are plotted, but are mostly smaller than the symbol size.
In the text
thumbnail Fig. 6

Constraints on the fireball parameters for the afterglow of GRB 151027B. Small black dots delineate the allowed phase space by the four constraints at 31 ks, and colored crosses indicate different possible solutions for seven different values of the kinetic energy. The solution with the lowest kinetic energy is marked with the red-filled octagon: Ekin =11.2 × 1052 erg, external density n = 5 cm−3, ϵe = 0.9, and ϵB = 1.5 × 10−5. The thick-lined triangles enclose the allowed parameter range if Ekin < 5 × Eγ,iso.
In the text
thumbnail Fig. 7

Model light curves for the afterglow of GRB 151027B at X-rays (red), optical (green, extinction-corrected), and radio (blue; open and filled symbols are 5.5 and 9 GHz, respectively; circles = detection, triangles = upper limits) for the lowest-Ekin parameter set. Data are drawn with error bars, which are mostly smaller than the symbol size. The vertical line denotes the break time of 22.5 ks (see Sect. 3.2). Radio data have not been used in deriving the model, so the blue curve is actually a “predicted” light curve.
In the text

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