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Context. Afterglows of gamma-ray bursts (GRBs) are simple in the most basic model, but can show many complex features. The ultra-long duration GRB 111209A, one of the longest GRBs ever detected, also has the best-monitored afterglow in this rare class of GRBs. Aims. We want to address the question whether GRB 111209A was a special event beyond its extreme duration alone, and whether it is a classical GRB or another kind of high-energy transient. The afterglow may yield significant clues. Methods. We present afterglow photometry obtained in seven bands with the GROND imager as well as in further seven bands with the Ultraviolet/Optical Telescope (UVOT) on-board the Neil Gehrels Swift Observatory. The light curve is analysed by multi-band modelling and joint fitting with power-laws and broken power-laws, and we use the contemporaneous GROND data to study the evolution of the spectral energy distribution. We compare the optical afterglow to a large ensemble we have analysed in earlier works, and especially to that of another ultra-long event, GRB 130925A. We furthermore undertake a photometric study of the host galaxy. Results. We find a strong, chromatic rebrightening event at ≈0.8 days after the GRB, during which the spectral slope becomes redder. After this, the light curve decays achromatically, with evidence for a break at about 9 days after the trigger. The afterglow luminosity is found to not be exceptional. We find that a double-jet model is able to explain the chromatic rebrightening. The afterglow features have been detected in other events and are not unique. Conclusions. The duration aside, the GRB prompt emission and afterglow parameters of GRB 111209A are in agreement with the known distributions for these parameters. While the central engine of this event may differ from that of classical GRBs, there are multiple lines of evidence pointing to GRB 111209A resulting from the core-collapse of a massive star with a stripped envelope. [ABSTRACT FROM AUTHOR]

We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 galaxies selected by long γ-ray bursts (GRBs) at 0.1 med ~ 1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFR), visual attenuations (AV), oxygen abundances (12 + log (O/H)), and emission-line widths (σ). We study GRB hosts up to z ~ 3.5 and find a strong change in their typical physical properties with redshift. The median SFR of our GRB hosts increases from SFRmed ~ 0.6 M⊙ yr-1 at z ~ 0.6 up to SFRmed ~ 15 M⊙ yr-1 at z ~ 2. A higher ratio of [OIII]/[OII] at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the Baldwin-Phillips-Terlevich diagram. There is weak evidence for a redshift evolution in AV and σ, with the highest values seen at z ~ 1.5(AV) or z ~ 2 (σ). Oxygen abundances of the galaxies are distributed between 12 + log (O/H) = 7.9 and 12 + log (O/H) = 9.0 with a median 12 + log (O/H)med ~ 8.5. The fraction of GRB-selected galaxies with super-solar metallicities is ~20% at z< 1 in the adopted metallicity scale. This is significantly less than the fraction of total star formation in similar galaxies, illustrating that GRBs are scarce in high metallicity environments. At z ~ 3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z ≲ 0.5 Z⊙. Together with a high incidence of Z ~ 0.5 Z⊙ galaxies at z ~ 1.5, this indicates that a metallicity dependence at low redshift will not be dominant at z ~ 3. Significant correlations exist between the hosts' physical properties. Oxygen abundance, for example, relates to AV (12 + log (O/H) ∝ 0.17⋅AV), line width (12 + log(O/H) ∝ σ0.6), and SFR (12 + log(O/H) ∝ SFR0.2). In the last two cases, the normalization of the relations shift to lower metallicities at z> 2 by~0.4 dex. These properties of GRB hosts and their evolution with redshift can be understood in a cosmological context of star-forming galaxies and a picture in which the hosts' properties at low redshift are influenced by the tendency of GRBs to avoid the most metal-rich environments. [ABSTRACT FROM AUTHOR]

Context. After the discovery of the first connection between γ-ray bursts (GRBs) and supernovae (SNe) almost two decades ago, tens of SN-like rebrightenings have been discovered and about seven solid associations have been spectroscopically confirmed to date. Aims. We determine the luminosity, evolution, and origin of three SN rebrightenings in GRB afterglow light curves at z ∼ 0.5 along with accurate determinations of the host-galaxy extinction. We estimate physical parameters of the SN explosions, such as synthesised 56Ni mass, ejecta mass, and kinetic energy. Methods. We employ GROND optical/NIR data and Swift X-ray/UV data to estimate the host-galaxy extinction by modelling the afterglow spectral energy distribution, to determine the SN luminosity and evolution, and to construct quasi-bolometric light curves. The latter are corrected for the contribution of the NIR-bands using data available in the literature and black-body fits. We employ Arnett's analytic approach to obtain the physical parameters of the explosion. Results. The SNe 2008hw, 2009nz, and 2010ma observed by GROND exhibit 0.80, 1.15, and 1.78 times the optical (r'-band) luminosity of SN 1998bw, respectively. While SN 2009nz exhibits an evolution similar to SN 1998bw, SNe 2008hw and 2010ma show earlier peak times. The quasi-bolometric light curves (340-2200 nm) confirm the large luminosity of SN 2010ma (1.4 x 1043 erg s-1), while SNe 2008hw and 2009nz reached a peak luminosity closer to that of SN 1998bw. The modelling indicates in 56Ni masses of around 0.4-0.5 M☉. Conclusions. By means of a very comprehensive data set, we found that the luminosity and the 56Ni mass of SNe 2008hw, 2009nz, and 2010ma resembles those of other known GRB-associated SNe. These findings strengthens previous claims of GRB-SNe being brighter than stripped-envelope SNe unaccompanied by GRBs. [ABSTRACT FROM AUTHOR]

The mechanism that causes the prompt-emission episode of gamma-ray bursts (GRBs) is still widely debated despite there being thousands of prompt detections. The favoured internal shock model relates this emission to synchrotron radiation. However, it does not always explain the spectral indices of the shape of the spectrum, which is often fit with empirical functions, such as the Band function. Multi-wavelength observations are therefore required to help investigate the possible underlying mechanisms that causes the prompt emission. We present GRB 121217A, for which we were able to observe its near-infrared (NIR) emission during a secondary prompt-emission episode with the Gamma-Ray burst Optical Near-infrared Detector (GROND) in combination with the Swift and Fermi satellites, which cover an energy range of 5 orders of magnitude (10-3 keV to 100 keV). We determine a photometric redshift of z = 3:1 ± 0:1 with a line-of-sight with little or no extinction (AV ∼ 0 mag) utilising the optical/NIR SED. From the afterglow, we determine a bulk Lorentz factor of Γ0 ∼ 250 and an emission radius of R < 1018 cm. The prompt-emission broadband spectral energy distribution is well fit with a broken power law with β1 = -0:3 ± 0:1 and β2 = 0:6 ± 0:1 that has a break at E = 6:6 ± 0:9 keV, which can be interpreted as the maximum injection frequency. Self-absorption by the electron population below energies of Ea < 6 keV suggest a magnetic field strength of B ∼ 105 G. However, all the best fit models underpredict the flux observed in the NIR wavelengths, which also only rebrightens by a factor of ∼2 during the second prompt emission episode, in stark contrast to the X-ray emission, which rebrightens by a factor of ∼100. This suggests an afterglow component is dominating the emission. We present GRB 121217A, one of the few GRBs that has multi-wavelength observations of the prompt-emission period and shows that it can be understood with a synchrotron radiation model. However, due to the complexity of the GRB's emission, other mechanisms that result in Band-like spectra cannot be ruled out. [ABSTRACT FROM AUTHOR]

Context. In the past few years the number of well-sampled optical to near-infrared (NIR) light curves of long gamma-ray bursts (GRBs) has greatly increased, particularly due to simultaneous multi-band imagers such as GROND. Combining these densely sampled ground-based data sets with the Swift UVOT and XRT space observations unveils a much more complex afterglow evolution than what was predicted by the most commonly invoked theoretical models. GRB100814A represents a remarkable example of these interesting well-sampled events, showing a prominent late-time rebrightening in the optical to NIR bands and a complex spectral evolution. This represents a unique laboratory to test the different afterglow emission models. Aims. Here we study the nature of the complex afterglow emission of GRB100814A in the framework of different theoretical models. Moreover, we compare the late-time chromatic rebrightening with those observed in other well-sampled long GRBs. Methods. We analysed the optical and NIR observations obtained with the seven-channel Gamma-Ray burst Optical and Near-infrared Detector (GROND) at the 2.2 m MPG/ESO telescope together with the X-ray and UV data detected by the instruments onboard the Swift observatory. The broad-band afterglow evolution, achieved by constructing multi-instrument light curves and spectral energy distributions, is discussed in the framework of different theoretical models. Results. We find that the standard models that describe the broad-band afterglow emission within the external shock scenario fail to describe the complex evolution of GRB100814A, and therefore more complex scenarios must be invoked. The analysis of the very well sampled broad-band light curve of GRB100814A allowed us to deduce that models invoking late-time activity of the central engine in the observed afterglow emission are the preferred ones for all the different observed features. This late-time activity most likely has the form of a delayed reactivation of the ejecta emission process. However, a more detailed modelling of the radiative mechanisms associated with these scenarios is necessary to arrive at a firm conclusion on the nature of the optical rebrightenings that so often are detected in long GRBs. [ABSTRACT FROM AUTHOR]

Aims. With the afterglow of GRB 100621A being the brightest detected so far in X-rays, and superb GROND coverage in the optical/near-infrared during the first few hours, an observational verification of basic fireball predictions seemed possible. Methods. In order to constrain the broad-band spectral energy distribution of the afterglow of GRB 100621A, dedicated observations were performed in the optical/near-infrared with the 7-channel Gamma-Ray Burst Optical and Near-infrared Detector (GROND) at the 2.2mMPG/ESO telescope, in the sub-millimeter band with the large bolometer array LABOCA at APEX, and at radio frequencies with ATCA. Utilizing also Swift X-ray observations, we attempt an interpretation of the observational data within the fireball scenario. Results. The afterglow of GRB 100621A shows a very complex temporal and spectral evolution. We identify three different emission components, the most spectacular one causing a sudden intensity jump about one hour after the prompt emission. The spectrum of this component is much steeper than the canonical afterglow. We interpret this component using a two-shell collision prescription after the first shell has been decelerated by the circumburst medium. We use the fireball scenario to derive constraints on the microphysical parameters of the first shell. Long-term energy injection into a narrow jet seems to provide an adequate description. Another noteworthy result is the large (AV = 3.6 mag) line-of-sight host extinction of the afterglow in an otherwise extremely blue host galaxy. Conclusions. Some GRB afterglows have shown complex features, and that of GRB 100621A is another good example. Yet, detailed observational campaigns of the brightest afterglows promise to deepen our understanding of the formation of afterglows and the subsequent interaction with the circumburst medium. [ABSTRACT FROM AUTHOR]

We present the discovery of molecular hydrogen (H2), including the presence of vibrationally-excited H" 2 in the optical spectrum of the afterglow of GRB 120815A at z = 2:36 obtained with X-shooter at the VLT. Simultaneous photometric broad-band data from GROND and X-ray observations by Swift/XRT place further constraints on the amount and nature of dust along the sightline. The galactic environment of GRB 120815A is characterized by a strong DLA with log(N(H i)=cm-2) = 21:95 ± 0:10, prominent H2 absorption in the Lyman-Werner bands (log(N(H2)=cm-2) = 20:54 ± 0:13) and thus a molecular gas fraction log f (H2) = -1:14 ± 0:15. The distance d between the absorbing neutral gas and GRB 120815A is constrained via photo-excitation modeling of fine-structure and meta-stable transitions of Fe ii and Ni ii to d = 0:5 ± 0:1 kpc. The DLA metallicity ([Zn=H] = -1:15 ± 0:12), visual extinction (AV <± 0:15 mag) and dust depletion ([Zn=Fe] = 1:01 ± 0:10) are intermediate between the values of well-studied, H2-deficient GRB-DLAs observed at high spectral resolution, and the approximately solar metallicity, highly-obscured and H2-rich GRB 080607 sightline. With respect to N(H i), metallicity, as well as dust-extinction and depletion, GRB 120815A is fairly representative of the average properties of GRB-DLAs. This demonstrates that molecular hydrogen is present in at least a fraction of the more typical GRBDLAs, and H2 and H± 2 are probably more wide-spread among GRB-selected systems than the few examples of previous detections would suggest. Because H± 2 transitions are located redwards of the Lyman ± absorption, H± 2 opens a second route for positive searches for molecular absorption also in GRB afterglows at lower redshifts and observed at lower spectral resolution. Further detections of molecular gas in GRB-DLAs would allow statistical studies, and, coupled with host follow-up and sub-mm spectroscopy, provide unprecedented insights into the process and conditions of star-formation at high redshift. [ABSTRACT FROM AUTHOR]

Galaxies selected through long ?-ray bursts (GRBs) could be of fundamental importance when mapping the star formation history out to the highest redshifts. Before using them as efficient tools in the early Universe, however, the environmental factors that govern the formation of GRBs need to be understood. Metallicity is theoretically thought to be a fundamental driver in GRB explosions and energetics, but it is still, even after more than a decade of extensive studies, not fully understood. This is largely related to two phenomena: a dust-extinction bias, which prevented high-mass and thus likely high-metallicity GRB hosts from being detected in the first place, and a lack of efficient instrumentation, which limited spectroscopic studies, including metallicity measurements, to the low-redshift end of the GRB host population. The subject of this work is the very energetic GRB 110918A (EY,iso = 1.9 × 1054 erg), for which we measure a redshift of z = 0.984. GRB 110918A gave rise to a luminous afterglow with an intrinsic spectral slope of β = 0.70, which probed a sight-line with little extinction (AVGRB = 0.16 mag) and soft X-ray absorption (NH,X = (1.6±0.5)×1021 cm-2) typical of the established distributions of afterglow properties. However, photometric and spectroscopic follow-up observations of the galaxy hosting GRB 110918A, including optical/near-infrared photometry with the Gamma-Ray burst Optical Near-infrared Detector and spectroscopy with the Very Large Telescope/X-shooter, reveal an all but average GRB host in comparison to the z ∼ 1 galaxies selected through similar afterglows to date. It has a large spatial extent with a half-light radius of R1/2 ∼ 10 kpc, the highest stellar mass for z < 1.9 (log(M*/M⊚) = 10.68 ± 0.16), and an Hα-based star formation rate of SFRHa = 41+28-16 M⊚ yr-1. We measure a gasphase extinction of AVgas ∼ 1.8mag through the Balmer decrement and one of the largest host-integrated metallicities ever of around solar using the well-constrained ratios of [NII]/Hα and [NII]/[O III] (12 + log (O/H) = 8.93 ± 0.13 and 8.85+0.14-0.18, respectively). This presents one of the very few robust metallicity measurements of GRB hosts at z ∼ 1, and establishes that GRB hosts at z ∼ 1 can also be very metal rich. It conclusively rules out a metallicity cut-off in GRB host galaxies and argues against an anti-correlation between metallicity and energy release in GRBs. [ABSTRACT FROM AUTHOR]

There is evidence of an overdensity of strong intervening Mgll absorption line systems distributed along the lines of sight toward gamma-ray burst (ORB) afterglows relative to quasar sight-lines. If this excess is real, one should also expect an overdensity of field galaxies around ORB sight-lines, as strong MgII tends to trace these sources. In this work, we test this expectation by calculating the two point angular correlation function of galaxies within 120" (~470 h71-1 Kpc at (z) ~ 0.4) of GRB afterglows. We compare the gamma-ray burst optical and near-infrared detector (GROND) ORB afterglow sample - one of the largest and most homogeneous samples of ORB fields - with galaxies and active galactic nuclei found in the COSMOS-30 photometric catalog. We find no significant signal of anomalous clustering of galaxies at an estimated median redshift of z ~ 0.3 around ORB sight-lines, down to KAB < 19.3. This result is contrary to the expectations from the Mgll excess derived from ORB afterglow spectroscopy, although many confirmed galaxy counterparts to MgII absorbers may be too faint to detect in our sample - especially those at z > 1. We note that the addition of higher sensitivity SpitzerfIRAC or HST/WFC3 data for even a subset of our sample would increase this survey's depth by several orders of magnitude, simultaneously increasing statistics and enabling the investigation of a much larger redshift space. [ABSTRACT FROM AUTHOR]

We report on follow-up observations of 20 short-duration gamma-ray bursts (GRBs; T90 < 2 s) performed in g'r'i'z'JHKs with the Gamma-Ray Burst Optical Near-Infrared Detector (GROND) between mid-2007 and the end of 2010. This is one of the most comprehensive data sets on GRB afterglow observations of short bursts published so far. In three cases, GROND was on target within less than 10 min after the trigger, leading to the discovery of the afterglow of GRB 081226A and its faint underlying host galaxy. In addition, GROND was able to image the optical afterglow and follow the light curve evolution in five further cases: GRBs 090305, 090426, 090510, 090927, and 100117A. In all other cases, optical/near-infrared upper limits can be provided on the afterglow magnitudes. After shifting all light curves to a common redshift, we find that the optical luminosities of the six events with light curves group into two subsamples. GRBs 090426 and 090927 are situated in the regime occupied by long-duration events (collapsars), while the other four bursts occupy the parameter space typical for merger events, confirming that the short-burst population is contaminated by collapsar events. Three of the aforementioned six bursts with optical light curves show a break: GRBs 090426 and 090510 (Papers I and II) as well as GRB 090305. For GRB 090927, no break is seen in the optical/X-ray light curve until about 150 ks/600 ks after the burst. The GROND multi-color data support the view that this burst is related to a collapsar event. A decay slope of the optical afterglow of GRB 100117A could be measured. For all six GRBs a lower limit on the corresponding jet opening angle can be set. Using these data supplemented by about ten events taken from the literature, we compare the jet half-opening angles of long and short bursts. We find tentative evidence that short bursts have wider opening angles than long bursts. However, the statistics are still very poor. [ABSTRACT FROM AUTHOR]

Context. The first observational evidence of a connection between supernovae (SNe) and gamma-ray bursts (GRBs) was found about a decade ago. Since then, only half a dozen spectroscopically confirmed associations have been discovered and XRF 100316D/SN 2010bh is among the latest. Aims. We constrain the progenitor radius, the host-galaxy extinction, and the physical parameters of the explosion of XRF 100316D and its associated SN 2010bh at z = 0.059. We study the brightness and colours of SN 2010bh in the context of GRB-SNe. Methods. We began observations 12 h after the GRB trigger and continued until 80 days after the burst. The Gamma-Ray burst Optical and Near-infrared Detector (GROND) provided excellent photometric data of XRF 100316D/SN 2010bh in six filter bands covering a wavelength range from approximately 350 to 1800 nm, significantly expanding the pre-existing data set for this event. Combining GROND and Swift data, the early broad-band spectral energy distribution (SED) is modelled with a blackbody and afterglow component attenuated by dust and gas absorption. The temperature and radius evolution of the thermal component are analysed and combined with earlier measurements available from the literature. Templates of SN 1998bw are fitted to the SN itself to directly compare the light-curve properties. Finally, a two-component parametrised model is fitted to the quasi-bolometric light curve, which delivers physical parameters of the explosion. Results. The best-fit models to the broad-band SEDs imply moderate reddening along the line of sight through the host galaxy (AV,host = 1.2 ± 0.1 mag). Furthermore, the parameters of the blackbody component reveal a cooling envelope at an apparent initial radius of 7 × 1011 cm, which is compatible with a dense wind surrounding a Wolf-Rayet star. A multicolour comparison shows that SN 2010bh is 60-70% as bright as SN 1998bw. It proves to be the most rapidly evolving GRB-SNe to date, reaching maximum brightness at 8-9 days after the burst in the blue bands. Modelling of the quasi-bolometric light curve yields MNi = 0.21 ± 0.03 M☉ and Mej = 2.6 ± 0.2 M☉, typical of values within the GRB-SN population. The kinetic energy is Ek = (2.4 ± 0.7) × 1052 erg, which is making this SN the second most energetic GRB-SN after SN 1998bw. Conclusions. This supernova has one of the earliest peaks ever recorded and thereafter fades more rapidly than other GRB-SNe, hypernovae, or typical type-Ic SNe. This implies that a thin envelope is possibly expanding at very high velocities and is, therefore, unable to retain the γ-rays that would prolong the duration of the SN event. [ABSTRACT FROM AUTHOR]

Context. The Swift discovery of the short burst GRB 090510 has raised considerable attention mainly because of two reasons: first, it had a bright optical afterglow, and second it is among the most energetic events detected so far within the entire GRB population (long plus short). The afterglow of GRB 090510 was observed with Swift/UVOT and Swift/XRT and evidence of a jet break around 1.5 ks after the burst has been reported in the literature, implying that after this break the optical and X-ray light curve should fade with the same decay slope. Aims. As noted by several authors, the post-break decay slope seen in the UVOT data is much shallower than the steep decay in the X-ray band, pointing to a (theoretically hard to understand) excess of optical flux at late times. We assess here the validity of this peculiar behavior. Methods. We reduced and analyzed new afterglow light-curve data obtained with the multichannel imager GROND. These additional g'r'i'z' data were then combined with the UVOT and XRT data to study the behavior of the afterglow at late times more stringently. Results. Based on the densely sampled data set obtained with GROND, we find that the optical afterglow of GRB 090510 did indeed enter a steep decay phase starting around 22 ks after the burst. During this time the GROND optical light curve is achromatic, and its slope is identical to the slope of the X-ray data. In combination with the UVOT data this implies that a second break must have occurred in the optical light curve around 22 ks post burst, which, however, has no obvious counterpart in the X-ray band, contradicting the interpretation that this could be another jet break. Conclusions. The GROND data provide the missing piece of evidence that the optical afterglow of GRB 090510 did follow a post-jet break evolution at late times. The break seen in the optical light curve around 22 ks in combination with its missing counterpart in the X-ray band could be due to the passage of the injection frequency across the optical bands, as already theoretically proposed in the literature. This is possibly the first time that this passage has been clearly seen in an optical afterglow. In addition, our results imply that there is no more evidence for an excess of flux in the optical bands at late times. [ABSTRACT FROM AUTHOR]

Context. Observations of the γ-ray sky with Fermi led to significant advances towards understanding blazars, the most extreme class of active galactic nuclei. A large fraction of the population detected by Fermi is formed by BL Lacertae (BL Lac) objects, whose sample has always suffered from a severe redshift incompleteness due to the quasi-featureless optical spectra. Aims. Our goal is to provide a significant increase of the number of confirmed high-redshift BL Lac objects contained in the 2 LAC Fermi/LAT cataloge. Methods. For 103 Fermi/LAT blazars, photometric redshifts using spectral energy distribution fitting have been obtained. The photometry includes 13 broad-band filters from the far ultraviolet to the near-IR observed with Swift/UVOT and the multi-channel imager GROND at the MPG/ESO 2.2 m telescope. Data have been taken quasi-simultaneously and the remaining source-intrinsic variability has been corrected for. Results. We release the UV-to-near-IR 13-band photometry for all 103 sources and provide redshift constraints for 75 sources without previously known redshift. Out of those, eight have reliable photometric redshifts at z ≳ 1.3, while for the other 67 sources we provide upper limits. Six of the former eight are BL Lac objects, which quadruples the sample of confirmed high-redshift BL Lac. This includes three sources with redshifts higher than the previous record for BL Lac, including CRATES J0402-2615, with the best-fit solution at z ≈ 1.9. [ABSTRACT FROM AUTHOR]