Search

Aims. We investigate the optical/near-infrared light curve of the afterglow of GRB 080710 in the context of rising afterglows.

Context. The detection of GeV photons from gamma-ray bursts (GRBs) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions. The full exploitation of the high-energy measurements relies, however, on accurate knowledge of the distance to the events.

The AGILE gamma-ray burst GRB 080514B is the first detected to have emission above 30?MeV and an optical afterglow. However, no spectroscopic redshift for this burst is known. We report on our ground-based optical/NIR and millimeter follow-up observations of this event at several observatories, including the multi-channel imager GROND on La Silla, supplemented by Swift UVOT and Swift XRT data. The spectral energy distribution?(SED) of the optical/NIR afterglow is found to decline sharply bluewards to the UV?bands, which can be utilized in estimating the redshift. Fitting the SED from the Swift UVOT uvw2 band to the H?band, we estimate a photometric redshift of z=1.8+0.4-0.3, which is consistent with the reported pseudo-redshift based on gamma-ray data. We find that the afterglow properties of GRB?080514B do not differ from those exhibited by the global sample of long bursts. Compared with the long burst sample, we conclude that this burst was special because of its high-energy emission properties, even though both its afterglow and host galaxy are not remarkable in any way. Obviously, high-energy emission in the gamma-ray band does not automatically correlate with the occurrence of special features in the corresponding afterglow light.

Aims.Using high-quality, broad-band afterglow data for GRB 091127, we investigate the validity of the synchrotron fireball model for gamma-ray bursts (GRBs), and infer physical parameters of the ultra-relativistic outflow.

Context.The afterglows and host galaxies of long gamma-ray bursts (GRBs) offer unique opportunities to study star-forming galaxies in the high-zUniverse. Until recently, however, the information inferred from GRB follow-up observations was mostly limited to optically bright afterglows, biasing all demographic studies against sight-lines that contain large amounts of dust.

Context.The link between the duration of GRBs and the nature of their progenitors remains disputed. Short bursts (with durations of less than  ~2 s) are less frequently observed, technically more difficult to localize, and exhibit significantly fainter afterglows.

Context.After the launch of the Swift satellite, the gamma-ray burst (GRB) optical light-curve smoothness paradigm has been questioned thanks to the faster and better sampled optical follow-up, which has unveiled a very complex behaviour. This complexity is triggering the interest of the whole GRB community. The GROND multi-channel imager is used to study optical and near-infrared (NIR) afterglows of GRBs with unprecedented optical and near-infrared temporal and spectral resolution. The GRB 081029 has a very prominent optical rebrightening event and is an outstanding example of the application of the multi-channel imager to GRB afterglows.

We present the results of a comprehensive study of the gamma-ray burst 080928 and of its afterglow. GRB 080928was a long burst detected by Swift/BAT and Fermi/GBM. It is one of the exceptional cases where optical emission had already been detected when the GRB itself was still radiating in the gamma-ray band. For nearly 100 s simultaneous optical, X-ray and gamma-ray data provide a coverage of the spectral energy distribution of the transient source from about 1 eV to 150 keV. In particular, we show that the SED during the main prompt emission phase agrees with synchrotron radiation. We constructed the optical/near-infrared light curve and the spectral energy distribution based on Swift/UVOT, ROTSE-IIIa (Australia), and GROND (La Silla) data and compared it to the X-ray light curve retrieved from the Swift/XRT repository. We show that its bumpy shape can be modeled by multiple energy-injections into the forward shock. Furthermore, weinvestigate whether the temporal and spectral evolution of the tail emission of the first strong flare seen in the early X-ray light curve can be explained by large-angle emission (LAE). We find that a nonstandard LAE model is required to explain the observations. Finally, we report on the results of our search for the GRB host galaxy, for which only a deep upper limit can be provided.

Aims.We report on the discovery and properties of the fading afterglow and underlying host galaxy of GRB 071028B, thereby facilitating a detailed comparison between these two.

Aims.We present a framework to obtain photometric redshifts (photo-zs) for gamma-ray burst afterglows. Using multi-band photometry from GROND and Swift/UVOT, photo-zs are derived for five GRBs for which spectroscopic redshifts are not available.

Context.Thirteen years after the discovery of the first afterglows, the nature of dark gamma-ray bursts (GRB) still eludes explanation: while each long-duration GRB typically has an X-ray afterglow, optical/NIR emission is only seen for 40–60% of them.

Aims.The quick and precise localization of GRBs by the Swift telescope allows the early evolution of the afterglow light curve to be captured by ground-based telescopes. With GROND measurements we can investigate the optical/near-infrared light curve of the afterglow of gamma-ray burst 080413B in the context of late rebrightening.

Aims. Fermican measure the spectral properties of gamma-ray bursts over a very large energy range and is opening a new window on the prompt emission of these energetic events. Localizations by the instruments on Fermiin combination with follow-up by Swiftprovide accurate positions for observations at longer wavelengths leading to the determination of redshifts, the true energy budget, host galaxy properties and facilitate comparison with pre-Fermi bursts.Methods. Multi-wavelength follow-up observations were performed on the afterglows of four bursts with high energy emission detected by Fermi/LAT: GRB 090323, GRB 090328, GRB 090510 and GRB 090902B. They were obtained in the optical/near-infrared bands with GROND mounted at the MPG/ESO 2.2 m telescope and additionally of GRB 090323 in the optical with the 2 m telescope in Tautenburg, Germany. Three of the events are classified as long bursts while GRB 090510 is a well localized short GRB with GeV emission. In addition, host galaxies were detected for three of the four bursts. Spectroscopic follow-up was initiated with the VLT for GRB 090328 and GRB 090510.Results. The afterglow observations in 7 bands are presented for all bursts and their host galaxies are investigated. Knowledge of the distance and the local dust extinction enables comparison of the afterglows of LAT-detected GRBs with the general sample. The spectroscopic redshifts of GRB 090328 and GRB 090510 were determined to be z= 0.7354 ±0.0003 and z= 0.903 ±0.001 and dust corrected star-formation rates of 4.8 $M_\odot$yr-1and 0.60 $M_\odot$yr-1were derived for their host galaxies, respectively.Conclusions. The afterglows of long bursts exhibit power-law decay indices (α) from less than 1 to ~2.3 and spectral indices ($\beta_{\rm opt}$) values from 0.65 to ~1.2 which are fairly standard for GRB afterglows. Constraints are placed on the jet half opening angles of $\la$2.1° to $\ga$6.4°, which allows limits to be placed on the beaming corrected energies. These range from $\la$5 × 1050erg to the one of the highest values ever recorded, $\ga$2.2 × 1052erg for GRB 090902B, and are not consistent with a standard candle. The extremely energetic long Fermibursts have optical afterglows which lie in the top half of the brightness distribution of all optical afterglows detected in the Swiftera or even in the top 5% if incompleteness is considered. The properties of the host galaxies of these LAT detected bursts in terms of extinction, star formation rates and masses do not appear to differ from previous samples.