Search

Context. Gamma-ray bursts (GRBs) can provide information about star formation at high redshifts. Even in the absence of a bright optical/near-infrared/radio afterglow, the high detection rate of X-ray afterglows by Swift/XRT and its localization precision of 2-3 arcsec facilitates the identification and the study of GRB host galaxies. Aims. We focus on the search for the host galaxies of 17 bursts with arcsec-sized XRT error circles but no detected long-wavelength afterglow, in spite of their deep and rapid follow-up observations. Three of these events can also be classified as truly dark bursts, i.e., the observed upper limit on the optical flux of the afterglow was less than expected based on the measured X-ray flux. Our goals are to identify the GRB host galaxy candidates and characterize their phenomenological parameters. Methods. Our study is based on deep RC and K s-band observations performed with FORS1, FORS2, VIMOS, ISAAC, and HAWK-I at the ESO/VLT, partly supported by observations with the seven-channel imager GROND at the 2.2-m telescope on La Silla, and supplemented by observations with NEWFIRM at the 4-m telescope on Kitt Peak. To be conservative, we searched for host galaxy candidates within an area of twice the radius of each associated 90% c.l. Swift/XRT error circle. Results. For 15 of the 17 bursts, we find at least one galaxy within the searching area, and in the remaining two cases only a deep upper limit to RC and Ks can be provided. In seven cases, we discover extremely red objects in the error circles, at least four of which might be dust-enshrouded galaxies. The most remarkable case is the host of GRB 080207, which has a color of (R C-Ks)AB ~ 4.7 mag, and is one of the reddest galaxies ever associated with a GRB. As a by-product of our study we identify the optical afterglow of GRB 070517. Conclusions. Only a minority of optically dim afterglows are due to Lyman dropout (≤ 1/3). Extinction by dust in the host galaxies might explain all other events. Thereby, a seemingly non-negligible fraction of these hosts are globally dust-enshrouded, extremely red galaxies. This suggests that at least a fraction of GRB afterglows trace a subpopulation of massive starburst galaxies, which are markedly different from the main body of the GRB host galaxy population, namely the blue, subluminous, compact galaxies

(Abridged) Until recently the information inferred from gamma-ray burst follow-up observations was mostly limited to optically bright afterglows, biasing all demographic studies against sight-lines that contain large amounts of dust. Here, we present GRB afterglow and host observations for a sample of bursts that are exemplary of previously missed ones because of high visual extinction along the sight-line. This facilitates an investigation of the properties, geometry and location of the absorbing dust of these poorly-explored host galaxies, and a comparison to hosts from optically-selected samples. The hosts of the dustiest afterglows are diverse in their properties, but on average redder, more luminous and massive than the hosts of optically-bright events. We hence probe a different galaxy population, suggesting that previous host samples miss most of the massive, chemically-evolved and metal-rich members. This also indicates that the dust along the sight-line is often related to host properties, and thus probably located in the diffuse ISM or interstellar clouds and not in the immediate GRB environment. Some of the hosts in our sample, are blue, young or of small stellar mass illustrating that even apparently non-extinguished galaxies possess very dusty sight-lines due to a patchy dust distribution. The presented observations establish a population of luminous, massive and correspondingly chemically-evolved GRB hosts. This suggests that GRBs trace the global star-formation rate better than studies based on optically-selected host samples indicate, and the previously-claimed deficiency of high-mass host galaxies was at least partially a selection effect., 17 pages, 13 figures, resubmitted to A&A after referee report

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. Aims: It is of critical importance to establish whether the burst duration can reliably distinguish the different GRB population models of collapsars and compact stellar mergers. The Swift GRB 090426 provides an unique opportunity to address this question. Its duration (T_90=1.28 s) places GRB 090426 firmly in the short burst population, while the high redshift (z=2.609), host galaxy properties, and prompt emission spectral characteristics are more similar to those of long-duration GRBs. Methods: On the basis of data obtained with the Tautenburg 2m telescope (Germany) and the 7-channel imager GROND (La Silla, Chile), we compiled the most finely sampled light curve available for a short burst optical/NIR afterglow. The light curve was then analysed in a standard fashion. GROND and XRT data were used to determine the broad-band spectral energy distribution of the afterglow across more than three orders of magnitude. Results: Our data show that a light curve break exists at 0.4 days, which is followed by a steep decay. This light curve decay is achromatic in the optical/NIR bands, and interpreted as a post-jet break phase. The X-ray data do not disagree with this interpretation. Conclusions: The half-opening angle of the suspected jet as well as the luminosity of the optical afterglow provide additional evidence that GRB 090426 is probably linked to the death of a massive star rather than to the merger of two compact objects., Comment: letter to A&A, in press

Using high-quality, broad-band afterglow data for GRB 091029, we test the validity of the forward-shock model for gamma-ray burst afterglows. We used multi-wavelength (NIR to X-ray) follow-up observations obtained with the GROND, BOOTES-3/YA and Stardome optical ground-based telescopes, and the UVOT and the XRT onboard the Swift satellite. To explain the almost totally decoupled light curves in the X-ray and optical/NIR domains, a two-component outflow is proposed. Several models are tested, including continuous energy injection, components with different electron energy indices and components in two different stages of spectral evolution. Only the last model can explain both the decoupled light curves with asynchronous peaks and the peculiar SED evolution. However, this model has so many unknown free parameters that we are unable to reliably confirm or disprove its validity, making the afterglow of GRB 091029 difficult to explain in the framework of the simplest fireball model., Comment: Accepted to A&A

Using high-quality, broad-band afterglow data for GRB 091127, we investigate the validity of the synchrotron fireball model for gamma-ray bursts, and infer physical parameters of the ultra-relativistic outflow. We used multi-wavelength follow-up observations obtained with GROND and the XRT onboard the Swift satellite. The resulting afterglow light curve is of excellent accuracy, and the spectral energy distribution is well-sampled over 5 decades in energy. These data present one of the most comprehensive observing campaigns for a single GRB afterglow and allow us to test several proposed emission models and outflow characteristics in unprecedented detail. Both the multi-color light curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a cooling break moving from high to lower energies. The early light curve is well described by a broken power-law, where the initial decay in the optical/NIR wavelength range is considerably flatter than at X-rays. Detailed fitting of the time-resolved SED shows that the break is very smooth with a sharpness index of 2.2 +- 0.2, and evolves towards lower frequencies as a power-law with index -1.23 +- 0.06. These are the first accurate and contemporaneous measurements of both the sharpness of the spectral break and its time evolution. The measured evolution of the cooling break (nu_c propto t^-1.2) is not consistent with the predictions of the standard model, wherein nu_c propto t^-0.5 is expected. A possible explanation for the observed behavior is a time dependence of the microphysical parameters, in particular the fraction of the total energy in the magnetic field epsilon_B. This conclusion provides further evidence that the standard fireball model is too simplistic, and time-dependent micro-physical parameters may be required to model the growing number of well-sampled afterglow light curves., accepted to A&A, 13 pages, 5 figures