Context. Our knowledge of the intrinsic properties of short duration Gamma-Ray Bursts has relied, so far, only upon a few cases for which the estimate of the distance and an extended, multiwavelength monitoring of the afterglow have been obtained. Aims. We carried out multiwavelength observations of the short GRB 061201 aimed at estimating its distance and studying its properties. Methods. We performed a spectral and timing analysis of the prompt and afterglow emission and discuss the results in the context of the standard fireball model. Results. A clear temporal break was observed in the X-ray light curve about 40 min after the burst trigger. We find that the spectral and timing behaviour of the X-ray afterglow is consistent with a jet origin of the observed break, although the optical data can not definitively confirm this and other scenarios are possible. No underlying host galaxy down to R ∼ 26 mag was found after fading of the optical afterglow. Thus, no secure redshift could be measured for this burst. The nearest galaxy is at z = 0.111 and shows evidence of star formation activity. We discuss the association of GRB 061201 with this galaxy and with the ACO S 995 galaxy cluster, from which the source is at an angular distance of 17 and 8.5 , respectively. We also test the association with a possible undetected, positionally consistent galaxy at z ∼ 1. In all these cases, in the jet interpretation, we find a jet opening angle of 1-2 degrees.Key words. gamma rays: bursts IntroductionShort duration Gamma-ray Bursts (GRBs) are historically defined as those GRBs with burst duration less than two seconds and hard spectra (Kouveliotou et al. 1993). As the sample of short GRBs increases, in order to take into account all the spectral and temporal properties of the prompt emission, alternative empirical definitions of short GRBs have been introduced (e.g. Norris & Bonnell 2006;Zhang et al. 2007).The expected progenitor for short GRBs is a merging binary system of compact objects. However, our present knowledge of the intrinsic properties of short bursts mainly relies upon a few cases for which the distance could be derived and an extendedThe results reported in this paper are partially based on observations carried out at ESO telescopes under program 078.D-0809.INAF personnel resident at ASDC. multiwavelength monitoring of the afterglow was carried out.From these it appears that short bursts are less energetic and less collimated than long GRBs (e.g. Fox et al. 2005;Burrows et al. 2006;Soderberg et al. 2006). Intrinsic spectral parameters such as the peak energy E p of the EF(E) spectrum of the burst and the equivalent isotropic energy E iso , do not match the E p − E iso correlation for long GRBs (e.g. Amati 2007).The short bursts for which unambiguous hosts have been found are at redshifts between z = 0.10 and z = 0.55. The optical afterglows of seven short bursts and their likely host galaxies have been recently observed with telescopes such as ESO/VLT, Gemini, Magellan and HST, but no firm estimation of...

We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, estimated from the presence of the Lyman break falling between the I and J filters. This is by far the most distant GRB known to date. Its isotropic-equivalent energy is 3.4 × 10 53 erg in the rest-frame 110−1100 keV energy band. Despite the high redshift, both the prompt and the afterglow emission are not peculiar with respect to other GRBs. We find a break in the J-band light curve at t b = 2.6 ± 1.0 d (observer frame). If we assume this is the jet break, we derive a beaming-corrected energy E γ ∼ (4 ÷ 12) × 10 51 erg. This limit shows that GRB 050904 is consistent with the Amati and Ghirlanda relations. This detection is consistent with the expected number of GRBs at z > 6 and shows that GRBs are a powerful tool to study the star formation history up to very high redshift.

We present optical observations of the short/hard gamma-ray burst GRB 050709, the first such event with an identified optical counterpart. The object is coincident with a weak X-ray source and is located inside a galaxy at redshift z = 0.1606 ± 0.0002. Multiband photometry allowed us to study the broad-band spectral energy distribution. Late-time monitoring places strong limits on any supernova simultaneous with the GRB. The host galaxy is not of early type. Spectra show that the dominant stellar population is relatively young (∼1 Gyr), and that ongoing star formation is present at a level of 2-3 L/L * M yr −1 . This is at least 2 orders of magnitude larger than that observed in the elliptical hosts of the short GRB 050509B and GRB 050724. This shows that at least some short GRBs originate in a young population. Short/hard GRB models based on the merger of a binary degenerate system are compatible with the host galaxy characteristics, although there is still the possibility of a connection between young stars and at least a fraction of such events.

Abstract. We present an optical-to-X-ray spectral analysis of the afterglow of GRB 020405. The optical spectral energy distribution not corrected for the extragalactic extinction is significantly below the X-ray extrapolation of the single powerlaw spectral model suggested by multiwavelength studies. We investigate whether considerable extinction could explain the observed spectral "mismatch" by testing several types of extinction curves. For the first time we test extinction curves computed with time-dependent numerical simulations of dust grains destruction by the burst radiation. We find that an extinction law weakly dependent on wavelength can reconcile the unabsorbed optical and X-ray data with the expected synchrotron spectrum. A gray extinction law can be provided by a dust grain size distribution biased toward large grains.

We present a possible OVIII X-ray absorption line at z = 0.117 ± 0.001 which, if confirmed, will be the first one associated with a broad HI Lyβ (BLB: FWHM=160 +50 −30 km s −1 ) absorber. The absorber lies along the line of sight to the nearby (z = 0.1372) Seyfert 1 galaxy PKS 0558-504, consistent with being a WHIM filament. The X-ray absorber is marginally detected in two independent XMM-Newton spectra of PKS 0558-504, a long ∼ 600 ks Guest-Observer observation and a shorter, ∼ 300 ks total, calibration observation, with a combined single line statistical significance of 2.8σ (2.7σ and 1.2σ in the two spectra, respectively). When fitted with our self-consistent hybrid-photoionization WHIM models, the combined XMM-Newton spectrum is consistent with the presence of OVIII Kα at z = (0.117 ± 0.001). This model gives best fitting temperature and equivalent H column density of the absorber of logT = 6.56 +0.19 −0.17 K, and logN H = (21.5 ± 0.3)(Z/Z 0.01⊙ ) −1 cm −2 , and predicts the marginal contribution of only two more lines within the XMM-Newton RGS band pass, NeIX Kα (λ = 13.45Å) and FeXVII L (λ = 15.02Å), both with equivalent widths well within the 1σ sensitivity of the combined XMM-Newton spectrum of PKS 0558-504 (EW 1σ < 3 mÅ). The lack of detection of associated OVI in the archival FUSE spectrum of PKS 0558-504, allows us to infer a tighter lower limit on the temperature, of logT > 6.52 K (at 1σ). -2 -The statistical sigificance of this single X-ray detection is increased by the detection of broad and complex HI Lyβ absorption in archival FUSE spectra of PKS 0558-504, at redshifts z = 0.1183 ± 0.0001 consistent with the bestfitting redshift of the X-ray absorber. The FUSE spectrum shows a broad (FWHM= 160 +50 −30 km s −1 ) absorption complex, which we identify as HI Lyβ z BLB = (0.1183 ± 0.0001). The single line statistical significance of this line is 4.1σ (3.7σ if systematics are considered). A possible HI Lyα is marginally hinted in an archival low-resolution (∆λ ∼ 6Å) IUE spectrum of PKS 0558-504, at a redshift of z = (0.119 ± 0.001) and with single line significance of 1.7σ. Thus, the combined significance of the three (XMM-Newton, FUSE, and IUE) independent tenative detections, is 5.2σ (5.0σ if the HI Lyα is not considered, and 4.6σ if the systematics in FUSE are considered).The detection of both metal and H lines at a consistent redshift, in this hot absorbing system, allows us to speculate on its metallicity. By associating the bulk of the X-ray absorber with the BLB line detected in the FUSE spectrum at z BLB = 0.1183 ± 0.0001, we obtain a metallicity of 1-4% Solar.Although the absorber is only blueshifted by ∼ −6000 km s −1 from the systemic redshift of PKS 0558-504, the identification of the absorbing gas with a high velocity nuclear ionized outflow, is unlikely. The physical, chemical and dynamical properties of the detected absorber are all quite different from those typically found in the Warm Absorber (WA) outflows, commonly detected in Seyferts and higher luminosity quasars. WA outflow velocit...