Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars(1), and some are bright enough that they should be observable out to redshifts of z > 20 using current technology(2-4). Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy(5). Here we report that GRB 090423 lies at a redshift of z approximate to 8.2, implying that massive stars were being produced and dying as GRBs similar to 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.

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A γ-ray burst at a redshift of z ≈ 8.2

We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall.

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The Afterglows of Swift-era Gamma-ray Bursts. I. Comparing pre-Swift and Swift-era Long/Soft (Type II) GRB Optical Afterglows

We invert directly the redshift-luminosity distribution of observed long Swift gamma-ray bursts (GRBs) to obtain their rate and luminosity function. Our best-fitting rate is described by a broken power law that rises like (1 + 2) 2.1+0.5 -0.6 for 0 3. The local rate is ρ 0 ≃ 1.3 +0.6 -0.7 (Gpc -3 yr -1 ). The luminosity function is well described by a broken power law with a break at L* ≃ 10 52.5±0.2 (erg s -1 ) and with indices α = 0.2 +0.2 -0.1 and β = 1.4 +0.3 -0.6 . The recently detected GRB 090423, with z ≈ 8, fits nicely into the model's prediction, verifying that we are allowed to extend our results to high redshifts. While the GRB rate may follow the star formation rate (SFR) for z < 3, the high-redshift slope is shallower than the steep decline in the SFR for 4 < z. However, we cannot rule out a GRB rate that follows one of the recent SFR models.

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The luminosity function and the rate of Swift's Gamma Ray Bursts

We present the second Swift Burst Alert Telescope (BAT) catalog of gamma-ray bursts (GRBs), which contains 476 bursts detected by the BAT between 2004 December 19 and 2009 December 21. This catalog (hereafter the BAT2 catalog) presents burst trigger time, location, 90% error radius, duration, fluence, peak flux, time-averaged spectral parameters and time-resolved spectral parametert:; measured by the BAT. In the correlation study of various observed parameters extracted from the BAT prompt emission data, we distinguish among long-duration GRBs (L-GRBs), short-duration GRBs (S-GRBs), and short-duration GRBs with extended emission (S-GRBs with E.E.) to investigate differences in the prompt emission properties. The fraction of L-GRBs, S-GRBs and S-GRBs with E.E. in the catalog are 89%, 8% and 2% respectively. We compare the BAT prompt emission properties with the BATSE, BeppoSAX and HETE-2 GRB samples. We also correlate the observed prompt emission properties with the redshifts for the GRBs with known redshift. The BAT T90 and T50 durations peak at 70 s and 30 s, respectively. We confirm that the spectra of the BAT S-GRBs are generally harder than those of the L-GRBs. The time-averaged spectra of the BAT S GRBs with E.E. are similar to those of the L-GRBs. Whereas, the spectra of the initial short spikes of the S-GRBs with E.E. are similar to those of the S-GRBs. We show that the BAT GRB samples are significantly softer than the BATSE bright GRBs, and that the time-averaged E obs/peak of the BAT GRBs peaks at 80 keV which is significantly lower energy than those of the BATSE sample which peak at 320 keV. The time-averaged spectral properties of the BAT GRB sample are similar to those of the HETE-2 GRB samples. By time-resolved spectral analysis, we find that 10% of the BAT observed photon indices are outside the allowed region of the synchrotron shock model. The observed durations of the BAT high redshift GRBs are not systematically longer than those of the moderate red shift GRBs. Furthermore, the observed spectra of the BAT high red shift GRBs are similar to or harder than the moderate red shift GRBs. The T90 and T50 distributions measured at the 140-220 keY band in the GRB rest frame form the BAT known redshift GRBs peak at 19 sand 8 s, respectively. We also provide an update on the status of the on-orbit BAT calibrations.

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The second swift burst alert telescope gamma-ray burst catalog

We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (d_l=2.3\pm 0.6) or in density (d_d=1.7\pm 0.5) is required in order to account for the observations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.

A complete sample of bright Swift Long Gamma-Ray Bursts: Sample presentation, Luminosity Function and evolution

(Abridged). We present a sample of 77 optical afterglows (OAs) of Swift detected GRBs for which spectroscopic follow-up observations have been secured. We provide linelists and equivalent widths for all detected lines redward of Ly-alpha. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray selected sample of Swift bursts with optimal conditions for ground-based follow up from the period March 2005 to September 2008; 146 bursts fulfill our sample criteria. We derive the redshift distribution for this sample and conclude that less than 19% of Swift bursts are at z>7. We compare the high energy properties for three sub-samples of bursts in the sample: i) bursts with redshifts measured from OA spectroscopy, ii) bursts with detected OA, but no OA-based redshift, and iii) bursts with no detection of the OA. The bursts in group i) have significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fraction of dark bursts is 14% in group i), 38% in group ii) and > 39% in group iii). From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight-lines. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular DLAs. On average GRB absorbers are characterized by significantly stronger EWs for HI as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. Based on the z>2 bursts in the sample we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies.

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Low-Resolution Spectroscopy of Gamma-ray Burst Optical Afterglows: Biases in the Swift Sample and Characterization of the Absorbers

We present the first search for galaxy counterparts of intervening high-z (2<z< 3.6) sub-DLAs and DLAs towards GRBs. Our final sample comprises of five intervening sub-DLAs and DLAs in four GRB fields. To identify candidate galaxy counterparts of the absorbers we use deep optical and near-infrared imaging, and low-, mid- and high-resolution spectroscopy acquired with 6 to 10-m class telescopes, the Hubble and the Spitzer space telescopes. Furthermore, we use the spectroscopic information and spectral-energy-distribution fitting techniques to study them in detail. Our main result is the detection and spectroscopic confirmation of the galaxy counterpart of the intervening DLA at z=3.096 in the field of GRB 070721B (z_GRB=3.6298) as proposed by other authors. We also identify good candidates for the galaxy counterparts of the two strong MgII absorbers at z=0.6915 and 1.4288 towards GRB 050820A (z_GRB=2.615). The properties of the detected DLA galaxy are typical for Lyman-break galaxies (LBGs) at similar redshifts; a young, highly starforming galaxy that shows evidence for a galactic outflow. This supports the hypothesis that a DLA can be the gaseous halo of an LBG. In addition, we report a redshift coincidence of different objects associated with metal lines in the same field, separated by 130-161 kpc. The high detection rate of three correlated structures on a length scale as small as ~150 kpc in two pairs of lines of sight is intriguing. The absorbers in each of these are most likely not part of the same gravitationally bound structure. They more likely represent groups of galaxies.

G. Bjoernsson

Papers

Low-Resolution Spectroscopy of Gamma-ray Burst Optical Afterglows: Biases in the Swift Sample and Characterization of the Absorbers

Galaxy counterparts of intervening high-z sub-DLAs/DLAs and MgII absorbers towards gamma-ray bursts