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.

Aims. With this paper we want to investigate the highly variable afterglow light curve and environment of gamma-ray burst (GRB) 060526 at z = 3.221.Methods. We present one of the largest photometric datasets ever obtained for a GRB afterglow, consisting of multi-color photometric data from the ultraviolet to the near infrared. The data set contains 412 data points in total to which we add additional data from the literature. Furthermore, we present low-resolution high signal-to-noise spectra of the afterglow. The afterglow light curve is modeled with both an analytical model using broken power law fits and with a broad-band numerical model which includes energy injections. The absorption lines detected in the spectra are used to derive column densities using a multi-ion single-component curve-of-growth analysis from which we derive the metallicity of the host of GRB 060526.Results. The temporal behaviour of the afterglow follows a double broken power law with breaks at t = 0.090 +/- 0.005 and t = 2.401 +/- 0.061 days. It shows deviations from the smooth set of power laws that can be modeled by additional energy injections from the central engine, although some significant microvariability remains. The broadband spectral-energy distribution of the afterglow shows no significant extinction along the line of sight. The metallicity derived from S II and Fe II of [S/H] = -0.57 +/- 0.25 and [Fe/H] = -1.09 +/- 0.24 is relatively high for a galaxy at that redshift but comparable to the metallicity of other GRB hosts at similar redshifts. At the position of the afterglow, no host is detected to F775W(AB) = 28.5 mag with the HST, implying an absolute magnitude of the host M(1500 angstrom) > -18.3 mag which is fainter than most long-duration hosts, although the GRB may be associated with a faint galaxy at a distance of 11 kpc.

In this paper, Figure 14 is incomplete due to an error during production. We here provide the missing sub-figures.

Aims. With this paper we want to investigate the highly variable afterglow light curve and environment of Gamma-Ray Burst (GRB) 060526 at z=3.221. Methods. We present one of the largest photometric datasets ever obtained for a GRB afterglow, consisting of multi-color photometric data from the optical to the NIR. The data set contains 218 data points in total to which we add additional data from the literature. Furthermore, we present low-resolution high signal-to-noise spectra of the afterglow. The afterglow light curve is modeled with both an analytical model using broken power-law fits and with a broadband numerical energy injection model. The absorption lines detected in the spectra are used to derive column densities using a multi-ion single-component curve-of-growth analysis from which we derive the metallicity of the host of GRB 060526. Results. The overall light curve follows a broken power-law with a break at t = 2:401 0:061 days. It shows deviations from the smooth power-law that can be explained by additional energy injections from the central engine. The broadband SED of the afterglow shows little extinction along the line of sight. The metallicity derived from S II and Fe II is relatively high for a galaxy at that redshift but comparable to the metallicity of other GRB hosts at similar redshifts. There is a candidate host galaxy at a relatively large o set of 7.7 kpc from the afterglow position with R =26.4 mag which would imply a rather luminous host.

(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.

We present deep optical and infrared (IR) observations of the short-duration GRB 050906. Although no X-ray or optical/IR afterglow was discovered to deep limits, the error circle of the gamma-ray burst (GRB) (as derived from the Swift Burst Alert Telescope, or BAT) is unusual in containing the relatively local starburst galaxy IC328. This makes GRB 050906 a candidate burst from a soft gamma-ray repeater (SGR), similar to the giant flare from SGR 1806-20. The probability of chance alignment of a given BAT position with such a galaxy is small (less than or similar to 1 per cent), although the size of the error circle (2.6 arcmin radius) is such that a higher z origin cannot be ruled out. Indeed, the error circle also includes a moderately rich galaxy cluster at z = 0.43, which is a plausible location for the burst given the apparent preference that short-duration GRBs have for regions of high mass density. No residual optical or IR emission has been observed, in the form of either an afterglow or a later time emission from any associated supernova-like event. We discuss the constraints these limits place on the progenitor of GRB 050906 based on the expected optical signatures from both SGRs and merging compact object systems.

The optical afterglow spectrum of GRB050401 (at z = 2.8992±0.0004) shows the presence of a large damped Lyα absorber (DLA), with log NH I = 22.5± 0.3. This is the highest column density DLA ever observed, and is nearly an order Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark; darach@astro.ku.dk European Southern Observatory, Casilla 19001, Santiago 19, Chile Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720, USA Nordic Optical Telescope, Apartado 474, Santa Cruz de La Palma, Spain Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate (LC), Italy Dipartimento di Fisica e Matematica, Universitá dell’Insubria, via Valleggio 1 1, I-22100 Como, Italy Instituto de Astrofisica de Andalucia, CSIC, c/ Camino Bajo de Huetor 24, 18008 Granada, Spain Joint Astronomy Centre, 660 North A’Ohoku Place, Hilo, HI 96720, USA Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan ALMA Project Office, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK Space Research Institute (IKI) 117997, 84/32 Profsoyuznaya Str, Moscow, Russia Laboratory for High Energy Astrophysics, Code 662, NASA Goddard Space Flight Center, Greenbelt Road, Greenbelt, MD 20771, USA Universities Space Research Association Crimean Astrophysical Observatory, Ukraine Instituto Astrofisica de Canarias, c/ Via Lactea s/n, 38200 La Laguna, Tenerife, Spain Ulugh Beg Astronomical Institute, Tashkent 700052, Uzbekistan Research Associate, FNRS, Belgium Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands

Aims. We present optical photometry and spectroscopy of the afterglow and host galaxy of gamma-ray burst GRB040924. This GRB had a rather short duration of T-90 similar to 2.4 s, and a well sampled optical afterglow light curve. We aim to use this dataset to find further evidence that this burst is consistent with a massive star core-collapse progenitor.Methods. We combine the afterglow data reported here with those from the literature and compare the host properties with survey data.Results. We find that the global behaviour of the optical afterglow is well fit by a broken power-law, with a break at similar to 0.03 days. We determine the redshift z = 0.858 +/- 0.001 from the detected emission lines in our spectrum. Using the spectrum and photometry we derive global properties of the host, showing it to have similar properties to other long GRB hosts. We detect the [Ne III] emission line in the spectrum, and compare the fluxes of this line of a sample of 15 long GRB host galaxies with survey data, showing the long GRB hosts to be comparable to local metal-poor emission line galaxies in their [Ne III] emission. We fit the supernova bump accompanying this burst, and find that it is similar to other long GRB supernova bumps, but fainter.Conclusions. All properties of GRB 040924 (the associated supernova, the spectrum and SED of host and afterglow) are consistent with an origin in the core-collapse of a massive star.

Aims. We present optical photometry and spectroscopy of the afterglow and host galaxy of gamma-ray burst GRB 040924. This GRB had a rather short duration of T90 ∼ 2.4 s, and a well sampled optical afterglow light curve. We aim to use this dataset to find further evidence that this burst is consistent with a massive star core-collapse progenitor. Methods. We combine the afterglow data reported here with those from the literature and compare the host properties with survey data. Results. We find that the global behaviour of the optical afterglow is well fit by a broken power-law, with a break at ∼0.03 days. We determine the redshift z = 0.858 ± 0.001 from the detected emission lines in our spectrum. Using the spectrum and photometry we derive global properties of the host, showing it to have similar properties to other long GRB hosts. We detect the [Ne iii] emission line in the spectrum, and compare the fluxes of this line of a sample of 15 long GRB host galaxies with survey data, showing the long GRB hosts to be comparable to local metal-poor emission line galaxies in their [Ne iii] emission. We fit the supernova bump accompanying this burst, and find that it is similar to other long GRB supernova bumps, but fainter. Conclusions. All properties of GRB 040924 (the associated supernova, the spectrum and SED of host and afterglow) are consistent with an origin in the core-collapse of a massive star.

We present HST/STIS observations of the optical counterpart (OT) of the γ-ray burster GRB 000301C obtained on 2000 March 6, five days after the burst. CCD clear aperture imaging reveals a R ≃ 21.50± 0.15 source with no apparent host galaxy. An 8000 s, 1150 < λ/Å < 3300 NUV–MAMA prism spectrum shows a relatively flat continuum (in fλ) between 2800 and 3300 Å, with a mean flux 7.3 −1.8 ± 0.6 10 −18 ergs s cm Å −1 , and a sharp break centered at 2761 ± 25 Å. We interpret it as H I Lyman break at z = 2.028 ± 0.025 indicating the presence of a cloud with a H I column density log (NHI cm ) > 18 on the line-of-sight to the OT. This value is conservatively a lower limit to the GRB redshift. However, the facts that large NHI system are usually considered as progenitors of present day galaxies and that other OTs are found associated with star forming galaxies strongly suggest that it is the GRB redshift. In any case, this represents the largest direct redshift determination of a γ-ray burster to date. Our data are compatible with an OT spectrum represented by a power-law with an intrinsic index α = 1.2 (fν ∝ ν ) and no extinction in the host galaxy or with α = 0.5 and extinction by a SMC-like dust in the OT rest-frame with AV = 0.15. The large NHI and the lack of detected host is similar to the situation for damped Ly-α absorbers at z > 2. Subject headings: gamma rays: bursts NASA Goddard Space Flight Center, Greenbelt MD 20771, USA; asmette@band3.gsfc.nasa.gov, gull@sea.gsfc.nasa.gov, cline@lheavx.gsfc.nasa.gov, u1jit@lepvax.gsfc.nasa.gov National Optical Astronomy Observatories, P.O. Box 26732, 950 North Cherry Avenue, Tucson AZ 85726-6732, USA. Collaborateur Scientifique, FNRS, Belgium. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA; fruchter@stsci.edu, sahu@stsci.edu, petro@stsci.edu ferguson@stsci.edu, rhoads@stsci.edu, gibbons@stsci.edu, livio@stsci.edu, macchetto@stsci.edu, Advanced Computer Concepts, Inc./Goddard Space Flight Center, Code 681, Greenbelt MD 20771, USA; lindler@rockit.gsfc.nasa.gov Institute for Advanced Study, Princeton NJ 08540, USA; hogg@ias.edu Hubble Fellow. NASA Marshall Space Flight Center, ES-84, Huntsville AL 35812, USA; chryssa.kouveliotou@msfc.nasa.gov Universities Space Research Association. Affiliated to the Astrophysics Division, Space Science Department, European Space Agency. Department of Astronomy, Caltech, MS 105-24, Pasadena CA 91125, USA; mrm@grus.caltech.edu Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, D-2100, Copenhagen Ø, Denmark; holger@ursa.astro.ku.dk, jens@astro.ku.dk, brian j@astro.ku.dk Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri, C.N.R., Via Gobetti 101, I–40129 Bologna, Italy; pian@tesre.bo.cnr.it Department of Astronomy and Astrophysics, Uni-

We present optical lightcurves and spectra of the afterglow of the gamma-ray burst (GRB) of February 18, 2006. GRB 060218 is a nearby long GRB which is clearly associated with a supernova – dubbed SN2006aj. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U , V and R bands, and we compare these data with data for other relevant supernovae. SN2006aj evolved quite fast, somewhat similar to SN 2002ap, but not as fast as SN1994I. Our spectra show the evolution of the supernova over the peak, when the U -band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic Type Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12 days after burst in the rest frame is similar to somewhat later spectra of both SN1998bw and SN2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z = 0.0331± 0.0007. The flux of these lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy. Subject headings: gamma rays: bursts — supernovae Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK–2100 Copenhagen Ø, Denmark Stockholm Observatory, Department of Astronomy, AlbaNova, S-106 91 Stockholm, Sweden 4 Institute of theoretical astrophysics, PO Box 1029, N0315 Oslo, Norway Instituto de Astrofisica de Andalucia (IAA-CSIC), PO Box 03004, 18080 Granada, Spain Astrophysikalisches Institut, An der Sternwarte 16, 14482 Potsdam, Germany ESO, Karl-Schwarzschild-Strasse 2, 85744 Garching, Germany University Observatory Munich, Scheinerstr. 1, 81679 Munich, Germany This paper is based on observations from the ESO/Danish 1.5-m telescope at the La Silla Observatory and on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

Aims. Our aim is to investigate the nature of the X-Ray Flash (XRF) of August 24, 2005. Methods. We present comprehensive photometric R-band observations of the fading optical afterglow of XRF 050824, from 11 minutes to 104 days after the burst. In addition we present observations ta ken during the first day in the BRIK bands and two epochs of spectroscopy. We also analyse available X-ray data. Results. The R-band lightcurve of the afterglow resembles the lightcurves of long duration Gamma-Ray Bursts (GRBs), i.e., a power-law, albeit with a rather shallow slope ofα = 0.6 (Fν ∝ t−α). Our late R-band images reveal the host galaxy. The rest-frame B-band luminosity is ∼ 0.5 L∗. The star-formation rate as determined from the (O II) emission line is∼ 1.8 M⊙ yr−1. When accounting for the host contribution, the slope isα = 0.65± 0.01 and a break in the lightcurve is suggested. A potential lightcurve bump at 2 weeks can be interpreted as a supernova only if this is a supernova with a fast rise and a fast decay. However, the overall fit still shows excess scatter in the light curve in the form of wiggles and bumps. The flat lightcurves in the optical and X-r ays could be explained by a continuous energy injection scenario, with an on-axis viewing angle and a wide jet opening angle (θ j> ∼ 10 ◦ ). If the energy injections are episodic this could potentia lly help explain the bumps and wiggles. Spectroscopy of the afterglow gives a redshift of z = 0.828± 0.005 from both absorption and emission lines. The spectral energy distribution (SED) of the afterglow has a power-law (Fν∝ν−β) shape with slopeβ = 0.56± 0.04. This can be compared to the X-ray spectral index which isβX = 1.0± 0.1. The curvature of the SED constrains the dust reddening towards the burst to Av< 0.5 mag.

We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hr after the trigger shows a continuum break at λ ≈ 8070 Å, produced by neutral hydrogen absorption at z ≈ 5.6. We also detect an absorption line at 8158 Å, which we interpret as Si II λ1260 at z = 5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyα profile with a column density with log(NH /cm-2) = 22.50 ± 0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: (1) GRB afterglows originating from z ≳ 6 should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; (2) the presence of large H I column densities in some GRB host galaxies at z > 5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; and (3) GRBs appear crucial to locate typical star-forming galaxies at z > 5, and therefore the type of galaxies responsible for the reionization of the universe.

Aims. Our aim is to investigate the nature of the X-Ray Flash (XRF) of August 24, 2005.Methods. We present comprehensive photometric R-band observations of the fading optical afterglow of XRF 050824, from 11 min to 104 days after the burst. In addition we present observations taken during the first day in the BRIK bands and two epochs of spectroscopy. We also analyse available X-ray data.Results. The R-band lightcurve of the afterglow resembles the lightcurves of long duration Gamma-Ray Bursts (GRBs), i.e., a power-law, albeit with a rather shallow slope of alpha = 0.6 (F-v proportional to t(-alpha)). Our late R-band images reveal the host galaxy. The rest-frame B- band luminosity is similar to 0.5 L*. The star-formation rate as determined from the [O II] emission line is similar to 1.8 M-circle dot yr(-1). When accounting for the host contribution, the slope is alpha = 0.65 +/- 0.01 and a break in the lightcurve is suggested. A potential lightcurve bump at 2 weeks can be interpreted as a supernova only if this is a supernova with a fast rise and a fast decay. However, the overall fit still shows excess scatter in the lightcurve in the form of wiggles and bumps. The flat lightcurves in the optical and X-rays could be explained by a continuous energy injection scenario, with an on-axis viewing angle and a wide jet opening angle (theta(j) greater than or similar to 10 degrees). If the energy injections are episodic this could potentially help explain the bumps and wiggles.Spectroscopy of the afterglow gives a redshift of z = 0.828 +/- 0.005 from both absorption and emission lines. The spectral energy distribution ( SED) of the afterglow has a power-law (F-v proportional to v(-beta)) shape with slope beta = 0.56 +/- 0.04. This can be compared to the X-ray spectral index which is beta(X) = 1.0 +/- 0.1. The curvature of the SED constrains the dust reddening towards the burst to A(v) < 0.5 mag.

The optical afterglow spectrum of GRB 050401 (at z 1⁄4 2:8992 0:0004) shows the presence of a damped Ly absorber (DLA), with logNH i1⁄4 22:6 0:3. This is the highest column density ever observed in a DLA and is about 5 times larger than the strongest DLA detected so far in any QSO spectrum. From the optical spectrum, we also find a very large Zn column density, implying an abundance of 1⁄2Zn/H 1⁄4 1:0 0:4. These large columns are supported by the early X-ray spectrum from Swift XRT, which shows a column density (in excess of Galactic) of log NH 1⁄4 22:21þ0:06 0:08 assuming solar abundances (at z 1⁄4 2:9). The comparison of this X-ray column density, which is dominated by absorption due to -chain elements, and the H i column density derived from the Ly absorption line allows us to derive a metallicity for the absorbing matter of 1⁄2 /H 1⁄4 0:4 0:3. The optical spectrum is reddened and can be well reproduced with a power law with SMC extinction, where AV 1⁄4 0:62 0:06. But the total optical extinction can also be constrained independent of the shape of the extinction curve: from the optical to X-ray spectral energy distribution, we find 0:5PAV P 4:5. However, even this upper limit, independent of the shape of the extinction curve, is still well below the dust column that is inferred from the X-ray column density, i.e., AV 1⁄4 9:1þ1:4 1:5. This discrepancy might be explained by a small dust content with high metallicity ( low dust-to-metals ratio). ‘‘Gray’’ extinction cannot explain the discrepancy, since we are comparing the metallicity to a measurement of the total extinction (without reference to the reddening). Little dust with highmetallicity may be produced by sublimation of dust grains or may naturally exist in systems younger than a few hundred megayears. Subject headingg s: dust, extinction — galaxies: high-redshift — galaxies: ISM — gamma rays: bursts — quasars: absorption lines — X-rays: general Online material: color figures

Aims. We present early optical spectroscopy of the afterglow of the gamma-ray burst GRB060206 with the aim of determining the metallicity of the GRB absorber and the physical conditions in the circumburst medium. We also discuss how GRBs may be important complementary probes of cosmic chemical evolution.Methods. Absorption line study of the GRB afterglow spectrum.Results. We determine the redshift of the GRB to be z = 4.04795 +/- 0.00020. Based on the measurement of the neutral hydrogen column density from the damped Lyman-alpha line and the metal content from weak, unsaturated S II lines we derive a metallicity of [S/H] = -0.84 +/- 0.10. This is one of the highest metallicities measured from absorption lines at z similar to 4. From the very high column densities for the forbidden Si II*, O I*, and O I** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H-2 molecules with log N(H-2) similar to 17.0, translating into a molecular fraction of log f approximate to 3.5 with f = 2N(H-2)/(2N(H-2) + N(H-I)). Even if GRBs are only formed by single massive stars with metallicities below similar to 0.3 Z(circle dot), they could still be fairly unbiased tracers of the bulk of the star formation at z > 2. Hence, metallicities as derived for GRB060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

We identify the fading X-ray afterglow of GRB 001025A from XMM-Newton observations obtained 1.9-2.3 days, 2 yr, and 2.5 yr after the burst. The nondetection of an optical counterpart to an upper limit of R = 25.5, 1.20 days after the burst, makes GRB 001025A a "dark" burst. Based on the X-ray afterglow spectral properties of GRB 001025A, we argue that some bursts appear optically dark because their afterglow is faint and their cooling frequency is close to the X-ray band. This interpretation is applicable to several of the few other dark bursts where the X-ray spectral index has been measured. The X-ray afterglow flux of GRB 001025A is an order of magnitude lower than for typical long-duration gamma-ray bursts. The spectrum of the X-ray afterglow can be fitted with an absorbed synchrotron emission model, an absorbed thermal plasma model, or a combination thereof. For the latter, an extrapolation to optical wavelengths can be reconciled with the R-band upper limit on the afterglow, without invoking any optical circumburst absorption, provided the cooling frequency is close to the X-ray band. Alternatively, if the X-ray afterglow is due to synchrotron emission only, 7 mag of extinction in the observed R-band is required to meet the R-band upper limit, making GRB 001025A much more obscured than bursts with detected optical afterglows. Based on the column density of X-ray-absorbing circumburst matter, an SMC gas-to-dust ratio is insufficient to produce this amount of extinction. The X-ray tail of the prompt emission enters a steep temporal decay excluding that the tail of the prompt emission is the onset of the afterglow. To within the astrometric uncertainty, this afterglow was coincident with an extended object, seen in a deep VLT R-band image, which we identify as the likely host galaxy of GRB 001025A.