Search

We present high-resolution spectroscopic observations of GRB 060418, obtained with VLT/UVES. These observations were triggered using the VLT Rapid-Response Mode (RRM), which allows for automated observations of transient phenomena, without any human intervention. This resulted in the first UVES exposure of GRB 060418 to be started only 10 min after the initial Swiftsatellite trigger. A sequence of spectra covering 330-670 nm were acquired at 11, 16, 25, 41 and 71 minutes (mid-exposure) after the trigger, with a resolving power of 7 km s-1, and a signal-to-noise ratio of 10-15. This time-series clearly shows evidence for time variability of allowed transitions involving Fe II fine-structure levels (6D7/2, 6D5/2, 6D3/2, and 6D1/2), and metastable levels of both Fe II (4F9/2and 4D7/2) and Ni II (4F9/2), at the host-galaxy redshift $z=1.490$. This is the first report of absorption lines arising from metastable levels of Fe II and Ni II along any GRB sightline. We model the observed evolution of the level populations with three different excitation mechanisms: collisions, excitation by infra-red photons, and fluorescence following excitation by ultraviolet photons. Our data allow us to reject the collisional and IR excitation scenarios with high confidence. The UV pumping model, in which the GRB afterglow UV photons excite a cloud of atoms with a column density N, distance d, and Doppler broadening parameter b, provides an excellent fit, with best-fit values: log N(Fe II) = $14.75^{+0.06}_{-0.04}$, log N(Ni II$)=13.84\pm0.02$, $d=1.7\pm0.2$kpc, and $b=25\pm3$km s-1. The success of our UV pumping modeling implies that no significant amount of Fe II or Ni II is present at distances smaller than ~1.7 kpc, most likely because it is ionized by the GRB X-ray/UV flash. Because neutral hydrogen is more easily ionized than Fe II and Ni II, this minimum distance also applies to any H I present. Therefore the majority of very large H I column densities typically observed along GRB sightlines may not be located in the immediate environment of the GRB. The UV pumping fit also constrains two GRB afterglow parameters: the spectral slope, β= $-0.5^{+0.8}_{-1.0}$, and the total rest-frame UV flux that irradiated the cloud since the GRB trigger, constraining the magnitude of a possible UV flash.

Context.Before the launch of the Swiftsatellite, the majority of the gamma-ray burst (GRB) afterglows for which Lyαwas redshifted into the observable spectrum showed evidence for a damped Lyαabsorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities.

Aims.To investigate the physical nature of the afterglow emission. We report on the photometric, spectroscopic and polarimetric observations of the optical afterglow of Gamma–Ray Burst (GRB) 030328 detected by HETE-2.

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB 991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at $z=0.80$and $z=1.02$, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at $z=0.77$. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0$.\!\!^{\prime\prime}$6 away. The spectrum shows a depression in flux in between 4000 Å and 5500 Å. This could be the result of a 2175 Å-type extinction feature in the host of GRB 991216, but at a rather red wavelength of 2360 Å. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB 011211 are identified with lines originating in a single absorption system at $z=2.142\pm0.002$, the redshift of the GRB 011211 host galaxy. We also detect the Lyαabsorption line in the host, to which we fit a neutral hydrogen column density of log N($\ion{H}{i}$) = $20.4\pm0.2$, which indicates that it is a damped Lyαsystem. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB 011211 redshift to be [Si/H] = $-0.9^{+0.6}_{-0.4}$, $\rm [Fe/H]=-1.3\pm0.3$, and [Al/H] = $-1.0^{+0.5}_{-0.3}$. For GRB 021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [$\ion{O}{ii}$] λ3727 at $z=1.006$. The corresponding unobscured [$\ion{O}{ii}$] star-formation rate is 1.4 M$_{\odot}$yr-1.

We present the results of an optical and near-infrared (NIR) monitoring campaign of the counterpart of Gamma-Ray Burst (GRB) 000911, located at redshift $z = 1.06$, from 5 days to more than 13 months after explosion. Our extensive dataset is a factor of 2 larger and spans a time interval ~4 times longer than the ones considered previously for this GRB afterglow; this allows a more thorough analysis of its light curve and of the GRB host galaxy properties. The afterglow light curves show a single power-law temporal decline, modified at late times by light from a host galaxy with moderate intrinsic extinction, and possibly by an emerging supernova (SN). The afterglow evolution is interpreted within the classical “fireball” scenario as a weakly collimated adiabatic shock propagating in the interstellar medium. The presence of a SN light curve superimposed on the non-thermal afterglow emission is investigated: while in the optical bands no significant contribution to the total light is found from a SN, the NIR J-band data show an excess which is consistent with a SN as bright as the known hypernova SN1998bw. If the SN interpretation is true, this would be the farthest GRB-associated SN, as well as the farthest core-collapse SN, discovered to date. However, other possible explanations of this NIR excess are also investigated. Finally, we studied the photometric properties of the host, and found that it is likely to be a slightly reddened, subluminous, extreme starburst compact galaxy, with luminosity ~0.1 $L^\star$, an age of ~0.5 Gyr and a specific Star Formation Rate (SFR) of ≈30 $M_\odot$yr-1($L/L^\star$)-1. This is the highest specific SFRvalue for a GRB host inferred from optical/NIR data.

We present the results of a VLT polarimetric monitoring campaign of the GRB 020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days after the gamma-ray event. The mean values of the degree of linear polarization (P) and its position angle (θ) resulting from our campaign are $\langle P\rangle=1.18 \pm 0.10 \%$and $\langle \theta\rangle = 148.7^{\circ} \pm 2.3^{\circ}$, after correcting for Galactic interstellar polarization. Our VLT data set is most naturally described by a constant degree of linear polarization and position angle, nonetheless a slow θevolution cannot be entirely ruled out by our data. The VLT monitoring campaign did not reveal either a significant θrotation respect to the Keck spectropolarimetric observations performed ~0.25 days after the GRB (Barth et al. [CITE]). However, $\langle P\rangle $is a factor of two lower than the polarization degree measured from Keck. Additionally, the VLT polarization data allowed us to construct an accurate V-band light curve. The V-band photometric data revealed a smooth light curve with a break located between the last Keck and our first VLT polarimetric measurement, $0.33 < t_{{\rm break}, V} < 0.88$days after the GRB. The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag, 0.010 mag and 0.016 mag for our three observing nights, respectively. We speculate that the stability of θmight be related to the smoothness of the light curve.

The polarization curve of GRB 020813 is discussed and compared to different models for the structure, evolution and magnetisation properties of the jet and the interstellar medium onto which the fireball impacts. GRB 020813 is best suited for this kind of analysis for the smoothness of its afterglow light curve, ensuring the applicability of current models. The polarization dataset allows us to rule out the standard GRB jet, in which the energy and Lorentz factor have a well defined value inside the jet opening angle and the magnetic field is generated at the shock front. We explore alternative models finding that a structured jet or a jet with a toroidal component of the magnetic field can fit equally well the polarization curve. Stronger conclusions cannot be drawn due to the incomplete sampling of the polarization curve. A more dense sampling, especially at early times, is required to pin down the structure of the jet and the geometry of its magnetic field.

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lyα(DLA) absorption and low- and high-ionization lines at a redshift $z=3.3718\pm0.0005$. The inferred neutral hydrogen column density, $\log N({\ion{H}{i}})=21.90\pm0.07$, is larger than any (GRB- or QSO-) DLA $\ion{H}{i}$column density inferred directly from Lyαin absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: $A_{\rm V}<0.5$mag. The iron abundance is $\rm [Fe/H]=-1.47\pm0.11$, while the metallicity of the gas as measured from sulphur is $\rm [S/H]=-1.26\pm0.20$. We derive an upper limit on the H2molecular fraction of $2N({\rm H_2)}/(2N{\rm (H_2)}+N({\ion{H}{i}}))\la10^{-6}$. In the Lyαtrough, a Lyαemission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 M$_{\odot}$yr-1. All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, $\ion{Si}{ii}$*, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the $\ion{H}{i}$volume density to be $n_{\ion{H}{i}}=10^2{-}10^4$cm-3. HST/ACS imaging 4 months after the burst shows an extended ${\it AB}(F606W)=28.0\pm0.3$mag object at a distance of 014 (1 kpc) from the early afterglow location, which presumably is the host galaxy of GRB 030323.

We report on the results of a search for Lyaemission from the host galaxy of the $z=2.140$GRB 011211 and other galaxies in its surrounding field. We detect Lyaemission from the host as well as from six other galaxies in the field. The restframe equivalent width of the Lyaline from the GRB 011211 host is about 21 Å. This is the fifth detection of Lyaemission out of five possible detections from GRB host galaxies, strongly indicating that GRB hosts, at least at high redshifts, are Lyaemitters. This is intriguing as only ~25% of the Lyman-Break selected galaxies at similar redshifts have Lyaemission lines with restframe equivalent width larger than 20 Å. Possible explanations are i)a preference for GRB progenitors to be metal-poor as expected in the collapsar model, ii)an optical afterglow selection bias against dusty hosts, and iii)a higher fraction of Lyaemitters at the faint end of the luminosity function for high-zgalaxies. Of these, the current evidence seems to favour i).

We report on photometric, spectroscopic and polarimetric monitoring of the optical and near-infrared (NIR) afterglow of GRB020405. Ground-based optical observations, performed with 8 different telescopes, started about 1 day after the high-energy prompt event and spanned a period of ~10 days; the addition of archival HST data extended the coverage up to ~150 days after the GRB. We report the first detection of the afterglow in NIR bands. The detection of Balmer and oxygen emission lines in the optical spectrum of the host galaxy indicates that the GRB is located at redshift $z =0.691$. Fe iiand Mg iiabsorption systems are detected at $z= 0.691$and at $z = 0.472$in the afterglow optical spectrum. The latter system is likely caused by absorbing clouds in the galaxy complex located ~2''southwest of the GRB020405 host. Hence, for the first time, the galaxy responsible for an intervening absorption line system in the spectrum of a GRB afterglow is spectroscopically identified. Optical and NIR photometry of the afterglow indicates that, between 1 and 10 days after the GRB, the decay in all bands is consistent with a single power law of index $\alpha = 1.54\pm 0.06$. The late-epoch VLT J-band and HST optical points lie above the extrapolation of this power law, so that a plateau (or “bump") is apparent in the VRIJlight curves at 10–20 days after the GRB. The light curves at epochs later than day ~20 after the GRB are consistent with a power-law decay with index $\alpha' = 1.85\pm 0.15$. While other authors have proposed to reproduce the bump with the template of the supernova (SN) 1998bw, considered the prototypical “hypernova”, we suggest that it can also be modeled with a SN having the same temporal profile as the other proposed hypernova SN2002ap, but 1.3 mag brighter at peak, and located at the GRB redshift. Alternatively, a shock re-energization may be responsible for the rebrightening. A single polarimetric R-band measurement shows that the afterglow is polarized, with $P = 1.5\pm 0.4$% and polarization angle $\theta = 172^{\circ}\pm 8^{\circ}$. Broad-band optical-NIR spectral flux distributions show, in the first days after the GRB, a change of slope across the Jband which we interpret as due to the presence of the electron cooling frequency $\nu_{\rm c}$. The analysis of the multiwavelength spectrum within the standard fireball model suggests that a population of relativistic electrons with index $p \sim 2.7$produces the optical-NIR emission via synchrotron radiation in an adiabatically expanding blastwave, with negligible host galaxy extinction, and the X–rays via Inverse Compton scattering off lower-frequency afterglow photons.

We have observed the host galaxy of GRB 990712 at 1.4 GHz with the Australia Telescope Compact Array, to obtain an estimate of its total star-formation rate. We do not detect a source at the position of the host. The 2σupper limit of 70 μJy implies that the total star-formation rate is lower than 100 $M_{\odot}$yr-1, using conservative values for the spectral index and cosmological parameters. This upper limit is in stark contrast with recent reports of radio/submillimeter-determined star-formation rates of ~500 $M_{\odot}$yr-1for two other GRB host galaxies. Our observations present the deepest radio-determined star-formation rate limit on a GRB host galaxy yet, and show that also from the unobscured radio point-of-view, not every GRB host galaxy is a vigorous starburst.

We present $BVR_{\rm c}I_{\rm c}$broad-band flux spectra for the host galaxies of GRB 970508, GRB 980613, GRB 980703, GRB 990123 and GRB 991208 obtained with the 6-m telescope of SAO RAS. The comparison of the broad-band flux spectra of these host galaxies with the template spectral energy distributions (SEDs) of local starburst galaxies of different morphological types shows that the $BVR_{\rm c}I_{\rm c}$of the hosts are best fitted by the spectral properties of template SEDs of starburst galaxies and that there is a significant internal extinction in these host galaxies. We derived the absolute magnitudes of the GRB host galaxies making use of SEDs for the starburst galaxies. To create theoretical templates we performed the population synthesis modeling of the continuum spectral energy distribution of the host galaxies of GRB 970508 and GRB 980703 using different extinction laws (Cardelli et al. [CITE] and Calzetti et al. [CITE]) and assuming burst and exponential scenarios of star formation. The comparison of $BVR_{\rm c}I_{\rm c}$broad-band flux spectra with the local starburst galaxies templates and theoretical templates as well as direct estimates (using Balmer emission lines) of the internal extinction shows that it is likely to be of great importance to take into account effects of the internal extinction in the host galaxies. From the energy distribution in the spectrum of the host galaxy of GRB 991208 and from the intensity of their spectral lines (with allowance for the effects of internal extinction) it follows that this is a GRB galaxy with the highest massive star-formation rate of all known GRB galaxies -up to hundreds of solar masses per year. The reduced luminosity of these dusty galaxies (e.g. for the host of GRB 970508 $A_{V}\sim 2$mag, for the host of GRB 980703 $A_{V}\sim 0.6$mag and for the host of GRB 991208 $A_{V}\sim 2$mag) could explain the observational fact (it results independently from our $BVR_{\rm c}I_{\rm c}$photometry and from calculated spectral distribution for the subset of galaxies having been observed with the 6-m telescope): none of the observed GRB host galaxies with known distances is brighter than the local galaxies with the luminosity $L_{*}$(where $L_{*}$is the "knee" of the local luminosity function).

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.