Abstract
We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB991216 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 0.6" away. The spectrum shows a depression in flux in between 4000 A and 5500 A. This could be the result of a 2175 A-type extinction feature in the host of GRB991216, but at a rather red wavelength of 2360 A. 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 GRB011211 are identified with lines originating in a single absorption system at z=2.142+/-0.002, the redshift of the GRB011211 host galaxy. We also detect the Lyman alpha absorption line in the host, to which we fit a neutral hydrogen column density of log N(HI)=20.4+/-0.2, which indicates that it is a damped Lyman alpha system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB011211 redshift. For GRB021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [OII] 3727 at z=1.006. The corresponding unobscured [OII] star-formation rate is 1.4 Msun/yr.
Highlights
Spectroscopy of the afterglows of long-duration Gamma-Ray Bursts (GRBs) has been essential for our understanding of theBased on observations collected at the European Southern Observatory, Chile; proposals Nos. 64.H-0313, 165.H-0464, 70.D-0523.physical mechanism that produces these powerful explosions
We present low-resolution Very Large Telescope (VLT) spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211
We present low-resolution Very Large Telescope (VLT) spectroscopy of three GRB afterglows: 991216, 011211 and 021211
Summary
Spectroscopy of the afterglows of long-duration Gamma-Ray Bursts (GRBs) has been essential for our understanding of the. For the GRB 991216 spectra a slit-loss correction was not necessary, as we flux calibrated with the standard that was taken with the same slit width as the afterglow spectra. This should in principle result in a correct flux calibration, provided that the seeing was the same during the afterglow and standard star observations. This was more or less the case for GRB 991216, with a seeing of 0. We do not include a contribution in the error from the uncertainty in the location of the continuum
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