Astronomy and Astrophysics, volume 447, 145-156 (2006/2-3)
Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211.
VREESWIJK P.M., SMETTE A., FRUCHTER A.S., PALAZZI E., ROL E., WIJERS R.A.M.J., KOUVELIOTOU C., KAPER L., PIAN E., MASETTI N., FRONTERA F., HJORTH J., GOROSABEL J., PIRO L., FYNBO J.P.U., JAKOBSSON P., WATSON D., O'BRIEN P.T. and LEDOUX C.
Abstract (from CDS):
We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of 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 4000Å and 5500Å. This could be the result of a 2175Å-type extinction feature in the host of GRB 991216, but at a rather red wavelength of 2360Å. 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 991216 are identified with lines originating in a single absorption system at z=2.142±0.002, the redshift of the GRB 011211 host galaxy. We also detect the Lyα absorption line in the host, to which we fit a neutral hydrogen column density of logN(HI)=20.4±0.2, which indicates that it is a damped Lyα system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the redshift to be [Si/H]=-0.9+0.6–0.4, [Fe/H]=-1.3±0.3, 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 [OII]l3727 at z=1.006. The corresponding unobscured [OII] star-formation rate is 1.4M☉/yr.
gamma rays: bursts - galaxies: abundances - galaxies: distances and redshifts - galaxies: quasars: absorption lines
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