Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

Abstract

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. 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 ± 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 log N(H i) = 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 GRB 011211 redshift to be [Si/H] = −0.9 Key words. gamma rays: bursts -galaxies: abundances -galaxies: distances and redshifts -galaxies: quasars: absorption lines IntroductionSpectroscopy of the afterglows of long-duration Gamma-Ray Bursts (GRBs) has been essential for our understanding of the Based on observations collected at the European Southern Observatory, Chile; proposals physical mechanism that produces these powerful explosions. Following the discovery of GRB afterglows (Costa et al. 1997;Van Paradijs et al. 1997), the first redshift determination by Metzger et al. (1997) provided conclusive evidence that the origin of the long-duration class of GRBs (Kouveliotou et al. 1993) is cosmological. Obviously, redshifts are required to deduce most meaningful quantities, such as the GRB energetics Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx

The fraction of ionizing radiation from massive stars that escapes to the intergalactic medium

“…We also note that in some spectra we detect metal fine-structure lines, which are thought to be the result of excitation by the burst itself of gas within its host galaxy, confirming the association (e.g. Vreeswijk et al 2006).…”

“…Fynbo et al (2002), (3)Vreeswijk et al (2006), (4)Hjorth et al (2003b), (5)Fynbo et al (2005), (6)Møller et al (2002), (7)Shin et al (2006),(8)Vreeswijk et al (2004), (9) Jakobsson et al (2004), (10) Fynbo et al (2009), (11) Berger et al (2006), (12) Totani et al (2006), (13) Chen et al (2007), (14) Chary et al (2007), (15) Ferrero et al (2009), (16) This work, (17) Wiseman et al (2017), (18) Patel et al (2010), (19) de Ugarte Postigo et al (2012), (20) Kuin et al (2009), (21) D'Avanzo et al (2010), (22) Savaglio et al (2012), (23) Levesque et al (2010a), (24) Selsing et al (2018), (25) Cucchiara et al (2011b), (26) Zafar et al in prep., (27) Cucchiara et al (2015), (28) Jeong et al (2014), (29) Chornock et al (2014), (30) Melandri et al (2015), (31) Pugliese et al in prep., (32) Chen et al (2009), (33) Perley (2011), (34) Schulze et al (2015), (35) Greiner et al (2015b), (36) McGuire et al (2016), (37) Tanvir et al (2012a), (38) McGuire et al in prep., (39) Thöne et al (2011), (40) Friis et al (2015), (41) Perley et al (2013), (42) Laskar et al (2011), (43) Perley et al (2016b), (44) Myers et al in prep. (in this case, the derived Spitzer photometry was transformed to stellar mass estimates following Perley et al 2016b).…”

DUST EXTINCTION BIAS IN THE COLUMN DENSITY DISTRIBUTION OF GAMMA-RAY BURSTS: HIGH COLUMN DENSITY, LOW-REDSHIFT GRBs ARE MORE HEAVILY OBSCURED

“…We would then require at least five such z < 1 systems for every z = 4 GRB, or a single system with W λ,0 > 6 Å, at z = 0.5 in most z > 4 GRBs, which is the largest equivalent width ever observed for any GRB to date. In other words, most GRBs would need to have absorption systems similar to, or larger than, GRB 991216 (Vreeswijk et al 2006), and this is not observed (Vergani et al 2009). In addition, we would have not to observe the corresponding galaxy at relatively low impact factor at such low redshifts.…”

A High Signal-to-Noise Ratio Composite Spectrum of Gamma-Ray Burst Afterglows

“…High resolution spectra of individual bursts have shown that GRBs reside in galaxies with large gas column densities and sub-solar metallicities (Vreeswijk et al 2004;Chen et al 2005). Host galaxy abundance ratios and metallicities have been measured (Savaglio et al 2003;Vreeswijk et al 2006), and evidence for galaxy outflows has been established from the presence of multiple absorption components separated in velocity space (Schaefer et al 2003) and from correlation of absorption line species with host galaxy luminosity (Chen et al 2009). The distances from the burst site to the absorbing material has been determined to be larger than a few tens of pc (Dessauges-Zavadsky et al 2006;Prochaska et al 2006;D'Elia et al 2010), while in other cases the time variations of finestructure lines (Vreeswijk et al 2007;D'Elia et al 2009a;Ledoux et al 2009), or the presence of vibrationally exited H 2 molecules (Sheffer et al 2009) indicated a distance of 0.4-2 kpc.…”