High-ion absorption in seven GRB host galaxies at z = 2–4

Abstract

Aims. We use VLT/UVES high-resolution optical spectroscopy of seven GRB afterglows (at z GRB between 2.20 and 3.97) to investigate circumburst and interstellar plasma in the host galaxies of the bursts. Methods. GRBs 050730, 050820, 050922C, 060607, 071031, and 080310 were each detected by the Swift satellite. With the minimum possible time delay (as short as eight minutes), follow-up optical spectroscopy at 6.0 km s −1 resolution began with UVES. We present Voigt-profile component fits and analysis of the high-ion absorption detected in these spectra and in the pre-Swift UVES spectrum of GRB 021004, at velocities between z GRB -5000 km s −1 and z GRB . Results. We identify several distinct categories of high-ion absorption at velocities close to z GRB : (i) strong high-ion absorption at z GRB itself is always seen in O vi, C iv, and Si iv, usually (in six of seven cases) in N v, and occasionally in S iv and S vi. Three of the cases show log N(N v) > 14 in a single-component, suggesting a circumburst origin, but we cannot rule out an interstellar origin. Indeed, using the non-detection of S iv * at z GRB toward GRB 050730 together with a UV photo-excitation model, we place a lower limit of 400 pc on the distance of the S iv-bearing gas from the GRB; (ii) complex, multi-component C iv and Si iv profiles extending over 100-400 km s −1 around z GRB are observed in each spectrum; these velocity fields are similar to those measured in C iv in damped Lyman-α systems at similar redshifts, suggesting a galactic origin; (iii) asymmetric, blueshifted, absorption-line wings covering 65-140 km s −1 are seen in the C iv, Si iv, and O vi profiles in four of the seven GRB afterglow spectra. The wing kinematics together with the "Galactic" C iv/Si iv ratios measured in two cases suggest that the wings trace outflowing interstellar gas in the GRB host galaxies; (iv) high-velocity (HV; 500-5000 km s −1 relative to z GRB ) components are detected in six of the seven spectra; these components are not necessarily a single homogeneous population, and many of them may arise in unrelated foreground galaxies.However, in the cases of GRBs 071031 and 080310, the ionization properties of the HV components (very high C iv/Si iv ratios and absence of neutral-phase absorption in Si ii or C ii) are suggestive of a circumburst origin; models of Wolf-Rayet winds from the GRB progenitors can explain both the kinematics and ionization level of these HV components.

Physical conditions in high-redshift GRB-DLA absorbers observed with VLT/UVES: implications for molecular hydrogen searches

“…Similar photo-excitation modelling applied to the observed column densities and upper limits of S 3+ energy levels has led to a lower limit on the distance of the S 3+ -bearing gas of 0.4 kpc (Fox et al 2008). From detailed calculations, we find that at such distances GRB afterglow photons cannot ionize H 0 nor dissociate H 2 significantly.…”

The missing gas problem in GRB host galaxies: evidence for a highly ionised component

“…Whereas low ion elements will only exist far from the GRB, such as Zn ii, Si ii, and Fe ii, more highly ionised metals, such as C iv, Si iv, N v, and O vi could survive Fox et al (2008), the N v data plotted as squares are from Prochaska et al (2008), and data taken from D 'Elia et al (2010) are plotted as triangles. In all four panels, the dashed line corresponds to where the normalised soft X-ray column density is equal to the column density of the corresponding highly ionised atom.…”

“…ionisation potentials between 50-150 eV) present in the hot gas (T ∼ 10 4 K) surrounding the GRB. Although these highly ionised absorption lines may also A&A 525, A113 (2011) originate in hot gas in the halo of the galaxy, measurements of O vi, C iv, Si iv and N v in a handful of GRB afterglow spectra have been used to probe the gas close to the GRB progenitor star (Fox et al 2008;Prochaska et al 2008).…”

Rapid-response mode VLT/UVES spectroscopy of GRB 060418 (Corrigendum)

“…4.1.3 of Fox et al (2008) and in Sect. 2.3 of Ledoux et al (2009), the distance estimates therein should also be scaled down by a factor of √ 4 π.…”