A Survey for NvAbsorption atz ≈ zGRBin GRB Afterglow Spectra: Clues to Gas Near the Progenitor Star

Abstract

We survey N V absorption in the afterglow spectra of long-duration gamma-ray bursts (GRBs) with the intent to study highly ionized gas in the galaxies hosting these events. We identify a high incidence (6/7) of spectra exhibiting N V gas with z ≈ zGRB, and the majority show large column densities N(N+ 4) 1014 cm −2. With one exception, the observed line profiles are kinematically cold; i.e., they are narrow and have small velocity offset (δ v 20 km s−1) from absorption lines associated with neutral gas. In addition, the N V absorption has similar velocity to that of the UV-pumped fine-structure lines, indicating that these high ions are located within ≈1 kpc of the GRB afterglow. These characteristics are unlike those for N V gas detected in the halo/disk of the Milky Way or along sight lines through high-z damped Lyα systems but resemble the narrow absorption line systems associated with quasars and some high-z starbursts. We demonstrate that GRB afterglows photoionize nitrogen to N+4 at r ≈ 10 pc. This process can produce N V absorption with characteristics resembling the majority of our sample, and we argue that it is the principal mechanism for N+4 along GRB sight lines. Therefore, the observations provide a snapshot of the physical conditions at this distance. In this scenario, the observations imply that the progenitor's stellar wind is confined to r 103 cm −3) environments, typical of molecular clouds. The observations, therefore, primarily constrain the physical conditions—metallicity, density, velocity fields—of the gas within the (former) molecular cloud region surrounding the GRB.