2008ApJ...685..344P

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~z_GRB, and the majority show large column densities N(N⁺⁴)≳10¹⁴/cm2. With one exception, the observed line profiles are kinematically ``cold''; i.e., they are narrow and have small velocity offset (δv≲20 km/s) 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<10 pc, which suggests that the GRB progenitors occur within dense (n>10³/cm3) 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.