A survey for N v absorption at z ≈ zGRB in GRB afterglow spectra:: Clues to gas near the progenitor star

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 approximate to z(GRB), and the majority show large column densities N(N+4) greater than or similar to 10(14) cm(-2). With one exception, the observed line profiles are kinematically cold''; i. e., they are narrow and have small velocity offset (delta v less than or similar to 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 approximate to 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 Ly alpha 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 approximate to 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 > 10(3) 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.