A Survey for NV Absorption at z~z_GRB in GRB Afterglow Spectra: Clues to Gas Near the Progenitor Star

TL;DR

This survey investigates NV absorption in GRB afterglow spectra to understand the ionized gas near progenitor stars, revealing that photoionization by afterglows explains the observed features and constrains the local environment.

Contribution

It provides the first comprehensive survey linking NV absorption characteristics to the physical conditions near GRB progenitors, emphasizing photoionization as the main mechanism.

Findings

High incidence of NV with large column densities near GRBs

NV gas is kinematically cold and close to the GRB (~1 kpc)

Progenitors are in dense molecular cloud environments

Abstract

We survey NV 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 NV gas with z~z_GRB and the majority show large column densities NV > 10^14 cm^-2. With one exception, the observed line-profiles are kinematically `cold', i.e. they are narrow and have small velocity offset (Dv < 20 km/s) from absorption lines associated with neutral gas. In addition, the NV absorption has similar velocity as the UV-pumped fine-structure lines indicating these high ions are located within ~1kpc of the GRB afterglow. These characteristics are unlike those for NV gas detected in the halo/disk of the Milky Way or along sightlines through high z damped Lya systems but resemble the narrow absorption line systems associated with quasars and some high z starbursts. We demonstrate that GRB afterglows photoionize nitrogen to NV at r~10pc. This process can produce NV absorption with characteristics resembling the majority of our sample and and we argue it is the principal mechanism for NV along GRB sightlines. Therefore, the observations provide a snapshot of the physical conditions at this distance. In this scenario, the observations imply the progenitor's stellar wind is confined to r<10pc which suggests 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.