Highly-ionized metals as probes of the circumburst gas in the natal regions of gamma-ray bursts

TL;DR

This study uses high-ionization absorption lines in GRB afterglow spectra to probe the circumburst environment, revealing correlations with neutral gas properties and exploring ionization mechanisms.

Contribution

It provides new insights into the ionization state and dynamics of the gas surrounding GRBs, especially regarding the origin of high-ionization lines like NV and OVI.

Findings

No correlation between NV column density and neutral gas metallicity or dust depletion.

A correlation between NV velocity shift and HI column density suggests an HII region expansion.

Tentative link between X-ray column density and NV indicates proximity of ionized gas to the GRB.

Abstract

We present here a survey of high-ionization absorption lines in the afterglow spectra of long-duration gamma-ray bursts (GRBs) obtained with the VLT/X-shooter spectrograph. Our main goal is to investigate the circumburst medium in the natal regions of GRBs. Our primary focus is on the NV 1238,1242 line transitions, but we also discuss other high-ionization lines such as OVI, CIV and SiIV. We find no correlation between the column density of NV and the neutral gas properties such as metallicity, HI column density and dust depletion, however the relative velocity of NV, typically a blueshift with respect to the neutral gas, is found to be correlated with the column density of HI. This may be explained if the NV gas is part of an HII region hosting the GRB, where the region's expansion is confined by dense, neutral gas in the GRB's host galaxy. We find tentative evidence (at 2-sigma significance) that the X-ray derived column density, N_H,X, may be correlated with the column density of NV, which would indicate that both measurements are sensitive to the column density of the gas located in the vicinity of the GRB. We investigate the scenario where NV (and also OVI) is produced by recombination after the corresponding atoms have been stripped entirely of their electrons by the initial prompt emission, in contrast to previous models where highly-ionized gas is produced by photoionization from the GRB afterglow.