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

We present here a survey of high-ionization absorption lines in the afterglow spectra of longduration 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 Nλλ 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 N and the neutral gas properties such as metallicity, H column density, and dust depletion; however, the relative velocity of N, typically a blueshift with respect to the neutral gas, is found to be correlated with the column density of H. This may be explained if the N gas is part of an H 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 2σ significance) that the X-ray derived column density, N, may be correlated with the column density of N, 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 N (and also O) 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.© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.We would like to thank the referee for a clear and thorough report, which greatly improved the presentation of the results in the paper. We also wish to thank K. Wiersema and P. Moller for enlightening discussions. KEH and PJ acknowledge support by a Project Grant (162948-051) from The Icelandic Research Fund. JK acknowledges financial support from the Danish Council for Independent Research (EU-FP7 under the Marie-Curie grant agreement no. 600207) with reference DFF-MOBILEX-5051-00115. JJ acknowledges support from NOVA and NWO-FAPESP grant for advanced instrumentation in astronomy. AG acknowledges the financial support from the Slovenian Research Agency (research core funding no. P1-0031 and project grant no. J1-8136). AdUP, CCT, and ZC acknowledge support from the Spanish Ministry of Economy and Competitivity under grant number AYA 2014-58381-P. AdUP and CCT acknowledge support from Ramon y Cajal fellowships (RyC-2012-09975 and RyC-2012-09984). AdUP acknowledges support from a grant from the BBVA foundation for researchers and cultural creators. ZC acknowledges support from the Juan de la Cierva Incorporacion fellowship IJCI-2014-21669 and from the Spanish research project AYA 2014-58381-P. RS-R acknowledges support from ASI (Italian Space Agency) through the contract no. 2015-046-R.0 and from European Union Horizon 2020 Programme under the AHEAD project (grant agreement no. 654215)

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

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