The Discovery of Vibrationally-Excited H_2 in the Molecular Cloud near GRB 080607

TL;DR

This paper reports the first detection of vibrationally-excited H_2 in a molecular cloud near a gamma-ray burst, revealing the presence of molecular material within hundreds of parsecs of the burst location.

Contribution

It provides the first observational evidence of vibrationally-excited H_2 in a GRB host galaxy and estimates the distance of the molecular cloud from the GRB.

Findings

Detection of vibrationally-excited H_2 with a column density of 10^{17.5} cm^{-2}

Estimated distance of 230-940 parsecs between the GRB and the molecular cloud

Substantial molecular material exists within hundreds of parsecs from the GRB

Abstract

GRB 080607 has provided the first strong observational signatures of molecular absorption bands toward any galaxy hosting a gamma-ray burst. Despite the identification of dozens of features as belonging to various atomic and molecular (H_2 and CO) carriers, many more absorption features remained unidentified. Here we report on a search among these features for absorption from vibrationally-excited H_2, a species that was predicted to be produced by the UV flash of a GRB impinging on a molecular cloud. Following a detailed comparison between our spectroscopy and static, as well as dynamic, models of H_2* absorption, we conclude that a column density of 10^{17.5+-0.2} cm^{-2} of H_2* was produced along the line of sight toward GRB 080607. Depending on the assumed amount of dust extinction between the molecular cloud and the GRB, the model distance between the two is found to be in the range 230--940 pc. Such a range is consistent with a conservative lower limit of 100 pc estimated from the presence of Mg I in the same data. These distances show that substantial molecular material is found within hundreds of pc from GRB 080607, part of the distribution of clouds within the GRB host galaxy.