GRB 090426: The Environment of a Rest-Frame 0.35-second Gamma-Ray Burst at Redshift z=2.609

TL;DR

This paper reports the discovery and analysis of a short-duration gamma-ray burst at high redshift, revealing its environment, host galaxy, and implications for GRB classification and progenitors.

Contribution

It provides the first firm redshift measurement for a short-duration GRB and analyzes its environment and host galaxy, challenging traditional classification.

Findings

GRB 090426 has a rest-frame duration of 0.35 s.

The burst's environment has very low HI column density.

The host galaxy is a luminous, star-forming galaxy.

Abstract

We present the discovery of an absorption-line redshift of z = 2.609 for GRB 090426, establishing the first firm lower limit to a redshift for a gamma-ray burst with an observed duration of <2 s. With a rest-frame burst duration of T_90z = 0.35 s and a detailed examination of the peak energy of the event, we suggest that this is likely (at >90% confidence) a member of the short/hard phenomenological class of GRBs. From analysis of the optical-afterglow spectrum we find that the burst originated along a very low HI column density sightline, with N_HI < 3.2 x 10^19 cm^-2. Our GRB 090426 afterglow spectrum also appears to have weaker low-ionisation absorption (Si II, C II) than ~95% of previous afterglow spectra. Finally, we also report the discovery of a blue, very luminous, star-forming putative host galaxy (~2 L*) at a small angular offset from the location of the optical afterglow. We consider the implications of this unique GRB in the context of burst duration classification and our understanding of GRB progenitor scenarios.