GRB090426: the farthest short gamma-ray burst?

TL;DR

This study presents a detailed analysis of GRB090426, a short-duration gamma-ray burst at a high redshift, revealing properties that challenge traditional classifications and suggesting a complex origin.

Contribution

The paper provides the first detailed optical and high-energy analysis of GRB090426, highlighting its unique combination of short duration and long-burst-like properties at high redshift.

Findings

GRB090426 has a redshift of z=2.61, the highest for a short GRB.

Its spectral and energy properties resemble long GRBs.

The host galaxy is a star-forming galaxy at the same redshift.

Abstract

Aims. With an observed and rest-frame duration of < 2s and < 0.5s, respectively, GRB090426 could be classified as a short GRB. The prompt detection, both from space and ground-based telescopes, of a bright optical counterpart to this GRB offered a unique opportunity to complete a detailed study. Methods. Based on an extensive ground-based observational campaign, we obtained the spectrum of the optical afterglow of GRB090426, measuring its redshift and obtaining information about the medium in which the event took place. We completed follow-up observation of the afterglow optical light curve down to the brightness level of the host galaxy that we firmly identified and studied. We also retrieved and analyzed all the available high-energy data of this event, and compared the results with our findings in the optical. This represents one of the most detailed studies of a short-duration event presented so far. Results. The time properties qualify GRB090426 as a short burst. In this case, its redshift of z = 2.61 would be the highest yet found for a GRB of this class. On the other hand, the spectral and energy properties are more similar to those of long bursts. LBT late-time deep imaging identifies a star-forming galaxy at a redshift consistent with that of the GRB. The afterglow lies within the light of its host and shows evidence of local absorption.