A short gamma-ray burst apparently associated with an elliptical galaxy at redshift z=0.225

Abstract

Gamma-ray bursts (GRBs) come in two classes(1): long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift ( z approximate to 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars(2). In contrast, no short GRB had been accurately (< 1000) and rapidly ( minutes) located. Here we report the detection of the X-ray afterglow from - and the localization of - the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect(3,4) if the origin of this GRB is through the merger of neutron-star or blackhole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.