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

Gamma Ray Bursts (GRBs) are bright, brief flashes of high energy photons that have fascinated scientists for 30 years. They come in two classes: long (>2 s), softspectrum bursts and short, hard events. The major progress to date on understanding GRBs has been for long bursts which are typically at high redshift (z ~ 1) and are in sub-luminous star-forming host galaxies. They are likely produced in core-collapse explosions of massive stars. Until the present observation, no short GRB had been accurately (<10") and rapidly (minutes) located. Here we report the detection of X-ray afterglow from and the localization of short burst GRB050509b. Its position on the sky is near a luminous, non-starforming elliptical galaxy at a redshift of 0.225, exactly the type of location one would expect if the origin of this GRB is the long-proposed fiery merger of neutron star (NS) or black hole (BH) binaries. The X-ray afterglow is found to be weak and fading below detection within a few hours and no optical afterglow is detected to stringent limits, explaining the past difficulty in localizing short GRBs.Comment: 16 pages, 3 figures updated figure