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


Gamma-ray bursts (GRBs) come in two classes1: 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 ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars2. In contrast, no short GRB had been accurately (< 10″) 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 expect3,4 if the origin of this GRB is through the merger of neutron-star or black-hole 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.

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Figure 1: Optical images of the region of GRB 050509B showing the association with a large elliptical galaxy.
Figure 2: BAT light curves for the short GRB 050509B, showing the short duration of this GRB.
Figure 3: X-ray afterglow light curve for GRB 050509B showing weak flux falling off to undetectability after 10 4  s.


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The authors acknowledge support from ASI, NASA and PPARC.

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Gehrels, N., Sarazin, C., O'Brien, P. et al. A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225. Nature 437, 851–854 (2005).

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