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An origin for short γ-ray bursts unassociated with current star formation

Abstract

Two short (< 2 s) γ-ray bursts (GRBs) have recently been localized1,2,3,4 and fading afterglow counterparts detected2,3,4. The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual γ-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5), coincident with the position determined by ground-based optical and radio observations6,7,8. The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (refs 3, 31), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2–3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts—the coalescence of binary systems of neutron stars or a neutron star-black hole pair—are the progenitors of short bursts.

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Figure 1: BAT lightcurves for GRB 050724 showing the short duration of this GRB and the long softer emission.
Figure 2: VLT optical image 17 showing the association of GRB 050724 with the galaxy.
Figure 3: The smooth transition of the GRB phase into the X-ray phase.

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Acknowledgements

We acknowledge support from ASI, NASA and PPARC, and acknowledge benefits from collaboration within the EU FP5 Research Training Network ‘γ-Ray Bursts: An Enigma and a Tool’.

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Correspondence to S. D. Barthelmy.

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Barthelmy, S., Chincarini, G., Burrows, D. et al. An origin for short γ-ray bursts unassociated with current star formation. Nature 438, 994–996 (2005). https://doi.org/10.1038/nature04392

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