1. NASA/Goddard Space Flight Center Greenbelt, Maryland 20771, USA
2. INAF—Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy
3. Universita degli studi di Milano Bicocca, Piazza delle Scienze 3, I-20126 Milano, Italy
4. Department of Astronomy and Astrophysics, Penn State University, University Park, Pennsylvania 16802, USA
5. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
6. Department of Astronomy, University of Virginia, Charlottesville, Virginia 22903-0818, USA
7. NASA/Marshall Space Flight Center, NSSTC, XD-12, 320 Sparkman Drive, Huntsville, Alabama 35805, USA
8. Department of Physics, University of Nevada, Las Vegas, Las Vegas, Nevada 89154-4002, USA
9. Osservatorio Astronomico di Roma, Via di Frascati, 33 00040 Monte Porzio Catone, Italy
10. National Research Council, 2101 Constitution Avenue NW, Washington, DC 20418, USA
11. Universita degli Studi dell'Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
12. Lund Observatory, Box 43, SE-221 00 Lund, Sweden
13. ASI Science Data Center, Via Galileo Galilei, I-00044 Frascati, Italy
14. Universities Space Research Association, NSSTC, XD-12, 320 Sparkman Drive, Huntsville, Alabama 35805, USA
15. Department of Physics, Penn State University, University Park, Pennsylvania 16802, USA
16. Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Correspondence to: S. D. Barthelmy¹ Correspondence and requests for materials should be addressed to S.D.B. (Email: scott@lheamail.gsfc.nasa.gov).

Abstract

Two short (< 2 s) -ray bursts (GRBs) have recently been localized^{1, 2, 3, 4} and fading afterglow counterparts detected^{2, 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 -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 observations^{6, 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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