AAS 207th Meeting, 8-12 January 2006
Session 158 GRB: Short Bursts and Panchromatic Observations
Oral, Wednesday, 2:00-3:30pm, January 11, 2006, Virginia

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[158.03] Are the Properties of Host Galaxies of Short–Hard GRBs Consistent With an Origin of NS-NS Mergers?

E. Nakar, A. Gal-Yam (Caltech)

The recent detection of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) indicates that SHB progenitors belong to old stellar populations (in contrast to those of the long-soft GRBs) and provides a strong clue about their physical nature. Definitive conclusions however are limited by the small number of known hosts. Here, we present our investigation of SHBs previously localized by the interplanetary network (IPN) using new and archival optical and X-ray observations. We show that we can likely identify the host galaxies/clusters for two additional bursts, almost doubling the sample of SHBs with known hosts and/or distances. In particular, we determine that SHB 790613 occurred within the rich galaxy cluster Abell 1892 and that SHB 000607 is most likely associated with a bright galaxy at z=0.14. We rule out galaxy overdensities (down to ~21mag) near the locations of two other SHBs, and set a lower limit on their probable redshift. We combine our SHB sample with events discovered recently by the Swift and HETE-2 missions to investigate SHB progenitor lifetimes, following two routes. We consider the distribution of SHB host galaxy Hubble types. Comparison with type Ia supernova hosts shows that the progenitors of SHBs are typically older. We use the observed SHB redshifts and peak luminosities in conjunction with the cosmic star formation rate and SHB logN-logS distribution from BATSE to constrain their typical lifetime. Both methods indicate a typical lifetime of several Gyr. The second method also yields a lower limit on the local rate of SHBs. We show that the combination of the local rate and the progenitor lifetime are inconsistent with predictions from NS-NS merger models based on the observed sample of Galactic NS binaries. We discuss the implications of our results for gravitational wave detections with LIGO.

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