On the nature of the 'hostless' short GRBs

Abstract

A significant proportion (~30 per cent) of the short-duration gamma-ray bursts (SGRBs)localized by Swift have no detected host galaxy coincident with the burst location to deeplimits, and also no high-likelihood association with proximate galaxies on the sky. TheseSGRBs may represent a population at moderately high redshifts (z≳ 1), for which the hostsare faint, or a population where the progenitor has been kicked far from its host or is sited in anoutlying globular cluster. We consider the afterglow and host observations of three 'hostless'bursts (GRBs 090305A, 091109B and 111020A), coupled with a new observational diagnosticto aid the association of SGRBs with putative host galaxies to investigate this issue. Consideringthe well localized SGRB sample, 7/25 SGRBs can be classified as 'hostless' by our diagnostic.Statistically, however, the proximity of these seven SGRBs to nearby galaxies is higher thanis seen for random positions on the sky. This suggests that the majority of 'hostless' SGRBshave likely been kicked from proximate galaxies at moderate redshift. Though this result stillsuggests only a small proportion of SGRBs will be within the Advanced Laser InterferometerGravitational Wave Observatory horizon for neutron star-neutron star (NS) or neutron star-black hole (BH) inspiral detection (z~0.1), in the particular case of GRB 111020A a plausiblehost candidate is at z = 0.02.©2013 The Authors.