On the nature of the 'hostless' short GRBs

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 deep limits, and also no high-likelihood association with proximate galaxies on the sky. These SGRBs may represent a population at moderately high redshifts (z ≳ 1), for which the hosts are faint, or a population where the progenitor has been kicked far from its host or is sited in an outlying globular cluster. We consider the afterglow and host observations of three ‘hostless’ bursts (GRBs 090305A, 091109B and 111020A), coupled with a new observational diagnostic to aid the association of SGRBs with putative host galaxies to investigate this issue. Considering the 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 than is seen for random positions on the sky. This suggests that the majority of ‘hostless’ SGRBs have likely been kicked from proximate galaxies at moderate redshift. Though this result still suggests only a small proportion of SGRBs will be within the Advanced Laser Interferometer Gravitational 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 plausible host candidate is at z = 0.02.