A short gamma-ray burst "no-host" problem? Investigating large progenitor offsets for short GRBs with optical afterglows

Abstract

We investigate the afterglow properties and large-scale environments of several short-duration gamma-ray bursts (GRBs) with subarcsecond optical afterglow positions but no bright coincident host galaxies. The purpose of this joint study is to robustly assess the possibility of significant offsets, a hallmark of the compact object binary merger model. Five such events exist in the current sample of 20 short bursts with optical afterglows, and we find that their optical, X-ray, and γ-ray emission are systematically fainter. These differences may be due to lower circumburst densities (by about an order of magnitude), to higher redshifts (by Δz ≈ 0.5-1), or to lower energies (by about a factor of 3), although in the standard GRB model the smaller γ-ray fluences cannot be explained by lower densities. To study the large-scale environments, we use deep optical observations to place limits on underlying hosts and to determine probabilities of chance coincidence for galaxies near each burst. In four of the five cases, the lowest probabilities of chance coincidence (P(