A Short GRB "No-Host'' Problem? Investigating Large Progenitor Offsets for Short GRBs with Optical Afterglows

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[abridged] We investigate the afterglow properties and large-scale environments of several short-duration gamma-ray bursts (GRBs) with sub-arcsecond 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 gamma-ray emission are systematically fainter.
• These differences may be due to lower circumburst densities (by about an order of magnitude), to higher redshifts (by dz~0.5-1), or to lower energies, although in the standard GRB model the smaller gamma-ray fluences cannot be explained by lower densities.