2010ApJ...722.1946B

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(<δR) ∼ 0.1) are associated with bright galaxies at separations of δR ∼ 10'', while somewhat higher probabilities of chance coincidence are associated with faint galaxies at separations of ∼2''. By measuring redshifts for the brighter galaxies in three cases (z = 0.111, 0.473, 0.403), we find physical offsets of ~30-75 kpc, while for the faint hosts the assumption of z ≳ 1 leads to offsets of ∼15 kpc. Alternatively, the limits at the burst positions (≳26 mag) can be explained by typical short GRB host galaxies (L ~ 0.1-1 L*) at z ≳ 2-3. Thus, two possibilities exist: (1) ∼1/4 of short GRBs explode ∼50 kpc or ∼15 kpc from the centers of z ∼ 0.3 or z ≳ 1 galaxies, respectively, and have fainter afterglows due to the resulting lower densities; or (2) ∼1/4 of short GRBs occur at z ≳ 2 and have fainter afterglows due to their higher redshifts. The high-redshift scenario leads to a bimodal redshift distribution, with peaks at z ∼ 0.5 and z ∼ 3. The large offset scenario leads to an offset distribution that is well matched by theoretical predictions of NS-NS/NS-BH binary kicks, or by a hybrid population with globular cluster NS-NS binaries at large offsets and primordial binaries at offsets of ≲10 kpc (indicative of negligible kicks). Deeper constraints on any coincident galaxies to ≳28 mag (using the Hubble Space Telescope) will allow us to better exclude the high-redshift scenario.