A jet break in the x-ray light curve of short GRB111020A: Implications for energetics and rates

We present broadband observations of the afterglow and environment of the short GRB111020A. An extensive X-ray light curve from Swift/XRT, XMM-Newton, and Chandra, spanning ∼100 s to 10days after the burst, reveals a significant break at δt ≈ 2days with pre- and post-break decline rates of α_X,1 ≈ -0.78 and α_X,2 ≲ -1.7, respectively. Interpreted as a jet break, we infer a collimated outflow with an opening angle of θ_j ≈ 3°-8°. The resulting beaming-corrected γ-ray (10-1000keV band) and blast-wave kinetic energies are (2-3) × 10⁴⁸erg and (0.3-2) × 10⁴⁹erg, respectively, with the range depending on the unknown redshift of the burst. We report a radio afterglow limit of <39 μJy (3σ) from Expanded Very Large Array observations that, along with our finding that ν_c < ν_X, constrains the circumburst density to n ₀ 0.01-0.1cm^-3. Optical observations provide an afterglow limit of i ≳ 24.4mag at 18hr after the burst and reveal a potential host galaxy with i 24.3mag. The subarcsecond localization from Chandra provides a precise offset of 080 ± 011 (1σ) from this galaxy corresponding to an offset of 5-7kpc for z = 0.5-1.5. We find a high excess neutral hydrogen column density of (7.5 ± 2.0) × 10²¹cm^-2 (z = 0). Our observations demonstrate that a growing fraction of short gamma-ray bursts (GRBs) are collimated, which may lead to a true event rate of ≳ 100-1000Gpc^-3yr^-1, in good agreement with the NS-NS merger rate of ≈200-3000Gpc^-3yr^-1. This consistency is promising for coincident short GRB-gravitational wave searches in the forthcoming era of Advanced LIGO/VIRGO.