2012ApJ...756..189F

We present broadband observations of the afterglow and environment of the short GRB 111020A. An extensive X-ray light curve from Swift/XRT, XMM-Newton, and Chandra, spanning ∼100 s to 10 days after the burst, reveals a significant break at δt ~ 2 days 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 (10-1000 keV band) and blast-wave kinetic energies are (2-3)x10⁴⁸ erg and (0.3-2)x10⁴⁹ erg, 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 n₀∼ 0.01-0.1/cm3. Optical observations provide an afterglow limit of i ≳ 24.4 mag at 18 hr after the burst and reveal a potential host galaxy with i ~ 24.3 mag. The subarcsecond localization from Chandra provides a precise offset of 0".80±0".11 (1σ) from this galaxy corresponding to an offset of 5-7 kpc for z = 0.5-1.5. We find a high excess neutral hydrogen column density of (7.5±2.0)x10²¹/cm2 (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 ≳ 100-1000 G/pc3/yr, in good agreement with the NS-NS merger rate of ~200-3000 G/pc3/yr. This consistency is promising for coincident short GRB-gravitational wave searches in the forthcoming era of Advanced LIGO/VIRGO.