A JET BREAK IN THE X-RAY LIGHT CURVE OF SHORT GRB 111020A: IMPLICATIONS FOR ENERGETICS AND RATES

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 similar to 100 s to 10 days after the burst, reveals a significant break at delta t approximate to 2 days with pre- and post-break decline rates of alpha(X, 1) approximate to -0.78 and alpha(X, 2) less than or similar to -1.7, respectively. Interpreted as a jet break, we infer a collimated outflow with an opening angle of theta(j) approximate to 3 degrees-8 degrees. The resulting beaming-corrected gamma-ray (10-1000 keV band) and blast-wave kinetic energies are (2-3) x 10(48) erg and (0.3-2) x 10(49) erg, respectively, with the range depending on the unknown redshift of the burst. We report a radio afterglow limit of <39 mu Jy (3 sigma) from Expanded Very Large Array observations that, along with our finding that nu(c) < nu(X), constrains the circumburst density to n(0) similar to 0.01-0.1 cm(-3). Optical observations provide an afterglow limit of i greater than or similar to 24.4 mag at 18 hr after the burst and reveal a potential host galaxy with i approximate to 24.3 mag. The subarcsecond localization from Chandra provides a precise offset of 0 ''.80+/-0 ''.11 (1 sigma) 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) x 10(21) cm(-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 greater than or similar to 100-1000 Gpc(-3) yr(-1), in good agreement with the NS-NS merger rate of approximate to 200-3000 Gpc(-3) yr(-1). This consistency is promising for coincident short GRB-gravitational wave searches in the forthcoming era of Advanced LIGO/VIRGO.