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

Abstract

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 αX,1 ≈ -0.78 and αX,2 ≲ -1.7, respectively. Interpreted as a jet break, we infer a collimated outflow with an opening angle of θj ≈ 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 νc