A Jet Break in the X-Ray Light Curve of Short GRB 111020A: Implications for Energetics and Rates

Abstract

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) × 10⁴⁸ erg and (0.3-2) × 10⁴⁹ 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 n ₀ ~ 0.01-0.1 cm^-3. 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 0farcs80 ± 0farcs11 (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) × 10²¹ 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 >~ 100-1000 Gpc^-3 yr^-1, in good agreement with the NS-NS merger rate of ≈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.