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 ∼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 n₀ ∼ 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 0″.80 ± 011 (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.