* Astronomy

Title: A Jet Break in the X-ray Light Curve of Short GRB 111020A: Implications for Energetics and Rates
Authors: Wen-fai Fong, Edo Berger, Raffaella Margutti, B. Ashley Zauderer (Harvard), Eleonora Troja (NASA-GSFC), Ian Czekala, Ryan Chornock (Harvard), Neil Gehrels, Takanori Sakamoto (NASA-GSFC), Derek B. Fox (PSU), Philipp Podsiadlowski (Oxford)

We present broad-band 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 seconds to 10 days after the burst, reveals a significant break at t~2 days with pre- and post-break decline rates of alphaX,1 ~ -0.78 and alphaX,224.4 mag at 18 hours after the burst, and reveal a potential host galaxy with i~24.3 mag. The sub-arcsecond localisation 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)e21 cm^(-2) (z=0). Our observations demonstrate that a growing fraction of short 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.

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