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

W. Fong, E. Berger, R. Margutti, B. A. Zauderer, E. Troja, I. Czekala, R. Chornock, N. Gehrels, T. Sakamoto, D. B. Fox, P. Podsiadlowski

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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 < νX, constrains the circumburst density to n 0 0.01-0.1cm-3. Optical observations provide an afterglow limit of i ≳ 24.4mag at 18hr after the burst and reveal a potential host galaxy with i 24.3mag. The subarcsecond localization from Chandra provides a precise offset of 080 ± 011 (1σ) from this galaxy corresponding to an offset of 5-7kpc for z = 0.5-1.5. We find a high excess neutral hydrogen column density of (7.5 ± 2.0) × 1021cm-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 ≳ 100-1000Gpc-3yr-1, in good agreement with the NS-NS merger rate of ≈200-3000Gpc-3yr-1. This consistency is promising for coincident short GRB-gravitational wave searches in the forthcoming era of Advanced LIGO/VIRGO.

Original languageEnglish (US)
Article number189
JournalAstrophysical Journal
Volume756
Issue number2
DOIs
StatePublished - Sep 10 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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