Jet Breaks in Short Gamma-Ray Bursts. II: The Collimated Afterglow of GRB 051221A

Abstract

We report the best evidence to date of a jet break in a short Gamma-Ray Burst (GRB) afterglow, using Chandra and Swift XRT observations of the X-ray afterglow of GRB 051221A. The combined X-ray light curve, which has three breaks, is similar to those commonly observed in Swift observations of long GRBs. A flat segment of the light curve at ~0.1 days after the burst represents the first clear case of strong energy injection in the external shock of a short GRB afterglow. The last break occurs at ~4 days post-burst and breaks to a power-law decay index of ~2. We interpret this as a jet break, with important implications for models of short GRBs, since it requires collimation of the afterglow into a jet with an initial opening angle ~4-8 degrees and implies a total jet kinetic energy of E_jet ~(1-5) x 10^{49} erg. Combined with the lack of a jet break in GRB 050724, this suggests a wide range in jet collimation in short GRBs, with at least some having collimation similar to that found in long GRBs, though with significantly lower jet energies.

10 pages, 4 figures. Submitted to The Astrophysical Journal 13 April 2006, accepted 7 August 2006. This revision is the accepted version

References in corpus (4)

• Gamma-Ray Bursts: Progress, Problems & Prospects
• X-Ray Flares from Postmerger Millisecond Pulsars
• The X-ray afterglow of the short gamma ray burst 050724
• The Swift X-ray flaring afterglow of GRB 050607

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