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 in the light curve occurs at ~4 days postburst 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 θ0~4deg-8deg and implies a total jet kinetic energy of Ejet~(1-5)×1049 ergs. 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, although with significantly lower jet energies.
The Astrophysical Journal
- Pub Date:
- December 2006
- Gamma Rays: Bursts;
- 10 pages, 4 figures. Submitted to The Astrophysical Journal 13 April 2006, accepted 7 August 2006. This revision is the accepted version