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

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 theta<SUB>0</SUB>~4<SUP>deg</SUP>-8<SUP>deg</SUP> and implies a total jet kinetic energy of E<SUB>jet</SUB>~(1-5)10<SUP>49</SUP> 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.