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 051221 A. 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 00 ∼ 4°-8° and implies a total jet kinetic energy of E jet ∼ (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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science