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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-rayafterglow of GRB 051221A. The combined X-ray light curve, which has threebreaks, is similar to those commonly observed in Swift observations of longGRBs. A flat segment of the light curve at ~0.1 days after the burst representsthe first clear case of strong energy injection in the external shock of ashort GRB afterglow. The last break occurs at ~4 days post-burst and breaks toa power-law decay index of ~2. We interpret this as a jet break, with importantimplications for models of short GRBs, since it requires collimation of theafterglow into a jet with an initial opening angle ~4-8 degrees and implies atotal jet kinetic energy of E_jet ~(1-5) x 10^{49} erg. Combined with the lackof a jet break in GRB 050724, this suggests a wide range in jet collimation inshort GRBs, with at least some having collimation similar to that found in longGRBs, though with significantly lower jet energies.Comment: 10 pages, 4 figures. Submitted to The Astrophysical Journal 13 April  2006, accepted 7 August 2006. This revision is the accepted versio

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