Swift observations of GRB 050128:: The early X-ray afterglow

Swift discovered GRB 050128 with the Burst Alert Telescope and promptly pointed its narrow field instruments to monitor the afterglow. X-ray observations started 108 s after the trigger time. The early decay of the afterglow is relatively flat, with a temporal decay modeled with a power-law index of similar to-0.3. A steepening occurs at later times (similar to 1500 s) with a power-law index of similar to-1.3. During this transition, the observed X- ray spectrum does not change. We interpret this behavior as either an early jet break or evidence of a transition from the fast cooling regime to the slow cooling regime in a wind environment.