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 ~-0.3. A steepening occurs at later times (~1500 s) with a power-law index of ~-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.