Swift Observations of Grb 050128: the Early X–ray Afterglow

Swift discovered GRB050128 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 with index ∼ −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 behaviour as either an early jet break or evidence for a transition from the fast cooling regime to the slow cooling regime in a wind environment. Subject headings: gamma rays: bursts; X-rays: individual (GRB050128)