  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.

Astrophysics