Accounting for the XRT early steep decay in models of the prompt GRB
  emission
  The Swift-XRT observations of the early X-ray afterglow of GRBs show that it
usually begins with a steep decay phase. A possible origin of this early steep
decay is the high latitude emission that subsists when the on-axis emission of
the last dissipating regions in the relativistic outflow has been switched-off.
We wish to establish which of various models of the prompt emission are
compatible with this interpretation. We successively consider internal shocks,
photospheric emission, and magnetic reconnection and obtain the typical decay
timescale at the end of the prompt phase in each case. Only internal shocks
naturally predict a decay timescale comparable to the burst duration, as
required to explain XRT observations in terms of high latitude emission. The
decay timescale of the high latitude emission is much too short in photospheric
models and the observed decay must then correspond to an effective and generic
behavior of the central engine. Reconnection models require some ad hoc
assumptions to agree with the data, which will have to be validated when a
better description of the reconnection process becomes available.
