  BAT and XRT observations of two recent well-covered GRBs observed by Swift,
GRB 050315 and GRB 050319, show clearly a prompt component joining the onset of
the afterglow emission. By fitting a power law form to the gamma-ray spectrum,
we extrapolate the time dependent fluxes measured by the BAT, in the energy
band 15-350 keV, into the spectral regime observed by the XRT 0.2-10 keV, and
examine the functional form of the rate of decay of the two light curves. We
find that the BAT and XRT light curves merge to form a unified curve. There is
a period of steep decay up to ~300 s, followed by a flatter decay. The duration
of the steep decay, ~100 s in the source frame after correcting for
cosmological time dilation, agrees roughly with a theoretical estimate for the
deceleration time of the relativistic ejecta as it interacts with circumstellar
material. For GRB 050315, the steep decay can be characterized by an
exponential form, where one e-folding decay time t_e(BAT) ~ 24+/-2 s, and
t_e(XRT) ~ 35+/-2 s. For GRB 050319, a power law decay -(d ln f/d ln t) = n,
where n is about 3, provides a reasonable fit. The early time X-ray fluxes are
consistent with representing the lower energy tail of the prompt emission, and
provide our first quantitative measure of the decay of the prompt gamma-ray
emission over a large dynamic range in flux. The initial steep decay is
expected due to the delayed high latitude photons from a curved shell of
relativistic plasma illuminated only for a short interval. The overall
conclusion is that the prompt phase of GRBs remains observable for hundreds of
seconds longer than previously thought.
