  Thermal X-ray emission which is simultaneous with the prompt gamma-rays has
been detected for the first time from a supernova connected with a gamma-ray
burst (GRB), namely GRB060218/SN2006aj. It has been interpreted as arising from
the breakout of a mildly relativistic, radiation-dominated shock from a dense
stellar wind surrounding the progenitor star. There is also evidence for the
presence of a mildly relativistic ejecta in GRB980425/SN1998bw, based on its
X-ray and radio afterglow. Here we study the process of repeated bulk Compton
scatterings of shock breakout thermal photons by the mildly relativistic
ejecta. During the shock breakout process, a fraction of the thermal photons
would be repeatedly scattered between the pre-shock material and the shocked
material as well as the mildly relativistic ejecta and, as a result, the
thermal photons get boosted to increasingly higher energies. This bulk motion
Comptonization mechanism will produce nonthermal gamma-ray and X-ray flashes,
which could account for the prompt gamma-ray burst emission in low-luminosity
supernova-connected GRBs, such as GRB060218. A Monte Carlo code has been
developed to simulate this repeated scattering process, which confirms that a
significant fraction of the thermal photons get "accelerated" to form a
nonthermal component, with a dominant luminosity. This interpretation for the
prompt nonthermal emission of GRB060218 may imply that either the usual
internal shock emission from highly relativistic jets in these low-luminosity
GRBs is weak, or alternatively, that there are no highly relativistic jets in
this peculiar class of bursts.
