Shock Dissipation in Magnetically Dominated Impulsive Flows
  We have revisited the issue of shock dissipation and emission and its
implications for the internal shock model of the prompt GRB emission and
studied it in the context of impulsive Poynting-dominated flows. Our results
show that unless the magnetization of GRB jets is extremely high, \sigma > 100
in the prompt emission zone, the magnetic model may still be compatible with
the observations. The main effect of reduced dissipation efficiency is merely
an increase in the size of the dissipation zone and even for highly magnetised
GRB jets this size may remain below the external shock radius, provided the
central engine can emit magnetic shells on the time scale well below the
typical observed variability scale of one second. Our analytical and numerical
results suggest that magnetic shells begin strongly interact with each other
well before they reach the coasting radius. As the result, the impulsive jet in
the dissipation zone is best described not as a collection of shells but as a
continuous highly magnetised flow with a high amplitude magnetosonic wave
component. How exactly the dissipated wave energy is distributed between the
radiation and the bulk kinetic energy of radial jets depends on the relative
rates of radiative and adiabatic cooling. In the fast radiative cooling regime,
the corresponding radiative efficiency can be as high as the wave contribution
to their energy budget, independently of the magnetization. Moreover, after
leaving the zone of prompt emission the jet may still remain
Poynting-dominated, leading to weaker emission from the reverse shock compared
to non-magnetic models.
