A study of prompt emission mechanisms in gamma-ray bursts

The principal paradigm for the generation of the nonthermal particles that are responsible for the prompt emission of gamma-ray bursts invokes diffusive shock acceleration at shocks internal to the dynamic ultra-relativistic outflow. This paper explores expectations for burst emission spectra arising from shock acceleration theory in the limit of particles cooling much slower than their acceleration. Parametric fits to burst spectra obtained by the Compton Gamma Ray Observatory (CGRO) are explored for the cases of the synchrotron, inverse Compton, and synchrotron self-Compton (SSC) radiation mechanisms, using a linear combination of thermal and nonthermal electron populations. These fits demand that the preponderance of electrons that are responsible for the prompt emission reside in an intrinsically nonthermal population, strongly contrasting particle distributions obtained from acceleration simulations. This implies a potential conflict for acceleration scenarios in which the nonthermal electrons are drawn directly from a thermal gas, unless radiative efficiencies only become significant at highly superthermal energies. It is also found that the CGRO data preclude effective spectroscopic discrimination between the synchrotron and inverse Compton mechanisms. This situation may be resolved with future missions probing gamma-ray bursts, namely Swift and GLAST. However, the SSC spectrum is characteristically too broad near the nuF(nu) peak to viably account for bursts such as GRB 910601, GRB 910814, and GRB 990123. We conclude that the SSC mechanism may be generally incompatible with differential burst spectra steeper than around E(-2.5) above the peak, unless the synchrotron component is strongly self-absorbed.