ar X iv : a st ro - p h / 97 11 10 7 v 1 1 1 N ov 1 99 7 Emission and Cooling Processes in a Hybrid Thermal - Nonthermal Plasma

In a hybrid thermal-nonthermal plasma, we find that the dominant emission and absorption mechanisms are synchrotron by nonthermal electrons and bremsstrahlung by thermal electrons. These two processes significantly change the spectrum from inverse Compton scatterings at low energies. We also find that Coulomb collisions are effective in cooling down the lower energy electrons but do not significantly alter the emission pattern. Compton cooling is more effective in changing emission and absorption coefficients when the photon energy density is high. INTRODUCTION The accurate positioning of GRBs by BeppoSAX has allowed observation of gamma ray bursts over a wide range of wavelengthes. Spectra from these observations provide crucial tests of different emission models, one of which is the inverse Compton scattering (ICS) model [1]. The ICS model explains from first principles most of the known spectral evolution properties of GRBs [1,2]. Under the ICS model, GRB spectra suggest that the sources are thermal-nonthermal hybrid plasma [1,2] whose electron distribution is described as