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P.S. Coppi (Department of Astronomy, Yale University)
Most model fits to the spectra of X-ray binaries currently rely on Comptonization by purely thermal or non-thermal electron energy distributions. However, in those cases where direct observations of the electron energy distributions have been possible (space and laboratory plasmas), the energy distribution never turns out to be exactly Maxwellian (thermal) or power-law (non-thermal). Recent broadband (0.5-500 keV) observations of X-ray binaries, particularly those of black hole candidates in the soft state, now appear to also require Comptonizing electron energy distributions that are ``hybrid," i.e., quasi-Maxwellian with a non-thermal tail. The pair plasma code of Coppi (1992) has been extended to accurately and self-consistently compute the electron and Comptonized photon distributions that result when some specified fraction of the electron power goes into heating particles that can thermalize effectively and the rest goes into producing a power law spectrum of accelerated electrons. (Such a parametrization of thermal vs. non-thermal ``efficiency" is often used in solar flare models.) The result is not a trivial superposition of thermal and non-thermal models. The model is rapid enough for data fitting, and a version suitable for use in XSPEC v.11 will be publically available by the time of this conference. Sample applications will be presented, in particular to the spectra of Cyg X-1 in its various states. An accompanying poster (Maccarone, Coppi, & Taam) shows the model applied to GRS 1915.
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