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Astron. Astrophys. 325, 401-413 (1997)

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10. Conclusion

We have presented the accurate formulas in the form of a simple expression or an one-fold integral for the energy losses and gains of particles scattered by a Maxwell-Boltzmann plasma. The processes concerned are the Coulomb scattering, [FORMULA] -, ee - and ep -bremsstrahlung as well as Comptonization in the Klein-Nishina regime.

The problem of positron propagation is treated in a Fokker-Planck approach, which can be easily generalized to include inelastic processes, stochastic acceleration etc. We have shown that the escape of positrons in the form of pair plasma has an effect on the positron distribution causing, in some cases, a strong deviation from a Maxwellian. When the energy of injected particles essentially exceeds the average one of plasma particles, the deviation appears as a deficit at lower energies and an extended tail of the distribution that leads to a widening and smoothing of the annihilation feature in the spectrum. In the case where the energy of particles injected is close to the average energy of plasma particles, the deviation appears as an injection bump and a deficit in the tail of the distribution. Meanwhile, it does not lead to visible distortions of the annihilation line shape which is similar to that of thermal plasmas.

The performed calculations allow us to obtain reliable estimations of the electron number density and the size of the emitting regions in Nova Muscae and the 1E 1740.7-2942 source, suggesting that spectral features in 300-600 keV region observed by SIGMA telescope are due to the electron-positron annihilation in thermal plasma. We conclude that in the case of Nova Muscae the observed radiation is coming from a pair plasma jet, [FORMULA], rather than from a gas cloud. The case of 1E 1740.7-2942 is not definitively clear, [FORMULA]. Although the observational data are consistent with annihilation in (hydrogen) plasma at rest, the redshift of the line suggests that it could be also a stream of pair plasma with matter captured from the source environment.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998