Bernoulli equation and the nonexistence of maximal jets

We discuss the idea of maximal jets introduced by Falcke & Biermann in 1995. According to it, the maximum possible jet power in its internal energy equals the kinetic power in its rest mass. We show this result is incorrect because of an unfortunate algebraic mistake.Comment: A&A, in pres

A study of spectra of Cyg X-3 observed by BeppoSAX

We model the ~1-200 keV spectra of Cygnus X-3 observed by BeppoSAX. The continuum, modeled by Comptonization in a hybrid plasma, is modified by the strongly ionized plasma of the stellar wind of the Wolf-Rayet companion star. Discrete absorption and emission spectral features are modeled with XSTAR. The model has been applied to phase-resolved spectra in the hard and soft spectral states.Comment: 3 pages, to appear in the Proceedings of the Conference "X-ray Diagnostics of Astrophysical Plasmas", Cambridge, November 15-17, 2004. Replacement: minor changes in text and acknowledgement

Thermal synchrotron radiation and its Comptonization in compact X-ray sources

We investigate thermal synchrotron radiation at semi-relativistic and relativistic temperatures. We find an analytic expression for the angle-averaged emission coefficient, and show that it is significantly more accurate that those derived previously. We also present analytic approximations to the synchrotron turnover frequency. Then, we treat Comptonization of the synchrotron radiation, and give simple expressions for the spectral shape and the emitted power. We also consider modifications of the above results by bremsstrahlung. The importance of Comptonization of synchrotron radiation in compact X-ray sources is then studied. We consider emission from hot accretion flows and from active coronae above optically-thick accretion discs in black-hole binaries and AGNs. Synchrotron Comptonization is found to be, in general, negligible in luminous sources, except for those with hardest X-ray spectra and stellar masses. Increasing the black-hole mass results in a strong reduction of the maximum Eddington ratio possible from to this process. X-ray spectra of intermediate-luminosity sources, e.g., low-luminosity AGNs, can be explained by synchrotron Comptonization only in the case of hot accretion flows. Then, bremsstrahlung emission always dominates X-ray spectra of very weak sources. Finally, we consider weakly-magnetized neutron stars and find that synchrotron Comptonization can account for the power-law X-ray spectra observed in their low states.Comment: 17 pages, 17 postscript figures. Accepted to MNRA