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

Bulk motion Comptonization in black-hole accretion flows

We study spectra generated by Comptonization of soft photons by cold electrons radially free-falling onto a black hole. We use a Monte Carlo method involving a fully relativistic description of Comptonization in the Kerr space-time. In agreement with previous studies, we find that Comptonization on the bulk motion of free fall gives rise to power-law spectra with the photon index of Gamma >~ 3. In contrast to some previous studies, we find that these power-law spectra extend only to energies << 511 keV. We indicate several effects resulting in generic cutoffs of such spectra at several tens of keV, regardless of any specific values of physical parameters in the model. This inefficiency of producing photons with energies > 100 keV rules out bulk motion Comptonization as a main radiative process in soft spectral states of black-hole binaries. The normalization of the power law (below the cutoff) with respect to the peak of the blackbody emission of the surrounding disc is typically very low, except for models with an overlap between the disc and the plasma, in which case the spectra are very soft, Gamma >~ 4.Comment: 10 pages, 4 figures, revised version, accepted for publication in MNRA

Hadronic models of blazars require a change of the accretion paradigm

We study hadronic models of broad-band emission of jets in radio-loud active galactic nuclei, and their implications for the accretion in those sources. We show that the models that account for broad-band spectra of blazars emitting in the GeV range in the sample of Boettcher et al. have highly super-Eddington jet powers. Furthermore, the ratio of the jet power to the radiative luminosity of the accretion disc is $\sim 3000$ on average and can be as high as $\sim 10^5$. We then show that the measurements of the radio core shift for the sample imply low magnetic fluxes threading the black hole, which rules out the Blandford-Znajek mechanism to produce powerful jets. These results require that the accretion rate necessary to power the modelled jets is extremely high, and the average radiative accretion efficiency is $\sim 4 \times 10^{-5}$. Thus, if the hadronic model is correct, the currently prevailing picture of accretion in AGNs needs to be significantly revised. Also, the obtained accretion mode cannot be dominant during the lifetimes of the sources, as the modelled very high accretion rates would result in too rapid growth of the central supermassive black holes. Finally, the extreme jet powers in the hadronic model are in conflict with the estimates of the jet power by other methods.Comment: MNRAS, in pres