1,721,493 research outputs found
The effect of radiation pressure on spherical accretion
No full textSpherical accretion onto a neutron star is analyzed, taking into account the effect of radiation pressure on the gas flow but neglecting the pressure of the infalling gas. The radiative-transfer equations are derived to first order in v/c, and the system is solved numerically with an approximate treatment of the opacity variation. Numerical results are given for a neutron star having a radius of 1 million cm and a mass of 1 solar mass. These results imply that the radiation flux in the comoving frame is greater than the Eddington limit and indicate that there are no transitions from supersonic to subsonic flow. Results are also discussed for a black hole of arbitrary mass
A MODEL FOR THE X-RAY AND ULTRAVIOLET EMISSION FROM SEYFERT GALAXIES AND GALACTIC BLACK HOLES
No full textThe effect of radiation pressure on accretion disks around black holes
Full text linkSteady-state disk accretion on to a black hole is studied for high accretion rates, where the dynamic effect of radiation pressure is important. The rotation of the disk is not assumed to be Keplerian, but is considered as an unknown in the Newtonian dynamic equation. The problem is reduced to a set of two differential equations which are solved numerically. It is found that steady-state solutions without mass-outflow exist for accretion rates greater than the critical rate. The radiated luminosity, however, is always of the order of the Eddington luminosity. For increasing accretion rates, the kinetic energy swallowed by the hole and the size of the radiating region increas
Unifying models for X-ray selected and radio selected BL Lac objects
No full textWe discuss alternative interpretations of the differences in the spectral energy distributions (SEDs) of BL Lacs found in complete radio or X-ray surveys.
A large body of observations in different bands suggests that the SEDs of BL Lac objects appearing in X-ray surveys differ from those appearing in radio surveys mainly in having a (synchrotron) spectral cut-off (or break) at much higher frequency.
In order to explain the different properties of radio- and X-ray-selected BL Lacs, Giommi and Padovani proposed a model based on a common radio luminosity function. At each radio luminosity, objects with high-frequency spectral cut-offs are assumed to be a minority. Nevertheless, they dominate the X-ray-selected population due to the larger X-ray-to-radio flux ratios. An alternative model explored here (reminiscent of the orientation models previously proposed) is that the X-ray luminosity function is 'primary', and that at each X-ray luminosity a minority of objects have larger radio-to-X-ray ratios. The predictions of the two scenarios, computed via a Monte Carlo technique, are compared with the observed properties of BL Lacs in the two samples extracted, respectively, from the 1-Jy radio survey and the Einstein Slew Survey. We show that both models can explain some but not all of the observed features.
We then propose a completely new approach, based on the idea that the physical parameter which governs the shape of the SEDs is (or is associated with) the bolometric luminosity. Assuming an empirical relation between spectral shape and luminosity, we show that the observational properties of the two surveys can be reproduced with at least the same accuracy as the two previous models
A model for the spectral variability of BL Lac objects at high frequencies
No full textThe average amplitude of variability of BL Lac objects is larger at higher frequencies, and the spectra in the X-ray range show a hardening with increasing intensity. This is shown to be a natural consequence of the relativistic jet model proposed by Ghisellini et al. (1985), where higher frequencies are produced nearer to the jet core. Time-dependent properties are computed assuming that a perturbation travels at fixed speed down the jet, producing enhancements of constant amplitude of the relativistic particle density and of the magnetic field, in a slab of self-similar cone geometry. The time-dependent spectral intensities due to synchrotron radiation and first-order Compton scattering are computed numerically, and approximate analytic formulae are given. The evolution of the spectral shape with time and the light curves at fixed frequencies are presented and discussed in detail
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