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Astron. Astrophys. 326, 99-107 (1997)

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1. Introduction

It is now generally agreed that the engine of high power emission in AGNs is a supermassive black hole of [FORMULA], accreting matter from a surrounding accretion disk (Shakura & Sunyaev 1973 ; Rees 1984 ). Besides, several satellites observations of radio quiet Seyfert galaxies have allowed to obtain an average high energy (X-ray/ [FORMULA] -ray) spectrum, better reproduced by a complex superposition of a primary power law, a reflected component from a cold thick gas, a fluorescent iron K line and an absorption by a hot medium (Pounds et al. 1990 ). On the other hand, Clavel et al. (1992 ) have shown a close simultaneity between UV and optical variations of some Seyfert galaxies, which cannot be reproduced by standard accretion disk models. Rather, these results are better explained if the UV-optical emission comes from the reprocessing of hard radiation emitted by a hot source above the disk. In Paper I, we have proposed a new model involving a point source of relativistic leptons (the hot source) emitting hard radiation by Inverse Compton (IC) process on soft photons produced by the accretion disk. The disk itself radiates only through the reprocessing of the hard radiation impinging on it. Such a geometry is highly anisotropic, which takes a real importance in the computation of IC process (Ghisellini et al. 1991 ; Paper I). Paper I dealt only with the Newtonian case and did not include the relativistic effects: these are first, the Doppler shift due to the rotation of the disk; second the gravitational shift, undergone by photons which follow a null geodesic, either from the disk to the hot source and inversely, or from the AGN to the observer at infinity; and third, the gravitational focusing, most important for rays of light skimming the black hole. Thus, the subject of this paper (Paper II) is to extend this simple model to the general relativistic formulation appropriate to a Kerr black hole. The organization is as follows. We will establish, in Sect. 2, the general equations which govern the radiative balance between the hot source and the accretion disk in an axisymmetric gravitational field. We will study the case of a rotating black hole in the Kerr metrics in Sect. 3. In Sect. 4, we will finally obtain the power spectra emitted by this model for different values of the inclination angles and the height of the hot source, and conclude on the importance of the gravitational effects on the overall spectrum.

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998