Astron. Astrophys. 348, 71-76 (1999)

1. Introduction

It is widely believed that Active Galactic Nuclei (AGN) are powered by accretion onto a supermassive black hole (Rees 1984, Blandford 1990, Ho 1998). This accretion probably operates through a geometrically thin disk, which is necessary to ensure enough radiation efficiency of the process (Rees 1984). Viscous release of energy in the disk, due to differential Keplerian rotation, is the most commonly invoked mechanism to explain the observed AGN luminosities, which are usually about . Through this mechanism, which generates mainly visual, UV, and soft X-ray continuum, up to 0.30 of the total mass-energy of the flow can be extracted (Thorne 1974). The observed hard X-ray radiation is probably produced by a compact source, located near the center (at , where , is the black hole mass). The compactness of this X-ray source follows from its rapid variability in many objects.

A thin accretion disk in AGN could not only be the energy source but also act as a fluorescent screen, producing a significant part of the broad emission lines in Seyfert 1 type nuclei due to the reprocessing of the central hard radiation (Collin-Souffrin 1987). Evidence for a disk reprocessing is given by reverberation mapping studies, which show a very small BLR radius (several light days), in a contradiction with the standard cloud models predictions (Osterbrock 1993 and references therein). Detailed analysis by Collin-Souffrin & Dumont (1989) shows that the low ionization lines (Balmer lines, for instance) from the disk could be successfully modeled, as this emission depends on the amount of hard X-ray radiation intercepted, instead of on the ionization parameter which is generally unknown. Two types of illuminating source geometry have been proposed (Dumont & Collin-Souffrin 1990a): a point source, located above/below the disk plane and a diffuse hot medium, reflecting hard radiation to the disk surface. The line profiles from the disk are broad, with , double-peaked in most cases, symmetric and non frequency shifted (Dumont & Collin-Souffrin 1990b). They depend mostly on disk structure, geometry and power of the ionizing source, as well as on the point of view.

All these profile calculations have been performed under a default assumption that the disk is a planar, axisymmetric structure, illuminated uniformly. This, however, might not always be true. If the accreting matter falls onto a rapidly rotating (Kerr ) black hole and the angular momenta of the hole, and of the accreting gas, are not aligned, the disk formed is a nonplanar structure (a twisted or warped disk). This is the well known Bardeen-Petterson effect (Bardeen & Petterson 1975, Macdonald et al. 1986). This effect is due to the differential Lense-Thirring precession (, a is the dimensionless spin parameter of the hole, ) of orbits around the spin axis of a rotating black hole (Misner et al. 1973). Although depends on the position within the disk, the disk is a steady structure as the viscosity prevents its disassembling. Near the hole, the flow is aligned with the equatorial plane of the black hole, while at larger distances it is smoothly tilted to its initial orbital plane. The alignment radius (the Bardeen-Petterson radius ) was originally been assumed to be of the same order as the radial distance, where the precession time scale () is comparable to the infall time scale (, is the radial velocity of the gas). When the internal hydrodynamics of the disk is fully taken into account it appears to be much smaller (Kumar & Pringle 1985).

Accretion disks spin up nonrotating, or slowly rotating, black holes because the angular momentum per mass unit of the accreting gas at the innermost stable orbit exceeds that of the hole (Misner et al. 1973; Shapiro & Teukolsky 1983). The Blandford-Znajek process (electromagnetic extraction of the black hole spin energy) is probably not efficient enough to reduce completely this accumulated spin momentum (Modersky et al. 1997, Livio et al. 1998). In other words, an AGN black hole is most probably fast rotating (). As a result, the accretion disk in AGN should be nonplanar in most cases, as there is no reason to suppose that the spin momentum of the accreting mass and the black hole spin momentum are always aligned. Evidence for the existence of nonplanar disks in AGN has been recently found by Nishiura et al. (1998).Obviously, such a nonplanar geometry of the disk makes it possible for the central radiation to reach the outer parts, as these can be directly "seen" from the center (Petterson 1977). The covering factor of a nonplanar disk in case of central irradiation is close to its initial inclination angle, measured in units.

In this paper, the broad Balmer emission due to reprocessing of the central high energy radiation by a warped accretion disk is investigated. Here we present HFI profiles. The profiles of other strong low ionization lines (HFF , for instance) could be slightly different because of different reprocessing properties of the medium. Our main purpose here is to demonstrate the effect of disk twisting on the line profiles, without making fits to observational data. That is why we choose the simplest model of a disk with small inclination and a point X-ray source located near the center. In the next sections we describe our method (Sect. 2) and results (Sect. 3). Discussion, comparison with the observations, and conclusions are given in Sect. 4

© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999
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