Dense Clouds Near the Center of Active Galactic Nuclei

Summary

Recently some interesting spectral features, such as an Fe line-edge system and a hard X-ray bump, have been observed in many Seyfert nuclei (e.g. Nandra et al., 1991, hereafter NPS91). These observed characteristics are naturally explained by the ‘two-component’ model which consists of a non-thermal hot plasma and a ‘cold’ plasma which reprocesses and reflects the non-thermal radiation. The presence of such cold matter in the vicinity of the central region is a natural consequence of the accretion model of active galactic nuclei (AGN) (e.g. Guilbert and Rees 1988, hereafter GR88). It is also supported by X-ray observations of many Seyfert nuclei by the EXOSAT and Ginga missions (e.g. NPS91, and references therein), for instance, the simultaneous rapid variations of the X-ray continuum and Fe line fluxes (Kunieda et al., 1990). The cold component is envisioned to be in a form of either a thin disc (or slab) or an assembly of clouds distributed over a three-dimensional configuration (GR88; Celotti, Fabian and Rees 1992, hereafter CFR92). For convenience, we shall call the former kind ‘the disc model’ and the latter ‘the cloud model’.

Here a model is presented which assumes the existence of ‘cold’ dense clouds near the central engine of AGN. In order for the cold clouds to exist near the central engine they must be sufficiently dense, and some physical pressure is required to confine them. This pressure may be supplied by equipartition magnetic fields (Rees 1987, CFR92).