Introduction

The intense X–ray emission of AGN (active galactic nuclei) can heat the gas in these objects to high temperatures, driving a wind from regions in which the thermal velocity is comparable to or greater than the escape velocity (Begelman et al. 1983). Other mechanisms, such as heating due to dissipation of magnetic fields, or acceleration by rotating magnetic fields or radiation pressure, can also produce winds in AGN; thus, X–ray heated winds may be considered to be the minimum required by observation. These winds are important both because they can alter the accretion rate onto the central object by extracting mass, and because they provide important diagnostics of the distribution and dynamics of gas in AGN (Begelman and McKee 1983).

The nature of the wind is determined by the geometry of the gas relative to the source of the X–rays. The variability of the X–ray emission in AGN indicates that the source of the emission is compact (e.g., Turner and Pounds 1988). The gas may be distributed around this compact source in several possible ways: First, it could be in an accretion disk, although direct observational evidence for this assumption is lacking at present; by contrast, there is good evidence for accretion disks in many binary X–ray sources in the Galaxy. A wind will be driven from an accretion disk either if the disk flares (as it does in the standard α disk—Shakura and Sunyaev 1973) or if the source of the X–rays is above the disk (as in Compton reflection models—Fabian, this volume).