5.4. Warped Disks

Accretion disks may not be flat in shape. If the angular momentum of the flow is misaligned with the spin axis, then the disk will be warped in such a way as to allow the inner parts to lie in the equatorial plane of the hole (Bardeen & Petterson 1975). It may be that the radiation-induced warping instability (Pringle 1997) could extend to the inner regions of accretion disks or that other warping modes exist there (see, e.g., Markovic & Lamb 1998). Such warping would immediately produce less overall polarization in the direct emitted spectrum, owing to the breaking of plane-parallel geometry (cf. the surface irregularities discussed by Coleman & Shields 1990). However, it would also open the possibility of scattering of optical/UV radiation by different parts of the disk, thereby producing different polarization signatures. In addition, if a warped disk intercepts and reprocesses radiation from the inner regions, the effective temperature distribution could flatten from the canonical T_eff r^-3/4 to r^-1/2, which would produce a long-wavelength SED of F ^-1, much redder than the canonical ^1/3 distribution, and perhaps more in line with observations. The theoretical problem of the spectral and polarization properties of the radiation emerging from a warped disk in an AGN remains to be explored.