GENERAL-RELATIVISTIC EFFECTS ON THE INFRARED-SPECTRUM OF THIN ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI - APPLICATION TO SAGITTARIUS-A

The possibility that some portion of the infrared (IR) radiation emana ting from active galactic nuclei (AGNs) may arise from disklike struct ures of ionized plasma accreting onto massive black holes motivates th e investigation of the effects of strong gravitational fields in the v icinity of emitting particles on the observed radiation. Numerous prev ious studies have been incomplete in several respects: (1) they have n eglected to take into account the observed specific power flux contrib ution of radiation emitted from the underside of the disk and gravitat ionally lensed into the upper half-hemisphere, (2) they have considere d only a limited range of observing positions and hole spins, and (3) many have been restricted to examination of the steady state flux aris ing from homogeneous disks. The present study develops, within the con text of the optically thick, geometrically thin accretion disk model, a set of new calculational techniques based on an analysis of the alge braic :properties of the effective potential functions governing photo n propagation in the Kerr metric; ancillary techniques, such as that o f ''extended images,'' are introduced and employed to illustrate aspec ts of general relativistic image formation that affect the observed ti me-dependent flux arising from a thermally inhomogeneous accretion dis k. The contribution of the first-orbit disk images, including the effe cts of disk self-blocking, to the observed flux are fully taken into a ccount for the entire range of observing positions and hole spins for both the steady state and time-dependent cases. The procedure is illus trated by application of the results to the paradigm case of the Galac tic center black hole candidate Sagittarius A. Current observations a re somewhat contaminated because of poor angular resolution, making th is exercise still only an illustrative examination of the method. Howe ver, the future deployment of the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on HST should provide data with sufficient sens itivity for direct comparison with our calculated K-band fluxes and li ght curves. Of particular interest in this comparison is the expected reversal of the disk's angular momentum in Sgr A on a timescale of 10 yr or so. We discuss the distinct spectral signature of a retrograde disk versus that of a prograde configuration and demonstrate the feasi bility of observing this transition.