General Relativistic Effects on the Infrared Spectrum of Thin Accretion Disks in Active Galactic Nuclei: Application to Sagittarius A*

Abstract

The possibility that some portion of the infrared (IR) radiation emanating from active galactic nuclei (AGNs) may arise from disklike structures of ionized plasma accreting onto massive black holes motivates the investigation of the effects of strong gravitational fields in the vicinity of emitting particles on the observed radiation. Numerous previous studies have been incomplete in several respects: (1) they have neglected to take into account the observed specific power flux contribution of radiation emitted from the underside of the disk and gravitationally lensed into the upper half-hemisphere, (2) they have considered only a limited range of observing positions and hole spins, and (3) many have been restricted to examination of the steady state flux arising from homogeneous disks. The present study develops, within the context of the optically thick, geometrically thin accretion disk model, a set of new calculational techniques based on an analysis of the algebraic properties of the effective potential functions governing photon propagation in the Kerr metric; ancillary techniques, such as that of "extended images," are introduced and employed to illustrate aspects of general relativistic image formation that affect the observed time-dependent flux arising from a thermally inhomogeneous accretion disk. The contribution of the first-orbit disk images, including the effects of disk self-blocking, to the observed flux are fully taken into account for the entire range of observing positions and hole spins for both the steady state and time-dependent cases. The procedure is illustrated by application of the results to the paradigm case of the Galactic center black hole candidate Sagittarius A*. Current observations are somewhat contaminated because of poor angular resolution, making this exercise still only an illustrative examination of the method. However, the future deployment of the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on HST should provide data with sufficient sensitivity for direct comparison with our calculated K-band fluxes and light 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 feasibility of observing this transition.