General Relativistic Effects on the Spectrum Reflected by Accretion Disks around Black Holes

Abstract

Making the usual assumption that the relatively cold matter within the central engine of an active galactic nucleus (or galactic black hole candidate) is in the form of a relativistic accretion disk, we compute the composite spectrum of the original disk plus a primary X-ray power-law source illuminating it from above, as well as the reflected emission from the disk. All special and general relativistic effects on both infalling photons and outgoing photons are considered in a Schwarzschild geometry. The strength, shape, and broadening of the reflected spectrum depend on the direction of the X-ray source relative to the disk and the observer's viewing angle. The reflected photons extract energy and angular momentum from the relativistically rotating accretion disk and are beamed in the direction of the disk velocity. The reflection hump could essentially disappear if viewed far from the symmetry axis because the X-ray photons are affected by gravity both approaching and leaving the disk. This may produce a difference between X-ray spectra for Seyfert 1 and Seyfert 2 galaxies. For a given observation angle, the reflection hump is most sensitive to the inclination of the source relative to the accretion disk. Thus the spectral shape may also shed light on the location of the primary X-ray source, which is probably either in a jet or in a corona; however, additional computations involving distributed sources will be necessary before detailed comparisons with observations are feasible.