The Effects of Complex Accretion Disk Geometry on Broadened Iron K$α$ Lines

Abstract

X-rays are emitted from the corona above the orbiting matter of the accretion disk and travel either directly to us or illuminate the disk. This illumination of the inner disk is enhanced by gravitational light bending, which focuses the rays towards the black hole and therefore towards the inner radii of the disk. These rays that hit the inner radii are reflected back to us, and we observe them in the X-ray reflection spectrum. In this work, we create novel general-relativistic ray-tracing simulations to investigate the effects of altering the geometry of the accretion disks of black holes on the most dominant part of the reflection spectrum, the iron K$α$ line. Work demonstrating the effect of disk geometry on the iron line has been performed, though many previous analyses have assumed a simplistic system, consisting of a point-source corona with a flat and infinitesimally thin accretion disk. We extend these models to more realistic accretion disk approximations. These include a constant-aspect-ratio disk, a radiation-pressure-dominated Shakura-Sunyaev disk, an expanded inner disk that has a nonnegligible scale height in its inner regions due to radiation pressure, as well as various warped-disk configurations. Using measurement uncertainties from XRISM, we find that nonnegligible thickness in accretion disks underestimates the black hole spin, coronal height, and inclination angle if fitted with a flat-disk model. The warped-disk model could not be fit with the flat-disk approximation.