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

TL;DR

This study uses advanced relativistic simulations to explore how complex accretion disk geometries influence the iron Kα line in X-ray spectra, revealing significant biases when simplified models are used.

Contribution

It introduces realistic accretion disk models into relativistic ray tracing simulations to assess their impact on X-ray reflection spectra.

Findings

Thick disks lead to underestimated black hole spin, corona height, and inclination when using flat disk models.

Warped disks cannot be accurately modeled with flat disk assumptions.

Realistic disk geometries significantly affect the interpretation of X-ray reflection spectra.

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$\alpha$ 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 non-negligible scale height in its inner regions due to radiation pressure, as well as various warped disks. Using measurement uncertainties from XRISM, we find that non-negligible thickness in accretion disks underestimates the black hole spin, corona height, and inclination angle if fitted with a flat disk model. The warped disk model could not be fit with the flat disk approximation.