General relativistic effects in black hole X-ray coronal models

Abstract

Black holes are geometrical objects that generate extreme gravitational environments and curve their surrounding spacetimes. The general relativistic effects in these regions produce substantial departures from their classical counterparts in almost every conceivable way. Models of X-ray spectra and variability originating from these regions can be used together with observation to constrain parameters of the black hole and its accretion, in both active galactic nuclei and black hole X-ray binaries. This thesis describes a new general relativistic ray-tracing tool that can be used to model the effects of spacetime geometry in radiative processes around black holes. It is used to compute line profiles that encapsulate relativistic broadening of emission reflected from an accretion disc when illuminated by a hot X-ray corona, as well as the soft X-ray reverberation lag associated with the light travel time between the corona and the disc. Assumptions related to the accretion disc solution in such models are questioned, and simulations for a Shakura-Sunyaev accretion disc that accounts for the disc thickness are computed, with their occasionally substantial differences relative to the infinitely-thin disc discussed. Methods for computing such results from geometrically extended coronal sources are given, including an analysis of the reflection and reverberation predictions calculated for a ring-like, disc-like, and shell-like extended corona. A novel umbrella corona is constructed that is compatible with popular point-like coronal models, but opens opportunities for constraining other consequences of an extended model, including its compactness, the seed-photon flux, and the coupling of coronal and disc variability. Difficulties and opportunities associated with these findings are discussed. Finally, the effects of modifying the underlying black hole solutions themselves on the spectra are examined, with ramifications for electromagnetic tests of the no-hair theorem.

Date of Award	9 Dec 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Andrew J Young (Supervisor)