Spectral-Timing Models of the Central Engines in Low Luminosity Accreting Black Holes

Abstract

Emission from accreting black holes of all sizes is often highly variable, and strongly dependent on the energy band of observation. Particularly at accretion rates close to a few percent of the Eddington rate, multiple components become important in constructing the energy spectrum, and the signals can vary greatly on short timescales. For both X-ray binaries and Active Galactic Nuclei (AGN), the accretion geometry at low luminosities has become controversial, as spectral contributions from the thermal disc, relativistic reflection and Comptonization continua are often highly degenerate, with instrumental effects throwing doubt on the strongest pieces of evidence. However, with the latest generation of instruments it has become possible to analyse both the time-variability of distinct energy bands, and the correlations between these bands, combining information from the orthogonal energy and time domains. For the X-ray binaries, the Rossi X-ray Timing Explorer (RXTE) and XMM-Newton have facilitated observations at high energy resolution down to the millisecond timescale - close to the dynamical period in these sources. Meanwhile for Active Galactic Nuclei, a series of high-cadence, long timescale campaigns using the Hubble Space Telescope, the Neil Gehrels Swift Observatory and a multitude of ground-based telescopes has produced data on the broadband continuum shape of a growing sample of accreting sources with unprecedented timing quality. In this thesis I will present modeling of these spectral-timing data, with a view to constraining the physical mechanisms and size scales of the accretion regimes in both the X-ray binaries and AGN. By jointly modeling the energetic and timing aspects of the data, the models in this thesis will aim to provide a more self-consistent picture of the accretion structure at low luminosities, and rule out those pictures which cannot simultaneously explain the spectral and temporal information.