X-ray quasi-periodic oscillations in Lense--Thirring precession model - I. variability of relativistic continuum

TL;DR

This paper models X-ray variability in accreting black holes using Lense-Thirring precession, accounting for relativistic effects and spectral state transitions, to explain observed quasi-periodic oscillations and energy-dependent variability.

Contribution

It introduces a Monte-Carlo simulation framework that combines relativistic effects with precession-induced variability in a comprehensive spectral model.

Findings

Fractional variability is low (~10%) in the hard state.

Variability increases with photon energy in the soft state.

Reflection variability depends on black hole spin, being higher for low spin.

Abstract

We develop a Monte-Carlo code to compute the Compton scattered X-ray flux arising from a hot inner flow which undergoes Lense-Thirring precession. The hot flow intercepts seed photons from an outer truncated thin disk. A fraction of the Comptonized photons will illuminate back the disk and the reflected/reprocessed photons will contribute to the observed spectrum. The total spectrum, including disk thermal emission, hot flow Comptonization, and disk reflection, is modelled within the framework of general relativity, taking light-bending and gravitational redshift into account. The simulations are performed in the context of the Lense-Thirring precession model for the low-frequency quasi-periodic oscillations, so the inner flow is assumed to precess, leading to periodic modulation of the emitted radiation. In this work, we concentrate on the energy-dependent X-ray variability of the model and, in particular, on the evolution of the variability during the spectral transition from hard to soft state, which is implemented by the decrease of the truncation radius of the outer disk towards Innermost Stable Circular Orbit (ISCO). In the hard state where the Comptonizing flow is geometrically thick, the Comptonization is weakly variable with the fractional variability amplitude of $\leq$10\%; in the soft state where the Comptonizing flow is cooled down and thus becomes geometrically thin, and the fractional variability of the Comptonization is highly variable, increasing with photon energy. The fractional variability of the reflection increases with energy, and the reflection emission for low spin is counterintuitively more variable than the one for high spin.