Modeling fast X-ray variability around an accreting black hole

TL;DR

This paper models the frequency-dependent X-ray time lags observed in black hole binaries by simulating the corona's geometry and accretion flow, successfully reproducing observed lag behaviors and evolution during outbursts.

Contribution

It introduces a Monte Carlo simulation approach that self-consistently reproduces both hard and soft X-ray lags and their evolution by varying corona geometry and disc properties.

Findings

Successfully reproduces low-frequency hard lags and high-frequency soft lags

Demonstrates lag evolution with changing corona size and disc viscosity

Highlights the importance of polarization measurements for geometry determination

Abstract

X-ray inter-band time lags are observed during the outbursts of black hole X-ray binaries (BHXRBs). Timing analysis of fast variability in low Fourier frequency bands shows that high-energy photons lag behind low-energy photons, a phenomenon referred to as hard lag. Conversely, in high Fourier frequency bands, low-energy photons lag behind high-energy photons, known as soft lag. This frequency-dependent lag spectrum suggests that the lags arise from different physical processes. Notably, a trend has been observed wherein the lags shift towards shorter timescales during the rising hard state, indicating an evolution in the inner accretion flow. In this study, we simulate these inter-band lags by conducting Monte Carlo simulations of the rapid variability within the geometry of a jet base corona. We consider both inward propagating accretion rate fluctuations and reverberation (light crossing) delays in our simulations. We successfully reproduce both low-frequency hard lags and high-frequency soft lags in a self-consistent manner. We replicate the observed evolution of the frequency-dependent lag spectra by varying the geometrical scale of the corona and the viscous frequency of the disc. Finally, we discuss the potential of a spherical corona and emphasize that polarization observations from the Imaging X-ray Polarimetry Explorer (IXPE) and the enhanced X-ray Timing and Polarimetry mission (eXTP) will be crucial for distinguishing the corona's geometry in future studies.