A Fourier Transformed Bremsstrahlung Flash Model for the Production of X-Ray Time Lags in Accreting Black Hole Sources

TL;DR

This paper introduces a new analytical model based on Fourier transforms to explain X-ray time lags in accreting black hole sources, successfully matching observations and providing insights into the physical processes involved.

Contribution

The authors develop an exact analytical Fourier transform solution for photon diffusion and Comptonization in a spherical corona, enabling detailed modeling of X-ray time lags.

Findings

Thermal Comptonization explains both X-ray time lags and spectra in Cyg X-1.

Bremsstrahlung seed photons produce frequency-dependent time lags.

Model matches observed lag behavior across various sources.

Abstract

Accreting black hole sources show a wide variety of rapid time variability, including the manifestation of time lags during X-ray transients, in which a delay (phase shift) is observed between the Fourier components of the hard and soft spectra. Despite a large body of observational evidence for time lags, no fundamental physical explanation for the origin of this phenomenon has been presented. We develop a new theoretical model for the production of X-ray time lags based on an exact analytical solution for the Fourier transform describing the diffusion and Comptonization of seed photons propagating through a spherical corona. The resulting Green's function can be convolved with any source distribution to compute the associated Fourier transform and time lags, hence allowing us to explore a wide variety of injection scenarios. We show that thermal Comptonization is able to self-consistently explain both the X-ray time lags and the steady-state (quiescent) X-ray spectrum observed in the low-hard state of Cyg X-1. The reprocessing of bremsstrahlung seed photons produces X-ray time lags that diminish with increasing Fourier frequency, in agreement with the observations for a wide range of sources.