Very fast X-ray spectral variability in Cygnus X-1: Origin of the hard and soft-state emission components

TL;DR

This study analyzes rapid X-ray spectral variability in Cygnus X-1, revealing different correlations in soft and hard states that shed light on the emission mechanisms and accretion geometry.

Contribution

Introduces a new method to measure photon index variability on millisecond scales, providing insights into the emission processes in different spectral states.

Findings

Soft state shows positive zero-lag correlation, indicating Compton cooling by seed photons.

Hard state exhibits anti-correlation, suggesting variations in electron energy rather than seed photon flux.

Identification of a lag between soft and hard X-ray emissions linked to accretion flow dynamics.

Abstract

The way in which the X-ray photon index, {\Gamma}, varies as a function of count rate is a strong diagnostic of the emission processes and emission geometry around accreting compact objects. Here we present the results from a study using a new, and simple, method designed to improve sensitivity to the measurement of the variability of {\Gamma} on very short time-scales. We have measured {\Gamma} in ~2 million spectra, extracted from observations with a variety of different accretion rates and spectral states, on time-scales as short as 16 ms for the high mass X-ray binary Cygnus X-1, and have cross-correlated these measurements with the source count rate. In the soft-state cross-correlation functions (CCFs) we find a positive peak at zero lag, stronger and narrower in the softer observations. Assuming that the X-rays are produced by Compton scattering of soft seed photons by high energy electrons in a corona, these results are consistent with Compton cooling of the corona by seed photons from the inner edge of the accretion disc, the truncation radius of which increases with increasing hardness ratio. The CCFs produced from the hard-state observations, however, show an anti-correlation which is most easily explained by variation in the energy of the electrons in the corona rather than in variation of the seed photon flux. The hard-state CCFs can be decomposed into a narrow anti-correlation at zero lag, which we tentatively associate with the effects of self-Comptonisation of cyclo-synchrotron seed photons in either a hot, optically thin accretion flow or the base of the jet, and a second, asymmetric component which we suggest is produced as a consequence of a lag between the soft and hard X-ray emission. The lag may be caused by a radial temperature/energy gradient in the Comptonising electrons combined with the inward propagation of accretion rate perturbations.