A synchrotron self-Compton -- disk reprocessing model for optical/X-ray correlation in black hole X-ray binaries

TL;DR

This paper introduces a new model combining synchrotron emission and disk reprocessing to explain complex optical/X-ray correlations in black hole X-ray binaries, accounting for observed variability and cross-correlation features.

Contribution

The novel model explains optical/X-ray variability and correlations by combining synchrotron and reprocessed emission, addressing limitations of previous models.

Findings

The model reproduces observed cross-correlation functions.

It explains the optical power spectral density features.

Time lags between optical and X-ray emissions are accounted for.

Abstract

Physical picture of the emission mechanisms operating in the X-ray binaries was put under question by the simultaneous optical/X-ray observations with high time resolution. The light curves of the two energy bands appeared to be connected and the cross-correlation functions observed in three black hole binaries exhibited a complicated shape. They show a dip of the optical emission a few seconds before the X-ray peak and the optical flare just after the X-ray peak. This behavior could not be explained in terms of standard optical emission candidates (e.g., emission from the cold accretion disk or a jet). We propose a novel model, which explains the broadband optical to the X-ray spectra and the variability properties. We suggest that the optical emission consists of two components: synchrotron radiation from the non-thermal electrons in the hot accretion flow and the emission produced by reprocessing of the X-rays in the outer part of the accretion disk. The first component is anti-correlated with the X-rays, while the second one is correlated, but delayed and smeared relative to the X-rays. The interplay of the components explains the complex shape of the cross-correlation function, the features in the optical power spectral density as well as the time lags.