# X-ray and optical orbital modulation of EXO 0748-676: A co-variability study using XMM-Newton

## TL;DR

This study analyzes simultaneous X-ray and optical observations of the binary EXO 0748-676, revealing weak correlations and disk structural changes affecting emission variability on hour-long timescales.

## Contribution

It provides the first detailed multi-wavelength timing analysis of EXO 0748-676 in a hard spectral state, highlighting accretion disk evolution and reprocessing effects.

## Key findings

- Optical and X-ray light curves are weakly correlated at few 1000s timescales.
- Large variability in light curves suggests dynamic accretion disk structures.
- Evidence of accretion disk evolution occurring over a few hours.

## Abstract

We present a multi-wavelength timing study of the eclipsing low mass X-ray binary EXO 0748-676 (UY Vol) using XMM-Newton when the source was in a hard spectral state. The orbital optical and X-ray light curves show a fairly large amount of intensity modulation in the 7 observations taken during September-November, 2003, covering 36 complete binary orbits of EXO 0748-676. While assessing the non-burst variability, simultaneously in the optical and X-ray light curves, we find that they are not correlated at reprocessing or orbital time-scales, but are weakly correlated at a few 1000s of seconds time-scales. Although a large fraction of the optical emission is likely to be due to reprocessing, the lack of significant correlation and presence of large variability in the orbital X-ray and optical light curves is probably due to structures and structural changes in the accretion disk that produce, and sometimes mask the reprocessed signal in varying amounts. These disk structures could be induced, at least partly, by irradiation. From the observed modulations seen in the optical light curves, there is strong evidence of accretion disk evolution at time scales of a few hours.