Long-Term X-Ray/UV Variability in ULXs

TL;DR

This study analyzes long-term X-ray and UV variability in ultraluminous X-ray sources (ULXs) using Swift data, revealing weak or non-linear correlations and discussing possible physical mechanisms behind these observations.

Contribution

It provides the first extensive analysis of coupled X-ray and UV variability in ULXs with a large sample, highlighting the complexity of their emission mechanisms.

Findings

Some ULXs show weakly correlated UV and X-ray variability.

Other ULXs exhibit non-linear relationships between bands.

Higher quality data may be needed for definitive conclusions.

Abstract

The focus of NASA's Swift telescope has been transients and target-of-opportunity observing, resulting in many observations of ultraluminous X-ray sources (ULXs) over the last ~20 years. For the vast majority of these observations, simultaneous data has been obtained using both the X-ray telescope (XRT) and the ultraviolet and optical telescope (UVOT), providing a unique opportunity to study coupled variability between these bands. Using a sample of ~40 ULXs with numerous repeat observations, we extract stacked images to characterise the spatial extent of the UV-Optical emission and extract long-term light curves to search for first-order linear correlations between the UV and X-ray emission. We find that a small subset may show weakly correlated joint variability, while other sources appear to display non-linear relationships between the bands. We discuss these observations in the context of several theoretical models: precession, irradiation of the outer accretion disc and irradiation of the companion star. We conclude that more complicated analysis or higher quality data may be required to accurately constrain the nature of the joint X-ray and UV/optical emission in these sources.