Long-term X-ray/UV variability in ULXs

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 & SIM;20 yr. For the vast majority of these observations, simultaneous data have 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 & SIM;40 ULXs with numerous repeat observations, we extract stacked images to characterize 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.

Automated summary

The NASA Swift telescope has been observing transients and target-of-opportunity with a focus on ULXs for the last 20 years. Simultaneous data from the XRT and UVOT have been obtained for the majority of these observations, allowing for the study of coupled variability. Through a sample of 40 ULXs with repeat observations, stacked images are extracted to analyze the spatial extent of UV/optical emission and long-term light curves to investigate correlations between UV and X-ray emission. The results suggest weakly correlated joint variability for a subset of ULXs and nonlinear relationships for other sources, requiring further analysis or higher quality data to accurately understand the nature of the emission.