Long-term X-ray Variability of Ultraluminous X-ray Sources

TL;DR

This study systematically analyzes long-term X-ray variability in four ultraluminous X-ray sources, revealing quasi-periodicities and suggesting that observed patterns may be related to superorbital periods and accretion disc structures.

Contribution

First systematic analysis of long-term X-ray variability in ULXs using Swift data, identifying quasi-periodicities and proposing precession as a cause for outburst recurrence time variations.

Findings

Detection of low-frequency quasi-periodicities in ULX light curves

Outburst recurrence time of ESO 243-49 HLX-1 varies over time

Possible link between long-term patterns and superorbital periods

Abstract

Long-term X-ray modulations on timescales from tens to hundreds of days have been widely studied for X-ray binaries located in the Milky Way and the Magellanic Clouds. For other nearby galaxies, only the most luminous X-ray sources can be monitored with dedicated observations. We here present the first systematic study of long-term X-ray variability of four ultraluminous X-ray sources (ESO 243-49 HLX-1, Holmberg IX X-1, M81 X-6, and NGC 5408 X-1) monitored with Swift. By using various dynamic techniques to analyse their light curves, we find several interesting low-frequency quasi-periodicities. Although the periodic signals may not represent any stable orbital modulations, these detections reveal that such long-term regular patterns may be related to superorbital periods and structure of the accretion discs. In particular, we show that the outburst recurrence time of ESO 243-49 HLX-1 varies over time and suggest that it may not be the orbital period. Instead, it may be due to some kinds of precession, and the true binary period is expected to be much shorter.