Swift-XRT 6-year monitoring of the ultraluminous X-ray source M33-X8

TL;DR

This study presents a 6-year Swift-XRT monitoring of the persistent ULX M33 X-8, revealing flux variability but stable spectral characteristics, and suggests the source's emission is consistent with thermal accretion models.

Contribution

First long-term soft X-ray monitoring of M33 X-8, providing insights into its spectral stability and flux variability over 16 years.

Findings

Flux varies by about 30% over the monitoring period.

Spectral shape remains consistent with thermal accretion models.

Average spectrum fits a slim disk or Comptonized corona plus disk model.

Abstract

The long term evolution of ULX with their spectral and luminosity variations in time give important clues on the nature of ULX and on the accretion process that powers them. We report here the results of a Swift-XRT 6-year monitoring campaign of the closest example of a persistent ULX, M33 X-8, that extends to 16 years the monitoring of this source in the soft X-rays. The luminosity of this source is a few 10^39 erg/s, marking the faint end of the ULX luminosity function. We analysed the set of 15 observations collected during the Swift monitoring. We searched for differences in the spectral parameters at different observing epochs, adopting several models commonly used to fit the X-ray spectra of ULX. The source exhibits flux variations of the order of 30%. No significant spectral variations are observed along the monitoring. The average 0.5-10 keV spectrum can be well described by a thermal model, either in the form of a slim disk, or as a combination of a Comptonized corona and a standard accretion disk.