Chandra Monitoring of a Population of Ultraluminous X-ray Sources

Ultraluminous X-ray pulsars (ULXPs) often demonstrate high levels of long-term variability and have been observed to exhibit either or both of a bimodal flux distribution which could be caused by the onset of the propeller effect, or a long-term (quasi-)periodicity of tens to hundreds of days which could be due to super-orbital precession of some part of the accreting system. While these individual sources of interest have undergone long-term monitoring, the long-term variability properties of the ULX population as a whole is less well understood, and the lack of such monitoring may cause us to miss other viable ULXP candidates. In order to explore the variability properties of ULXs on a population level, we used Chandra to monitor three ULX-rich galaxies: M51, NGC 253, and NGC 4485-90, with a total population of 23 ULXs. We found that the ULX population contains a range of variability behaviors, from persistent, to moderately variable, to highly variable sources. We used Bayesian Estimation of Hardness Ratios to calculate hardness ratios for all ULXs, and found that moderately-variable sources tend to exhibit correlations between their hardness and luminosity that suggest that their variability is driven by changes in mass accretion rate, whereas highly-variable sources do not demonstrate strong trends of hardness with luminosity and may be further examples of super-orbital periodicity.