Monitoring the Very-Long-Term Variability of X-ray Sources in the Giant Elliptical Galaxy M87

TL;DR

This study investigates long-term X-ray variability in M87's X-ray binaries using archival Chandra data, highlighting the need for dedicated monitoring to understand their modulation behaviors.

Contribution

It provides the first detailed analysis of very-long-term X-ray variability in M87's XRBs and discusses the limitations of current data for period detection.

Findings

Some sources exceed the ULX luminosity threshold.

Period detection is limited to periods less than 120 days.

Dedicated monitoring is necessary for understanding long-term modulations.

Abstract

We report on our search for very-long-term variability (weeks to years) in X-ray binaries (XRBs) in the giant elliptical galaxy M87. We have used archival Chandra imaging observations to characterise the long-term variability of 8 of the brightest members of the XRB population in M87. The peak brightness of some of the sources exceeded the ultra luminous X-ray source (ULX) threshold luminosity of ~ 10^{39} erg/s, and one source could exhibit dips or eclipses. We show that for one source, if it has similar modulation amplitude as in SS433, then period recoverability analysis on the current data would detect periodic modulations, but only for a narrow range of periods less than 120 days. We conclude that a dedicated monitoring campaign, with appropriately defined sampling, is essential if we are to investigate properly the nature of the long-term modulations such as those seen in Galactic sources.