Monitoring the very-long-term variability of X-ray sources in the giant elliptical galaxy M87

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 characterize the long-term variability of eight of the brightest members of the XRB population in M87. The peak brightness of some of the sources exceeded the ultraluminous X-ray source (ULXs) threshold luminosity of ∼10³⁹ erg s^-1, 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 d. 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.