2003A&A...411..553B

Four XMM-Newton observations of the core of M31, spaced at 6 month intervals, show that the brightest point X-ray source, RX J0042.6+4115, has a 0.4-10keV luminosity of ∼5x10³⁸erg/s, and exhibits significant variability in intensity and X-ray spectrum over a time scale of ∼100s including hard flares; such behaviour is only observed in Z-sources and transient black hole binaries in our Galaxy. The lightcurves, X-ray spectra and hardness-intensity data from the four XMM-Newton observations all strongly suggest that it is a Z-source, bringing the total number of known Z-sources to nine.