  Context: Power density spectra (PDS) that are characteristic of low mass
X-ray binaries (LMXBs) have been previously reported for M31 X-ray sources,
observed by XMM-Newton. However, we have recently discovered that these PDS
result from the improper addition/subtraction of non-simultaneous lightcurves.
Aims: To understand the properties and origins of the artefact. Methods: We
re-analysed our XMM-Newton observations of M31 with non-simultaneous and
simultaneous lightcurves, then combined simulated lightcurves at various
intensities with various offsets and found that the artefact is more dependent
on the offset than the intensity. Results: The lightcurves produced by the
XMM-Newton Science Analysis Software (SAS) are non-synchronised by default.
This affects not only the combination of lightcurves from the three EPIC
detectors (MOS1, MOS2 and pn), but also background subtraction in the same CCD.
It is therefore imperative that all SAS-generated lightcurves are synchronised
by time filtering, even if the whole observation is to be used. We also find
that the reported timing behaviour for NGC 4559 ULX-7 was also contaminated by
the artefact; there is no significant variability in the correctly-combined
lightcurves of NGC 4559 ULX-7. Hence, the classification of this source as an
intermediate-mass black hole is no longer justified. Conclusions: While
previous timing results from M31 have been proven wrong, and also the broken
power law PDS in NGC 4559 ULX-7, XMM-Newton was able to detect aperiodic
variability in just 3 ks of observations of NGC 5408 ULX1. Hence XMM-Newton
remains a viable tool for analysing variability in extra-galactic X-ray
sources.
