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On the artificial nature of aperiodic variability in XMM-Newton observations of M 31 X-ray sources and the ultraluminous X-ray source NGC 4559 ULX-7

Barnard, R.; Trudolyubov, S.; Kolb, U. C.; Haswell, C. A.; Osborne, J. P. and Priedhorsky, W. C. (2007). On the artificial nature of aperiodic variability in XMM-Newton observations of M 31 X-ray sources and the ultraluminous X-ray source NGC 4559 ULX-7. Astronomy and Astrophysics, 469 pp. 875–879.

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Context:Power density spectra (PDS) that are characteristic of low mass X-ray binaries (LMXBs) have been previously reported for M 31 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 M 31 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 M 31 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.

Item Type: Journal Article
ISSN: 1432-0746
Project Funding Details:
Funded Project NameProject IDFunding Body
Not SetNot SetParticle Physics and Astronomy Research Council (PPARC)
Not SetNot SetNASA [grant NAG5-12390]
Extra Information: Article published by EDP Sciences and available at or
Keywords: X-rays: binaries � galaxies: individual: M 31 � galaxies: individual: NGC 4559 � galaxies: individual: NGC 5408 � X-rays: general
Academic Unit/Department: Science > Physical Sciences
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 10856
Depositing User: Astrid Peterkin
Date Deposited: 23 Jun 2008
Last Modified: 08 Mar 2014 01:32
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