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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
journal contribution
posted on 2012-10-24, 09:06 authored by R. Barnard, U. C. Kolb, C. A. Haswell, S. Trudolyubov, J. P. Osborne, W. C. PriedhorskyContext.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.
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Astronomy & Astrophysics, 2007, 469 (3), pp. 875-879Version
- VoR (Version of Record)
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Astronomy & AstrophysicsPublisher
EDP Sciences for European Southern Observatory (ESO)issn
0004-6361eissn
1432-0746Copyright date
2007Available date
2012-10-24Publisher DOI
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http://www.aanda.org/articles/aa/abs/2007/27/aa6333-06/aa6333-06.htmlLanguage
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