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The Hunt for Pulsating Ultraluminous X-ray Sources

Song, X.; Walton, D.J.; Lansbury, G.B.; Evans, P.A.; Fabian, A.C.; Earnshaw, H.; Roberts, T.P.

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X. Song

D.J. Walton

G.B. Lansbury

P.A. Evans

A.C. Fabian

H. Earnshaw


Motivated by the recent discoveries that six Ultraluminous X-ray Sources (ULXs) are powered by highly super-Eddington X-ray pulsars, we searched for additional pulsating ULX (PULX) candidates by identifying sources that exhibit long-term flux variability of at least an order of magnitude (a common feature seen in the 6 known PULXs, which may potentially be related to transitions to the propeller regime). Expanding on previous studies, we used the available fluxes from XMM-Newton, Swift and Chandra, along with carefully computed upper limits in cases of a non-detection, to construct long-term lightcurves for a sample of 296 ULXs selected from the XMM-Newton archive. Among these 296, we find 25 sources showing flux variability larger than a factor of 10, of which 17 show some evidence for (or are at least consistent with) exhibiting bi-modal flux distributions, as would be expected for sources undergoing propeller transitions. These sources are excellent candidates for continued monitoring programs to further test for this behaviour. There are 3 sources in our final sample with fluxes similar to NGC 5907 ULX1, currently the faintest known PULX, which would also be good targets for deeper observations with current facilities to search for pulsations. For the rest of the PULX candidates identified here, the next generation of X-ray telescopes (such as Athena) may be required to determine their nature owing to their lower peak fluxes.


Song, X., Walton, D., Lansbury, G., Evans, P., Fabian, A., Earnshaw, H., & Roberts, T. (2020). The Hunt for Pulsating Ultraluminous X-ray Sources. Monthly Notices of the Royal Astronomical Society, 491(1), 1260-1277.

Journal Article Type Article
Acceptance Date Oct 24, 2019
Online Publication Date Oct 31, 2019
Publication Date Jan 31, 2020
Deposit Date Oct 29, 2019
Publicly Available Date Nov 15, 2019
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 491
Issue 1
Pages 1260-1277
Related Public URLs


Accepted Journal Article (630 Kb)

Copyright Statement
This article has been accepted for publication in the Monthly notices of the Royal Astronomical Society ©: 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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