A long-term X-ray monitoring of the ultraluminous X-ray source NGC 5408 X-1 with Swift reveals the presence of dips but no orbital period

Grisé, F.; Kaaret, P.; Corbel, S.; Cseh, D.; Feng, H.
Bibliographical reference

Monthly Notices of the Royal Astronomical Society, Volume 433, Issue 2, p.1023-1038

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8
2013
Number of authors
5
IAC number of authors
1
Citations
38
Refereed citations
38
Description
NGC 5408 X-1 is a well-studied ultraluminous X-ray source (ULX) that has been seen to emit in X-rays persistently above the Eddington limit of a stellar mass black hole for years. In this paper we report on the most extensive X-ray monitoring of a ULX, using more than 4 yr of observations from the Swift satellite. We find that the 115 d periodicity reported by Strohmayer disappeared after only a few cycles, confirming the suspicion of Foster et al. that the periodicity is most likely superorbital and not the orbital period of the system. We also report on a clear dipping behaviour of the source that may be related to a (super)orbital phenomenon. All these features are reminiscent of Galactic X-ray binaries and strengthen their link with ULXs. Deeper observations of a dip could help resolve the ambiguity about the interpretation of the spectral components of ULXs.
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