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
  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 four years of observations from the Swift
satellite. We find that the 115 day periodicity reported by Strohmayer (2009)
disappeared after only a few cycles, confirming the suspicion of Foster et al.
(2010) that the periodicity is most likely super-orbital 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.
