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

TL;DR

This study presents a four-year Swift X-ray monitoring of ULX NGC 5408 X-1, revealing the disappearance of a 115-day periodicity, the presence of dips, and suggesting the periodicity is super-orbital rather than orbital.

Contribution

It provides the most extensive long-term X-ray data of a ULX, clarifying the nature of observed periodicities and dips, and linking ULX behavior to Galactic X-ray binaries.

Findings

115-day periodicity disappeared after a few cycles

Detected dipping behavior potentially related to super-orbital phenomena

Periodicities are likely super-orbital, not orbital, based on observations

Abstract

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.