Quasi-periodic dipping in the ultraluminous X-ray source, NGC 247 ULX-1

Abstract

Most ultraluminous X-ray sources (ULXs) are believed to be stellar mass black holes or neutron stars accreting beyond the Eddington limit. Determining the nature of the compact object and the accretion mode from broadband spectroscopy is currently a challenge, but the observed timing properties provide insight into the compact object and details of the geometry and accretion processes. Here we report a timing analysis for an 800 ks XMM-Newton campaign on the supersoft ultraluminous X-ray source, NGC 247 ULX-1. Deep and frequent dips occur in the X-ray light curve, with the amplitude increasing with increasing energy band. Power spectra and coherence analysis reveals the dipping preferentially occurs on $\sim 5$ ks and $\sim 10$ ks timescales. The dips can be caused by either the occultation of the central X-ray source by an optically thick structure, such as warping of the accretion disc, or from obscuration by a wind launched from the accretion disc, or both. This behaviour supports the idea that supersoft ULXs are viewed close to edge-on to the accretion disc.

Accepted MNRAS version following minor comments. 8 pages, 6 figures

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• Ultraluminous X-ray sources
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• The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1
• Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1
• Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925
• Propeller states in locally super-critical ULXs
• Identification of a Helium Donor Star in NGC 247 ULX-1
• On the nature of the ultraluminous X-ray source Holmberg II X-1
• Winds in ultraluminous X-ray sources: new challenges
• A transient ultraluminous X-ray source in NGC 55
• The recent star formation history of NGC 628 on resolved scales
• X-ray reverberation models of the disc wind in ultraluminous X-ray source NGC 5408 X-1
• XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS
• Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1