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

TL;DR

This study analyzes the timing properties of NGC 247 ULX-1, revealing quasi-periodic dips in X-ray emission that suggest a near edge-on viewing angle and complex accretion structures.

Contribution

It provides the first detailed timing analysis of NGC 247 ULX-1, identifying characteristic dipping timescales and proposing possible physical causes.

Findings

Deep, energy-dependent dips occur on ~5 ks and ~10 ks timescales.

Dips may be caused by disc warping or wind obscuration.

Supports the view that supersoft ULXs are observed edge-on.

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.