Pulsations and constraints on time lags in two distinct ULXs: NGC 5204 X-1 and NGC 55 ULX-1

Ultra-luminous X-ray sources (ULXs) remain mysterious because they are believed to be point sources with luminosities above the classical Eddington limit for a 10-solar-mass black hole, though they generally fit into the classification tree of super-Eddington X-ray binaries. Studies of reverberation lags and lag-energy spectra can provide insight into viscous turbulence in optically thick, super-Eddington discs, and are useful alongside modelling of multi-wavelength spectra and covariance spectra.

This study focuses on the timing properties of two classically distinct ULXs: NGC 5204 X-1 and NGC 55 ULX-1. Both are known to emit relativistic winds, as inferred from modelling emission-line spectra with photo-ionisation models such as SPEX, but they are thought to be viewed at slightly different inclinations because of differences in the hardness ratio of their X-ray continua. The authors perform an extensive timing study of both sources to derive new constraints on the accretion disc size scale and structure.

They also carry out an accelerated search for pulsations, including a correction for orbital modulation of the signal. This search reveals significant spin periodicities in both NGC 5204 X-1 and NGC 55 ULX-1 at greater than 3σ significance, offering new observational clues to the accretion and spin properties of these super-Eddington systems.