Work

Detection of kilosecond hard lags in the new pulsating ULX candidate NGC 7456 ULX-1

Year: 2026

Type: article

Abstract: Context. Ultraluminous X-ray sources (ULXs) are thought to be powered, in many cases, by super-Eddington accretion onto compact objects. While soft X-ray lags, commonly interpreted as reverberation in optically thick outflows, have been detected in several ULXs, hard lags remain rare and poorly understood. Aims. We investigate the temporal and energy dependence of X-ray lags in NGC 7456 ULX-1 to constrain their physical origin and probe the structure of the super-Eddington accretion flow. Methods. We analysed the two deepest XMM-Newton observations of NGC 7456 ULX-1, taken in 2018 and 2023. Hard (1–10 keV) light curves were cross correlated with soft (0.3–1 keV) light curves using adaptive-binning techniques optimized for Poissonian low-count data. Time-resolved lag measurements were performed over consecutive 10 ks intervals, and the energy dependence of the delays was investigated. Broadband spectra were modelled using multi-component thermal and Comptonization models. Results. We detect significant hard X-ray lags in both observations, with the 1–10 keV emission delayed by ∼10 3 s relative to the soft band during phases of rapid flux variability. The energy-resolved analysis shows that the delays are primarily driven by the lowest-energy photons. Spectral modelling indicates a Comptonization-dominated super-Eddington flow comprising a cooler, extended outer region and a hotter, compact inner flow embedded in an optically thick wind. We interpret the observed delays as the combined effect of inward propagation of accretion-rate fluctuations and photon diffusion within the dense outflow. In this scenario, fluctuations first enhance the soft-emitting outer regions and subsequently propagate towards the hotter inner flow, where photons undergo stronger Comptonization before escaping with a kilosecond delay. The relatively small inferred inner emitting radius disfavours an intermediate-mass black hole accretor, while the observed delays imply diffusion through an extended optically thick wind. Conclusions. The sign, amplitude, and energy dependence of the observed delays disfavour a standard reverberation scenario. Instead, propagation-driven variability coupled with radiative transfer in optically thick winds appears to play a major role in shaping the timing properties of super-Eddington accretion flows.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202661582