Speaker
Description
We present a combined spectral and timing analysis of NGC 7456 ULX-1 based on the two deepest available XMM-Newton observations, obtained in 2018 and 2023. Using adaptive-binning cross-correlation techniques optimized for the low-count regime, we detect significant hard X-ray lags of order $\sim10^{3}$ s, with the 1--10 keV emission delayed with respect to the 0.3--1 keV band in both observations. The most significant lag episodes occur during intervals characterized by pronounced variations in the count-rate derivative, suggesting a link between the measured delays and rapid changes in the accretion flow. Energy-resolved timing further shows that the delay is mainly driven by the lowest-energy soft photons, with the 1--10 keV band lagging the $\sim$0.3--0.6 keV emission.
The broadband spectra are well described by a Comptonization-dominated model in which seed photons originate from two thermal regions: a cooler, extended component associated with the outer flow or wind photosphere, and a hotter, compact component linked to the inner accretion region. The source shows strong short-term flux variability but only weak spectral variability, a behaviour that can be naturally explained if the observed emission is largely reprocessed in a warm, optically thick medium, where multiple Compton/Thomson scatterings smear out intrinsic spectral changes. Combining timing and spectral information allows us to constrain the geometry and physical conditions of the system, including the characteristic size of the emitting regions, wind densities of order $\rho_{\rm wind}\sim10^{-5}$--$10^{-4}$ g cm$^{-3}$, and an inner characteristic radius consistent with a neutron-star accretor.
The sign, amplitude, temporal behaviour, and energy dependence of the lags disfavour a standard reverberation origin. Instead, they point to inward propagation of mass-accretion-rate fluctuations coupled with photon diffusion and Compton reprocessing in a dense super-Eddington wind. These results show that the combination of spectral modelling and advanced timing techniques can provide powerful constraints on the structure, radiative transfer, and variability mechanisms of super-Eddington accretion flows.