Spectral and timing variability of the transient ultraluminous X-ray source NGC 4631 X–4

Context. Ultraluminous X-ray sources (ULXs) are accreting compact objects exhibiting luminosities above the Eddington limit of stellar-mass black holes and are key laboratories for studying super-Eddington accretion physics. Aims. We aim to investigate the spectral and timing properties of the transient ULX NGC 4631 X–4 to constrain the nature of the accretion flow and the compact object. Methods. We analyzed archival Chandra , XMM-Newton , and Swift /XRT observations, focusing on long- and short-term variability, spectral evolution, and timing behavior. Spectral fitting was performed using standard absorbed models, while timing analysis was performed to search for variability and periodic or quasi-periodic signals using standard statistical methods. Results. X–4 exhibits strong spectral and flux variability on both short and long timescales, with luminosity changes exceeding two orders of magnitude. The X-ray spectra are adequately described by absorbed multicolored disk-blackbody ( diskbb ) and power-law models, with characteristic inner-disk temperatures of kT in ∼ 0.9–1.4 keV and relatively steep power-law photon indices of Γ ∼ 2–2.4. The source does not follow the standard L ∝ T 4 relation expected for a thin accretion disk. No coherent pulsations or quasi-periodic oscillations are detected. Short-term variability is dominated by aperiodic fluctuations and kilosecond peak-like structures, possibly associated with clumpy winds and geometric effects in a super-Eddington accretion flow. Conclusions. The spectral and timing properties of X–4 are consistent with super-Eddington accretion onto a stellar-mass compact object. The observed variability and spectral evolution can be explained by a combination of a variable accretion rate and a radiatively driven, optically thick wind. The nature of the compact object cannot be determined uniquely from the present data; both neutron-star and stellar-mass black-hole accretors remain consistent with the observed properties.