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arXiv · 2606.23498

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

Sinan Allak·Wei Yu·Lorenzo Ducci·Andrea Santangelo·Aysun Akyuz·Faruk Soydugan·Valery Suleimanov
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Abstract

Ultraluminous X-ray sources (ULXs) are among the best laboratories for studying super-Eddington accretion onto compact objects. We present a detailed spectral and timing analysis of the transient ULX NGC 4631 X-4 using archival Chandra, XMM-Newton, and Swift/XRT observations. The source exhibits pronounced spectral and flux variability on both short and long timescales, with luminosity variations exceeding two orders of magnitude. Its X-ray spectra are well described by absorbed multicolor disk blackbody and power-law models, with characteristic inner disk temperatures of 0.9-1.4 keV and photon indices of 2.0-2.4. The source does not follow the standard luminosity-temperature relation expected for a geometrically thin, optically thick accretion disk. No coherent pulsations or quasi-periodic oscillations are detected, while the short-term variability is dominated by aperiodic fluctuations and kilosecond peak-like structures, consistent with clumpy winds and geometric effects in a super-Eddington accretion flow. Overall, the spectral and timing properties support super-Eddington accretion onto a stellar-mass compact object, although the current data do not allow us to distinguish uniquely between a neutron star and a stellar-mass black hole accretor.

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Sinan Allak, Wei Yu, Lorenzo Ducci, Andrea Santangelo, Aysun Akyuz, Faruk Soydugan, Valery Suleimanov. 2026-06-22. Spectral and timing variability of the transient ultraluminous X-ray source NGC 4631 X-4. https://doi.org/10.1051/0004-6361%2F202660387

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