Spectral and timing variability of the transient ultraluminous X-ray source NGC 4631 X-4
ASTRONOMY & ASTROPHYSICS, cilt.712, 2026 (SCI-Expanded, Scopus)
- Yayın Türü: Makale / Tam Makale
- Cilt numarası: 712
- Basım Tarihi: 2026
- Doi Numarası: 10.1051/0004-6361/202660387
- Dergi Adı: ASTRONOMY & ASTROPHYSICS
- Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Artic & Antarctic Regions, Compendex, INSPEC, zbMATH, Directory of Open Access Journals, Academic Search Ultimate (EBSCO)
- Çanakkale Onsekiz Mart Üniversitesi Adresli: Evet
Özet
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) similar to 0.9-1.4 keV and relatively steep power-law photon indices of Gamma similar to 2-2.4. The source does not follow the standard L proportional to 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.