Work

Probing spectral variability in NGC 4490 ULX-8 over 24 yr of XMM–Newton, Chandra , and Swift -XRT observations

Year: 2026

Type: article

Abstract: ABSTRACT We present a spectral variability study of the ultraluminous X-ray source NGC 4490 ULX-8 based on 14 Chandra, 6 XMM–Newton, and 19 Swift-XRT observations obtained between 2000 and 2024. The X-ray spectra are modelled using absorbed power-law and absorbed multicolour disc blackbody models. The best-fitting photon indices span $\Gamma \simeq 0.9$–2.7, while the inferred inner disc temperatures lie in the range $kT_{\mathrm{in}} \simeq 1.0$–1.6 keV. We detect pronounced long-term variability in the unabsorbed X-ray luminosity on multiyear time-scales, while variability within individual observations is comparatively modest. A hardness–intensity Diagram of the source shows no clear transition between hard and soft states; however, two recent observations taken on 2022 December 1 and 2024 May 4 show a sharp increase in brightness. The spectra across all observations are dominated by smooth, single-component curvature in the 0.3–10 keV band, consistent with the broadened-disc regime of ultraluminous X-ray sources. A correlation analysis reveals a weak positive $L_{\mathrm{X}}$–$\Gamma$ trend that remains statistically supported after controlling for absorption-related degeneracies, indicating that it is not driven solely by fitting covariance. The $L_{\mathrm{X}}$–$T_{\mathrm{in}}$ relation is only weakly constrained, but remains compatible, within uncertainties, with both thin-disc and slim-disc scalings. Using disc parameters derived from higher quality XMM–Newton spectra, we obtain model-dependent estimates of the characteristic inner disc radius and compact-object mass as functions of inclination and spin. The reported results are consistent with a stellar-mass black hole accretor operating at or near the Eddington limit.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag976

Authors: Tarang Vashisht, Aru Beri, Tanuman Ghosh, Aman Upadhyay, Vikram Rana

Institutions: Indian Institute of Science Education and Research Mohali, Indian Institute of Astrophysics, University of Southampton, Inter-University Centre for Astronomy and Astrophysics, Raman Research Institute

Language: English

Topic: Astrophysical Phenomena and Observations

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $3,046

Funders: National Aeronautics and Space Administration, Science and Engineering Research Board