Identification of three new ultraluminous X-ray sources in NGC 4631 and NGC 1097: Evidence for stellar-mass black holes

¹ Institut für Astronomie und Astrophysik Sand 1 72076 Tübingen, Germany
² Department of Physics, University of Çukurova 01330 Adana, Türkiye
³ Space Science and Solar Energy Research and Application Center (UZAYMER), University of Çukurova 01330 Adana, Türkiye

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 September 2025
Accepted: 12 January 2026

Abstract

Context. Recent observations of galaxies continue to reveal new ultraluminous X-ray sources (ULXs), thereby increasing their known population. This growing sample provides improved statistics that are essential for advancing our understanding of ULX properties and their nature.

Aims. Our objective was to study the source populations using published Chandra, XMM-Newton, and Swift/XRT observations to identify new ULXs. In particular, we focused on investigating their X-ray and optical properties to constrain the nature of the compact objects and donor stars.

Methods. We analyzed archival X-ray observations spanning 2000–2025 for NGC 4631 and NGC 1097 to study their ULX populations. We performed spectral fitting for sources with sufficient counts using absorbed power-law and diskbb models to determine their physical properties. We conducted variability analyses, including hardness-intensity diagrams and light curves, to assess both short- and long-term variability. Additionally, we used optical color-magnitude diagrams and near-infrared (NIR) spectral energy distributions (SEDs) to identify and characterize possible donor stars.

Results. In NGC 4631, we identify two new transient ULXs, X-6 and X-7, whose X-ray count rates vary by more than an order of magnitude. The L_X ∝ T⁴ relation obtained from the diskbb fits provides strong evidence that NGC 4631 X-6 is powered by a stellar-mass black hole (BH) accreting via a standard disk. The optical sources within the X-6 and X-7 X-ray error circles are candidate optical counterparts, suggesting that these systems are candidate high-mass X-ray binaries. In NGC 1097, we report the discovery of a new transient ULX, designated ULX-3, which exhibits variations in X-ray luminosity (L_X) by a factor of about 30. For ULX-3, we detect spectral state transitions, which may indicate a compact object consistent with either a stellar-mass BH or a neutron star. Moreover, we identify a unique optical and NIR counterpart: while the optical emission is variable, the NIR emission remains stable. The NIR counterpart of ULX-3 shows an SED consistent with a blackbody temperature of ∼3300 K, which lies within the expected range for red supergiants with a stellar radius of ∼200 R_⊙.