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

TL;DR

This study discovers three new ultraluminous X-ray sources in NGC 4631 and NGC 1097, providing evidence that some are stellar-mass black holes, and analyzes their X-ray and optical properties to understand their nature.

Contribution

The paper reports the identification of three new ULXs and presents detailed multiwavelength analysis, including spectral fitting and donor star identification, advancing understanding of ULX origins.

Findings

NGC 4631 X-6 is powered by a stellar-mass black hole.

NGC 1097 ULX-3 shows spectral state transitions.

Optical and NIR counterparts suggest red supergiant donors.

Abstract

Recent observations of galaxies continue to reveal new ultraluminous X-ray sources (ULXs), increasing their known population and improving the statistics needed to understand their nature. We study the ULX populations of NGC 4631 and NGC 1097 using archival Chandra, XMM-Newton, and Swift/XRT observations to identify new ULXs and investigate their X-ray and optical properties. We analyzed X-ray data spanning 2000-2025. Spectral fitting was performed for sources with sufficient counts using absorbed power-law and diskbb models. Variability analyses, including hardness-intensity diagrams and light curves, were used to assess short- and long-term variability. Optical color-magnitude diagrams and near-infrared (NIR) spectral energy distributions (SEDs) were employed to identify possible donor stars. In NGC 4631, we identified two new transient ULXs, X-6 and X-7, with X-ray count rates varying by more than an order of magnitude. The LX-T^4 relation from diskbb fits provides strong evidence that NGC 4631 X-6 is powered by a stellar-mass black hole accreting via a standard disk. The optical sources within the X-ray error circles of X-6 and X-7 are candidate counterparts, suggesting these systems are candidate HMXBs. In NGC 1097, we discovered a new transient ULX, ULX-3, showing X-ray luminosity variations by a factor of about 30 and evidence for spectral state transitions, consistent with either a stellar-mass black hole or a neutron star. We identified a unique optical and NIR counterpart. While the optical emission is variable, the NIR emission is stable. The NIR SED is consistent with a blackbody temperature of about 3300 K, compatible with a red supergiant donor with a radius of about 200 solar radii.