Identification of three new ULXs in NGC 4631 and NGC 1097: evidence for stellar-mass black holes

TL;DR

This study identifies three new ULXs in NGC 4631 and NGC 1097 (X-6, X-7, and ULX-3) using archival Chandra, XMM-Newton, Swift/XRT, plus optical/NIR data from HST and JWST. Spectral fits favor disk-dominated, stellar-mass BH accretors for X-6 and X-7, with $L_{ m X}$ up to a few $\times10^{39}$ erg s$^{-1}$ and $T_{ m in}$ in the range $\sim$0.6–2 keV; X-6 shows $L_{ m X} \propto T_{ m in}^{\alpha}$ with $\alpha\approx3.68$, supporting a standard disk geometry, while X-7 indicates a slim-disk regime. ULX-3 exhibits strong long-term variability ($L_{ m X}$ from $3\times10^{39}$ to $8\times10^{40}$ erg s$^{-1}$) and state transitions consistent with a stellar-mass BH or NS in a super-Eddington accretion regime, with a unique optical/NIR counterpart whose SED is a $\sim3300$ K blackbody; the NIR color-magnitude and SED analysis points to a red supergiant donor with $\sim200\,R_{\odot}$. Optical counterparts for X-6/X-7 suggest HMXB-like donors of $\sim7$–$9\,M_{\odot}$ and ages of 30–40 Myr. Altogether, the results support a heterogeneous ULX population in nearby galaxies, spanning standard thin-disk accretion by stellar-mass BHs to super-Eddington regimes with diverse donor stars, and demonstrate the value of joint X-ray and multiwavelength analyses for unveiling ULX natures.

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.

Figures (12)

• Figure 1: X-ray color stacked Chandra images of the galaxies NGC 4631 (left) and NGC 1097 (right). The energy ranges used for the red, green, and blue colors in both images are 0.5-1.2, 1.2-2, and 2-8 keV, respectively. The positions of the ULXs are indicated with solid white circles.
• Figure 2: Chandra energy spectra of the NGC 4631 ULXs X-6 and X-7 (top two panels) and XMM-Newton (MOS) energy spectra (bottom two panels) with residuals shown as $R = (\text{data} - \text{model})/\text{error}$. In all panels, solid blue lines represent diskbb model.
• Figure 3: Energy spectra for NGC 1097 ULX-3 obtained from XMM-Newton observations. Blue and red plus symbols represent MOS1 and MOS2 data, respectively. The power-law + blackbody model fits are shown with green and red lines. All errors are at the 90% confidence level.
• Figure 4: Timing analysis of NGC 4631 ULX X-6 from the C2 observation. (a) Background-subtracted light curve in the 0.3--10 keV band with a bin size of 500 s. (b) Lomb--Scargle periodogram showing a peak at P = 5120 s ($\sim$ 1.42 h) at 2.6$\sigma$ confidence level. (c) Folded light curve at P = 5120 over two cycles, normalized to the mean count rate.
• Figure 5: The left panel shows the hardness ratios versus time, and the right panel presents the hardness–intensity diagram of ULXs X–6 and X–7. X–6 and X–7 are represented by filled circles and stars, respectively. Green and blue colors show the medium/soft and hard/medium ratios, respectively. All error bars represent the 1$\sigma$ level.