Spectral study of the X-ray sources in NGC 4472

Abstract

In this study, we investigated the spectral properties of the X-ray point sources in the giant elliptical galaxy NGC 4472 (M49), located at a distance of $\sim$16.7 Mpc in the equatorial constellation of Virgo. Utilizing all available Chandra data observed using the ACIS-S detector from the year 2000 to the year 2021, we identified a total of 57 X-ray point sources, each with data counts $\gtrsim$100. From the spectral study, we found 8 ultraluminous X-ray sources (ULXs) and 49 X-ray binaries (XRBs). Among the ULXs, source X-1 was found to be an extremely luminous X-ray source (ELXs) with bolometric luminosity, $L_x \sim 10^{40}$ erg s$^{-1}$, probably accreting at a super-Eddington rate. We also detected a super-soft source, X-3, accreting at $\sim$0.05 times the Eddington limit, with a disk temperature of $KT_{in} \sim 0.25^{+0.06}_{-0.05}$ keV. Assuming a disk blackbody model, the estimated black hole mass is ${\sim } 5.57^{+8.89}_{-3.10} \times 10^2 \ M_{\odot }$, placing it as a possible intermediate-mass black hole (IMBH) candidate. Source X-4 showed notable variability in luminosity, which closely correlated with changes in its inner disk temperature. Its luminosity increased as the disk temperature rose, and decreased as the temperature dropped. A similar trend was also observed in source X-14, although with comparatively less variation in luminosity. Most of the other ULXs in this study remained in a hard spectral state with consistent luminosity across observations, with the exception of source X-34, which displayed spectral variability, and source X-50, which was observed in a soft thermal state. Further, the majority of the detected X-ray binaries were found in a hard spectral state. While four of the XRBs exhibited a soft state and another four showed spectral transitions, their luminosities remained relatively stable. A color–color analysis of the X-ray sources revealed that both XRBs and ULXs generally reside within the low-mass X-ray binary (LMXB) region, albeit with enhanced spectral hardness. Notably, source X-1 displayed even greater hardness, potentially due to its higher luminosity.