On the Black Hole Masses In Ultra-luminous X-ray Sources

TL;DR

This study estimates black hole masses in ultra-luminous X-ray sources using X-ray properties, revealing some contain intermediate-mass black holes and most likely host stellar-mass black holes accreting at super-Eddington rates.

Contribution

It compares two methods for estimating BH masses in ULXs and discusses their implications for understanding ULX accretion and black hole types.

Findings

Some ULXs host black holes of 10^4-10^5 solar masses.

X-ray variability amplitude and photon index methods yield conflicting results for lower luminosity ULXs.

Most ULXs likely contain stellar-mass black holes accreting at super-Eddington rates.

Abstract

Ultra-luminous X-ray sources (ULXs) are off-nuclear X-ray sources in nearby galaxies with X-ray luminosities $\geq$ 10$^{39}$ erg s$^{-1}$. The measurement of the black hole (BH) masses of ULXs is a long-standing problem. Here we estimate BH masses in a sample of ULXs with XMM-Newton observations using two different mass indicators, the X-ray photon index and X-ray variability amplitude based on the correlations established for active galactic nuclei (AGNs). The BH masses estimated from the two methods are compared and discussed. We find that some extreme high-luminosity ($L_{\rm X} >5\times10^{40}$ erg s$^{-1}$) ULXs contain the BH of 10$^{4}$-10$^{5}$ $M_\odot$. The results from X-ray variability amplitude are in conflict with those from X-ray photon indices for ULXs with lower luminosities. This suggests that these ULXs generally accrete at rates different from those of X-ray luminous AGNs, or they have different power spectral densities of X-ray variability. We conclude that most of ULXs accrete at super-Eddington rate, thus harbor stellar-mass BH.