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

Abstract

Ultra-luminous X-ray sources (ULXs) are off-nuclear X-ray sources in nearby galaxies with X-ray luminosities $\ge$ 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 ($L_{\mathrm{X}}>5\times 10^{40}$ 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.

2 figures, 2 tables, accepted by New Astronomy

References in corpus (6)

• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• The Hard X-ray Spectral Slope as an Accretion-Rate Indicator in Radio-Quiet Active Galactic Nuclei
• X-ray observations of ultraluminous X-ray sources
• The hard X-ray spectral evolution in X-ray binaries and its application to constrain the black hole mass of ultraluminous X-ray sources
• Revisiting the radio/X-ray flux correlation in the black hole V404 Cyg: from outburst to quiescence
• Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313

Cited by in corpus (1)

• NuSTAR observations of WISE J1036+0449, a Galaxy at z$\sim 1$ obscured by hot dust