Spectral Analyses of the Nearest Persistent Ultraluminous X-Ray Source M33 X-8

Abstract

We provide a detailed analysis of 12 XMM observations of the nearest persistent extragalactic ultraluminous X-ray source (ULX), M33 X-8. No significant spectral evolution is detected between the observations, therefore we combine the individual observations to increase the signal-to-noise ratio for spectral fitting. The combined spectra are best fitted by the self-consistent p-free disk plus power-law component model with p = 0.571_{-0.030}^{+0.032}, kT_{in} = 1.38_{-0.08}^{+0.09} keV, and the flux ratio of the p-free disk component to the power-law component being 0.63:0.37 in the 0.3 -- 10 keV band. The fitting indicates that the black hole in M33 X-8 is of \sim 10 M_{\odot} and accretes at a super-Eddington rate (\sim 1.5 L_{Edd}), and the phase of the accretion disk is close to a slim disk (p = 0.5). We report, for the first time, that an extra power-law component is required in addition to the p-free disk model for ULXs. In super-Eddington cases, the power-law component may possibly result from the optically thin inner region f the disk or a comptonized corona similar to that of a standard thin disk.

11 pages, 1 table, 2 figures, accepted by PASJ

References in corpus (7)

• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• The Ultraluminous State
• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• Masses, Beaming and Eddington Ratios in Ultraluminous X-ray Sources
• Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313
• On the weakness of disc models in bright ULXs
• New flaring of an ultraluminous X-ray source in NGC 1365

Cited by in corpus (13)

• Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era
• The missing link: a low mass X-ray binary in M31 seen as an ultraluminous X-ray source
• X-ray spectral evolution in M33 X-8
• Observing the Galaxy's massive black hole with gravitational wave bursts
• The Chandra ACIS Survey of M33 (ChASeM33): The final source catalog
• Integral field spectroscopy of HII regions in M33
• Comparing the Accretion Disk Evolution of Black Hole and Neutron Star X-ray Binaries from Low to Super-Eddington Luminosity
• M - sigma relation between SMBHs and the velocity dispersion of globular cluster systems
• Super-Eddington Accretion in the Ultraluminous X-ray Source NGC1313 X-2: An Ephemeral Feast
• Suzaku investigation into the nature of the nearest ultraluminous X-ray source, M33 X-8
• Swift-XRT 6-year monitoring of the ultraluminous X-ray source M33-X8
• NuSTAR observations of the ultraluminous X-ray source M33 X-8: a black hole in a very high state?
• Recurrent outbursts revealed in 3XMM J031820.8-663034