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

TL;DR

This study analyzes 12 XMM observations of the ultraluminous X-ray source M33 X-8, revealing a super-Eddington accretion onto a ~10 solar mass black hole with a slim disk structure and the necessity of an additional power-law component.

Contribution

First detection of an extra power-law component in ULX spectra, indicating complex accretion physics beyond standard models.

Findings

Black hole mass estimated at ~10 solar masses.

Accretion rate is super-Eddington (~1.5 L_Edd).

Spectral analysis supports a slim disk model with p close to 0.5.

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.