Suzaku Investigation into the Nature of the Nearest Ultraluminous X-Ray Source, M33 X-8

Abstract

The X-ray spectrum of the nearest ultraluminous X-ray source, M 33 X-8, obtained by Suzaku during 2010 January 11-13, was closely analyzed in order to examine its nature. It is, by far, the only data with the highest signal statistic in the 0.4-10 keV range. Despite being able to reproduce the X-ray spectrum, Comptonization of the disk photons failed to give a physically meaningful solution. A modified version of the multi-color disk model, in which the dependence of the disk temperature on the radius is described as r^-p, with p being a free parameter, can also approximate the spectrum. From this model, the innermost disk temperature and bolometric luminosity were obtained as T_in = 2.00^+0.06_-0.05 keV and L_disk = 1.36 × 10^39</samll> (cos i )^-1 erg s^-1, respectively, where i is the disk inclination. A small temperature gradient of p = 0.535^-0.004_-0.005, together with the high disk temperature, is regarded as signatures of the slim accretion disk model, suggesting that M 33 X-8 was accreting at a high mass-accretion rate. With a correction factor for the slim-disk taken into account, the innermost disk radius, R_in = 81.9^+5.9_-6.5 (cos i )^-0.5 km, corresponds to a black-hole mass of M∼ 10_⊙ (cos i )^-0.5. Accordingly, the bolometric disk luminosity is estimated to be about 80(cos i )^-0.5% of the Eddington limit. A numerically calculated slim-disk spectrum was found to reach a similar result. Thus, the extremely super-Eddington luminosity is not required to explain the nature of M 33 X-8. This conclusion is utilized to argue for the existence of intermediate-mass black holes with M gtrsim 100_⊙ radiating at the sub/trans-Eddington luminosity, among ultraluminous X-ray sources with L_disc gtrsim 10⁴⁰ erg s^-1.