Suzaku Investigation into the Nature of the Nearest Ultraluminous X-Ray Source, M 33 X-8

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(-0.05)(+0.06) keV and L-disk = 1.36 x 10(39)(cos i)(-1) erg s(-1), respectively, where i is the disk inclination. A small temperature gradient of p = 0.535(-0.005)(+0:004), 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(-6.5)(+5.9)(cos i)(-0.5) km, corresponds to a black-hole mass of M similar to 10M(circle dot)(cosi)(-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 greater than or similar to 100 M-circle dot radiating at the sub/trans-Eddington luminosity, among ultraluminous X-ray sources with L-disk greater than or similar to 10(40) erg s(-1).