The X-ray spectrum of the nearest ultraluminous X-ray source, M33 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 Tin = 2.00+0:06 -0:05 keV and Ldisk = 1.36 × 1039(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 M33 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, Rin = 81.9+5:9 -6:5(cosi )-0:5 km, corresponds to a black-hole mass ofM ̃ 10M(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 M33 X-8. This conclusion is utilized to argue for the existence of intermediate-mass black holes with M and 100M radiating at the sub/trans-Eddington luminosity, among ultraluminous X-ray sources with L disk ≥ 1040 erg s-1.
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