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

Abstract

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_{\rm in}$$ =$ 2.00$ _{-0.05}^{+0.06}$ keV and $ L_{\rm disk}$$ =$ 1.36 $ \times$ 10$ ^{39}$ (cos $ i$ )$ ^{-1}$ erg s$ ^{-1}$ , respectively, where $ i$ is the disk inclination. A small temperature gradient of $ p$$ =$ 0.535$ _-0.005^{+0.004}$ , together with the high disk temperature, is regarded as signatures of the slim accretion disk model, suggesting that M 33 X-8 was accreting at a high mass-accretion rate. With a correction factor for the slim-disk taken into account, the innermost disk radius, $ R_{\rm in}$$ =$ 81.9$ _-6.5^{+5.9}$ (cos$ i$ )$ ^{-0.5}$ km, corresponds to a black-hole mass of $ M$$ \sim$ 10 $ M_{\odot}$ (cos $ i$ )$ ^{-0.5}$ . Accordingly, the bolometric disk luminosity is estimated to be about 80(cos $ i$ )$ ^{-0.5}$ % of the Eddington limit. A numerically calculated slim-disk spectrum was found to reach a similar result. Thus, the extremely super-Eddington luminosity is not required to explain the nature of M 33 X-8. This conclusion is utilized to argue for the existence of intermediate-mass black holes with $ M$$ \gtrsim$ 100$ M_{\odot}$ radiating at the sub/trans-Eddington luminosity, among ultraluminous X-ray sources with $ L_{\rm disk}$$ \gtrsim$ 10$ ^{40}$ erg s$ ^{-1}$ .