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 Tin = 2.00+0.06-0.05 keV and Ldisk = 1.36 × 1039</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, Rin = 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 Ldisc gtrsim 1040 erg s-1.
- Publication:
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Publications of the Astronomical Society of Japan
- Pub Date:
- December 2012
- DOI:
- 10.1093/pasj/64.6.119
- arXiv:
- arXiv:1205.5331
- Bibcode:
- 2012PASJ...64..119I
- Keywords:
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- accretion;
- accretion disks;
- black hole physics;
- X-rays: individual (M33 X-8);
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 10 pages, 4 figures, PASJ accepted