A Suzaku observation of an ultraluminous X-ray source, NGC 2403 Source 3, performed on 2006 March 16-17, is reported. The Suzaku XIS spectrum of Source 3 was described as multi-color black-body-like emission from an optically thick accretion disk. The innermost disk temperature and radius were measured to be Tin = 1.08+0.02-0.03keV andRin = 122.11+7.7-6.8α1/2km, respectively, where α = (cos 60°/cosi) with i being the disk inclination. Its bolometric luminosity was estimated to be Lbol = 1-82 × 10 39α erg s-1. Archival Chandra and XMM-Newton data of the source were analyzed. In almost all observations, the source showed multi-color black-body-like X-ray spectra with parameters similar to those of the Suzaku observation. In only one Chandra observation, however, Source 3 exhibited a power-law-like spectrum, with a photon index of Γ = 2.37 ±0.08, when it was fainter by ̃ 15% than in the Suzaku observation. This behavior is explained in terms of a transition between a slim-disk state and "very high" states, both found in Galactic black hole binaries, when their luminosity apporached the Eddington limit. These results are utilized to argue that ultraluminous X-ray sources generally have significantly higher masses than ordinary stellar-mass black holes.
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