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 with a multi-color black-body-like emission from an optically thick accretion disk .
The innermost temperature and radius of the accretion disk was measured to be T _ { in } = 1.08 _ { -0.03 } ^ { +0.02 } keV and R _ { in } = 122.1 _ { -6.8 } ^ { +7.7 } ~ { } \alpha ^ { 1 / 2 } km , respectively , where \alpha = ( \cos 60 ^ { \circ } / \cos i ) with i being the disk inclination .
The bolometric luminosity of the source was estimated to be L _ { bol } = 1.82 \times 10 ^ { 39 } \alpha ergs s ^ { -1 } .
Archival Chandra and XMM-Newton data of the source were analyzed for long-term spectral variations .
In almost all observations , the source showed multi-color black-body-like X-ray spectra with parameters similar to those in the Suzaku observation .
In only one Chandra observation , however , Source 3 was found to exhibit a power-law-like spectrum , with a photon index of \Gamma = 2.37 \pm 0.08 , when it was fainter by about \sim 15 \% than in the Suzaku observation .
The spectral behavior is naturally explained in terms of a transition between the slim disk state and the “ very high ” states , both found in Galactic black hole binaries when their luminosity approach the Eddington limit .
These results are utilized to argue that ultraluminous X-ray sources generally have significantly higher black-hole masses than ordinary stellar-mass black holes .