Spectral Transitions of an Ultraluminous X-Ray Source, NGC 2403 Source 3

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 Tin =1.08 +0.02 −0.03 keV and Rin =122.1 +7.7 −6.8 � 1/2 km, respectively, where �=(cos60 ◦ /cosi) with i being the disk inclination. The bolometric luminosity of the source was estimated to be Lbol = 1.82 ×10 39 � 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 = 2 .37 ±0.08, when it was fainter by about ∼ 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