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

• Published in: Publications of the Astronomical Society of Japan 2009-01, Vol.61 (sp1), p.S279-S289
• Main Authors: Isobe, Naoki, Makishima, Kazuo, Takahashi, Hiromitsu, Mizuno, Tsunefumi, Miyawaki, Ryouhei, Gandhi, Poshak, Kawaharada, Madoka, Senda, Atsushi, Yoshida, Tessei, Kubota, Aya, Kobori, Hiroshi
• Format: Article
• Language: English
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/61.sp1.S279

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 $T_{\rm in}$ $=$ 1.08 $_{-0.03}^{+0.02}$ keV and $R_{\rm in}$ $=$ 122.1 $_{-6.8}^{+7.7} \ \alpha^{1/2}$ km, respectively, where $\alpha$ $=$ (cos 60 $^\circ /$ cos $i$ ) with $i$ being the disk inclination. Its bolometric luminosity was estimated to be $L_{\rm bol}$ $=$ 1.82 $\times$ 10 $^{39}\alpha $ ergs $^{-1}$ . 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 $\Gamma$ $=$ 2.37 $\pm$ 0.08, when it was fainter by $\sim$ 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.