Spectral transition of an ultra-luminous X-ray source, NGC 2403 Source 3
Naoki Isobe (RIKEN) , Kazuo Makishima (RIKEN/University of Tokyo), Atsushi Senda (RIKEN), Madoka Kawaharada (RIKEN), Hiromitsu Takahashi (Hiroshima University), Tsunehumi Mizuno (Hiroshima University), Ryohei Miyawaki (University of Tokyo), Tessei Yoshida (Tokyo University of Science), Richard Mushotzky (NASA/GSFC)
We report the study of the spectral variation over 5 years, of an ultra-luminous X-ray source NGC 2403 Source 3, which is located in the arm region of a normal spiral galaxy NGC 2403. We performed a Suzaku observation of NGC 2403 during the Science working group phase on 2006 March 16. All the available archival Chandra and Newton data are also analysed. Except for one Chandra observation, the X-ray spectra of NGC 2403 source 3 are successfully described with a multi-color disk (MCD) model. The innermost disk temperature and bolometric luminosity of the source is found to be rather stable, Tin = 1.04 - 1.18 keV and
ergs/s, respectively. The innermost disk radius is determined to be 96 - 130 km, indicating that the mass of the source is about 10 - 15 times the solar masses. As a result, the source is inferred to be shining near the Eddington limit. In the remaining Chandra observation, conducted in 2004 November, we have discovered that the source showed a transition to a power-law (PL) spectrum with a photon index of Gamma = 
. In this observation, the lumineosity of the source is lower by abou 10 /