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Abstract: 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 Source3 was described with a multi-color black-body-like emission from an opticallythick accretion disk. The innermost temperature and radius of the accretiondisk was measured to be $T { m in} = 1.08 {-0.03}^{+0.02} $ keV and $R { min} = 122.1 {-6.8}^{+7.7} \alpha^{1-2}$ km, respectively, where $\alpha = \cos60^\circ -\cos i$ with $i$ being the disk inclination. The bolometricluminosity of the source was estimated to be $L { m bol} = 1.82 \times 10^{39}\alpha $ ergs s$^{-1}$. Archival Chandra and XMM-Newton data of the source wereanalyzed for long-term spectral variations. In almost all observations, thesource showed multi-color black-body-like X-ray spectra with parameters similarto 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 theSuzaku observation. The spectral behavior is naturally explained in terms of atransition between the slim disk state and the -very high- states, both foundin Galactic black hole binaries when their luminosity approach the Eddingtonlimit. These results are utilized to argue that ultraluminous X-ray sourcesgenerally have significantly higher black-hole masses than ordinarystellar-mass black holes.

Autor: Naoki Isobe RIKEN, Kazuo Makishima, Hiromitsu Takahashi, Tsunefumi Mizuno, Ryouhei Miyawaki, Poshak Gandhi, Madoka Kawaharada, At

Fuente: https://arxiv.org/

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