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arXiv 0810.5188astro-ph
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超铝X射线源NGC 2403的光谱跃迁

Spectral transitions of an ultraluminous X-ray source, NGC 2403 Source 3

Naoki Isobe, Kazuo Makishima, Hiromitsu Takahashi, Tsunefumi Mizuno, Ryouhei Miyawaki, Poshak Gandhi, Madoka Kawaharada, Atsushi Senda, Tessei Yoshida, Aya Kubota, Hiroshi Kobori

备注：
Accepted for PASJ 3nd Suzaku special issue
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摘要：

报道了2006年3月16- 17日对超亮X射线源NGC 2403 Source 3的朱雀观测。源3的朱雀XIS光谱被描述为来自光学厚吸积盘的多色黑体状发射。吸积盘最里面的温度和半径分别为$T_{\rm in} = 1.08_{-0.03}^{+0.02} $ keV和$R_{\rm in} = 122.1_{-6.8}^{+7.7} α^{1/2}$ km，其中$α= (\cos 60^\circ /\cos i)$和$i$是盘的倾角。辐射热光度的来源估计为$L_{\rm bol} = 1.82 \times 10^{39} α$尔格s $^{-1}$。档案钱德拉和XMM-牛顿数据的源进行了分析的长期光谱变化。在几乎所有的观测中，该源都显示出多色黑体状X射线光谱，其参数与朱雀观测中的相似。然而，只有在钱德拉的一次观测中，发现源3表现出类似幂律的光谱，光子指数为$Γ= 2.37 \pm 0.08$，而它比朱雀的观测要暗$\sim 15 %$。光谱行为自然解释为细长盘状态和“非常高”状态之间的过渡，这两种状态都存在于银河系黑洞双星中，当它们的光度接近爱丁顿极限时。这些结果被用来论证超亮X射线源通常具有比普通恒星质量黑洞高得多的黑洞质量。

英文摘要：

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_{\rm in} = 1.08_{-0.03}^{+0.02} $ keV and $R_{\rm in} = 122.1_{-6.8}^{+7.7} α^{1/2}$ km, respectively, where $α= (\cos 60^\circ /\cos i)$ with $i$ being the disk inclination. The bolometric luminosity of the source was estimated to be $L_{\rm bol} = 1.82 \times 10^{39} α$ 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 $Γ= 2.37 \pm 0.08$, 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.

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