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arXiv 1104.4264astro-ph.HE
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用INTEGRAL和RXTE观测~ 4U 1728-34的光谱态演化:非热成分探测

Spectral states evolution of 4U 1728-34 observed by INTEGRAL and RXTE: non-thermal component detection

A. Tarana, T. Belloni, A. Bazzano, M. Méndez, P. Ubertini

摘要:

我们报告了用INTEGRAL和RXTE对低质量X射线双星(LMXB)源(环礁型)4U 1728-34进行为期一年的监测的结果。INTEGRAL覆盖了三个时间间隔,在此期间,源显示出强烈的光谱演化。通过在硬度-强度图的不同部分拟合几个模型,详细研究了宽带X射线谱。软态的特点是突出的黑体发射加上贡献的康普顿发射。硬态的特征在于相对于50千电子伏以上的康普顿化模型的过量通量的存在,而软组分是微弱的。为了获得一个可接受的适合的数据,这种过剩的建模与光子指数伽玛~ 2的幂律或康普顿化(CompPS)光谱意味着混合的热电子和非热电子在电晕的存在。这使得4U 1728-34成为为数不多的环礁型LMXB之一,在高能量下显示出非热辐射。从我们的分析中,很明显,随着整个频谱变得更硬,硬尾的存在也更加突出。我们还讨论了替代模型,可以描述这些硬态。

英文摘要:

We report results of a one-year monitoring of the low mass X-ray binary (LMXB) source (atoll type) 4U 1728-34 with INTEGRAL and RXTE. Three time intervals were covered by INTEGRAL, during which the source showed strong spectral evolution. We studied the broad-band X-ray spectra in detail by fitting several models in the different sections of the hardness-intensity diagram. The soft states are characterised by prominent blackbody emission plus a contribution from a Comptonized emission. The hard states are characterised by the presence of an excess flux with respect to the Comptonization model above 50 keV while the soft component is fainter. To obtain an acceptable fit to the data this excess is modeled either with a power law with photon index Gamma ~ 2 or a Comptonization (CompPS) spectrum implying the presence of hybrid thermal and non-thermal electrons in a corona. This makes 4U 1728-34 one of the few LMXBs of atoll type showing non-thermal emission at high energy. From our analysis, it is also apparent that the presence of the hard tail is more prominent as the overall spectrum becames harder. We discuss also alternative models which can discribe these hard states.

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