QUIESCENT X-RAY EMISSION FROM Cen X-4: A VARIABLE THERMAL COMPONENT
QUIESCENT X-RAY EMISSION FROM Cen X-4: A VARIABLE THERMAL COMPONENT
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DOI：
10.1088/0004-637x/720/2/1325
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发表时间：
2010-07
期刊：
影响因子：
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通讯作者：
E. Cackett;E. Brown;Jonathan M. Miller;R. Wijnands
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文献类型：
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作者：
E. Cackett;E. Brown;Jonathan M. Miller;R. Wijnands
附近的中子星低质量x射线双星，半人马座X-4，自1979年爆发以来一直处于静止状态。通常，这些天体的静态发射包括热发射（可能来自中子星表面）和一个未知性质的硬幂律尾巴。在半人马座X-4的静止期间，在短至数百秒的时间尺度和长至数年的时间尺度上都观察到变率。然而，这种可变性的性质仍然未知。早期的观察似乎表明，这一切都是由于一个可变的硬x射线尾巴。在这里，我们提出了与此相矛盾的新的和档案观察结果。朱雀最近对半人马座X-4的观测发现，它处于历史上最低的状态，比最亮的静止观测暗淡4.4倍。由于在最亮观测期间的光谱由大约60%的热分量和40%的幂律分量组成，因此如此大的变化不能仅仅用幂律变异性来解释。具有可变热成分的光谱拟合可以很好地拟合数据，而允许柱密度和幂律变化的光谱拟合则不能，从而得出热成分必须是可变的结论。有趣的是，我们还发现热分数在所有时代之间保持一致，这意味着热和幂律通量的变化量大致相同。如果发射面积在两次观测之间保持不变，则有效表面温度必须改变。或者，如果温度保持不变，那么发射面积必须改变。这种热变异性的性质尚不清楚，但可以用可变的低层吸积来解释。
The nearby neutron star low-mass X-ray binary, Cen X-4, has been in a quiescent state since its last outburst in 1979. Typically, quiescent emission from these objects consists of thermal emission (presumably from the neutron star surface) with an additional hard power-law tail of unknown nature. Variability has been observed during quiescence in Cen X-4 on both timescales as short as hundreds of seconds and as long as years. However, the nature of this variability is still unknown. Early observations seemed to show it was all due to a variable hard X-ray tail. Here, we present new and archival observations that contradict this. The most recent Suzaku observation of Cen X-4 finds it in a historically low state, a factor of 4.4 fainter than the brightest quiescent observation. As the spectrum during the brightest observation was comprised of approximately 60% from the thermal component and 40% from the power-law component, such a large change cannot be explained by just power-law variability. Spectral fits with a variable thermal component fit the data well, while spectral fits allowing both the column density and the power law to vary do not, leading to the conclusion that the thermal component must be variable. Interestingly, we also find that the thermal fraction remains consistent between all epochs, implying that the thermal and power-law fluxes vary by approximately the same amount. If the emitting area remains unchanged between observations, then the effective surface temperature must change. Alternatively, if the temperature remains constant, then the emitting area must change. The nature of this thermal variability is unclear, but may be explained by variable low-level accretion.