NuSTAR DISCOVERY OF A LUMINOSITY DEPENDENT CYCLOTRON LINE ENERGY IN VELA X-1-猫眼课题宝

DOI：

10.1088/0004-637x/780/2/133

发表时间：

2013-11

期刊：

The Astrophysical Journal

影响因子：

通讯作者：

中科院分区：

其他

文献类型：

作者：

F. Fuerst;K. Pottschmidt;J. Wilms;J. Tomsick;M. Bachetti;S. Boggs;F. Christensen;W. Craig;B. Grefenstette;C. Hailey;F. Harrison;K. Madsen;Jonathan M. Miller;D. Stern;D. Walton;Will Zhang

我们介绍了船帆座X-1的努斯塔观测，船帆座X-1是一个持久的，但高度可变的，中子星星高质量X射线双星（HMXB）。在相似的轨道阶段进行了两次观测，但相隔近一年。它们显示出非常不同的3-79千电子伏通量水平，以及在每次观测期间的强可变性，几乎覆盖了通量的一个数量级。这些观测允许，有史以来第一次，调查的回旋共振散射功能（CRSFs）的质心能量如何依赖于通量的持久性HMXB的千秒时间尺度。我们发现，谐波CRSF的线能量与通量相关，正如预期的亚临界吸积制度。我们认为船帆座X-1有一个非常窄的吸积柱，半径约为0.4 km，在观测到的亮度为Lx = 1.3 × 1036 erg s−1时维持着库仑相互作用主导的激波。除了在55 keV的主要谐波线，25 keV左右的基本线被清楚地检测到。我们发现这两个CRSF的强度是反相关的，我们解释了光子产卵。这种反相关性是关于基本线是否存在的争论的一个可能的解释。线能量的比率随时间而变化，并且显著偏离2.0，这也是光子繁殖的可能结果，其改变了线的形状。在第二次观察期间，船帆座X-1显示出短暂的关闭状态，其中幂律变软，并且截止不再可测量。很可能是源切换到不同的吸积制度，在这些低质量吸积率，解释光谱形状的急剧变化。

We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3–79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of Lx ∼ 3 × 1036 erg s−1. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratio of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape.

NuSTAR DISCOVERY OF A LUMINOSITY DEPENDENT CYCLOTRON LINE ENERGY IN VELA X-1

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