A Low-Amplitude X-Ray and Optical Outburst from the Periodic Transient A0538–66: Accretion onto a Magnetosphere?
A Low-Amplitude X-Ray and Optical Outburst from the Periodic Transient A0538–66: Accretion onto a Magnetosphere?
复制标题
周期性瞬变 A0538-66 的低振幅 X 射线和光学爆发：吸积到磁层上？
DOI：
10.1086/303644
复制
发表时间：
1997
期刊：
影响因子：
--
通讯作者：
A. Smale
中科院分区：
文献类型：
--
作者：
R. Corbet;P. Charles;K. Southwell;A. Smale
我们目前ASCA的意见，在1995年2月左右的预测爆发的时间从16.65天周期的经常性瞬变A0538-66，连同同时光学测光。在大约2个月前预测的爆发时间附近也获得了光谱。我们发现A0538-66在X射线和光学上都很活跃，尽管其水平比过去看到的最大的超级爱丁顿爆发低得多。光学光谱显示出可变的巴耳末辐射，但不是伴随着大爆发的He II 4686的强辐射。X射线的平均光度约为5.5 × 1036尔格·s-1，我们发现了在X射线光谱中存在一条铁谱线的证据。这可以被解释为6.54 ± 0.04 keV处的单条线，或者替代地，被解释为6.4和6.7 keV处的发射的混合。一个简单的幂律，或幂律加上高能截止，不能很好地拟合连续谱。相反，需要一个更复杂的模型，我们发现，幂律加上黑体给出了一个很好的适合的连续，虽然其他模型不能完全排除。在我们的数据中没有发现以前检测到的69毫秒脉动存在的证据，虽然我们的灵敏度是有限的，在我们的观测和未知的轨道参数的低计数率。我们认为X射线的生产过程中，可能会考虑所观察到的行为。特别是，吸积到磁层，而不是表面的中子星星进行了讨论。然而，我们的数据并不能最终排除中子星星上的吸积仍然在发生的可能性，这要么是因为中子星星的磁场很低，要么是因为物质泄漏通过磁垒。如果X射线通量是由磁层释放的能量驱动的，那么在大于6.4千电子伏的光谱中存在一条铁谱线，这在Be星星X射线双星中是罕见的，可能是这样一个过程正在发生的一个指标。
We present ASCA observations made in 1995 February around the time of a predicted outburst from the 16.65 day period recurrent transient A0538-66, together with simultaneous optical photometry. Optical spectroscopy was also obtained near the time of a predicted outburst approximately 2 months earlier. We find that A0538-66 is active, both in X-rays and optically, although at a much lower level than during the largest super-Eddington outbursts that have been seen in the past. The optical spectra show variable Balmer emission, but not the strong He II 4686 Å emission that accompanies large outbursts. The average X-ray luminosity is ~5.5 × 1036 ergs s-1, and we find evidence for the presence of an iron line in the X-ray spectrum. This can be interpreted either as a single line at 6.54 ± 0.04 keV or, alternatively, as a blend of emission at both 6.4 and 6.7 keV. A simple power law, or power law plus high-energy cutoff, does not fit the continuum spectrum well. Instead, a more complicated model is required, and we find that a power law plus blackbody gives a good fit to the continuum, although other models cannot be excluded completely. No evidence for the presence of previously detected 69 ms pulsations is found in our data, although our sensitivity is limited by the low count rate during our observations and the unknown orbital parameters of this system. We consider X-ray production processes that may account for the observed behavior. In particular, accretion onto the magnetosphere rather than the surface of a neutron star is discussed. Our data do not, however, exclude conclusively the possibility that accretion onto the neutron star is still occurring either because the magnetic field of the neutron star is low or because material is leaking past the magnetic barrier. If the X-ray flux is powered by energy released at the magnetosphere, then the presence of an iron line in the spectrum at greater than 6.4 keV, which is rare for Be star X-ray binaries, may be an indicator that such a process is occurring.