Accretion heated atmospheres of X-ray bursting neutron stars
Accretion heated atmospheres of X-ray bursting neutron stars
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DOI：
10.1051/0004-6361/201833581
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发表时间：
2018-08
影响因子：
6.5
通讯作者：
V. Suleimanov;J. Poutanen;K. Werner
中科院分区：
文献类型：
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作者：
V. Suleimanov;J. Poutanen;K. Werner
在低质量X射线双星中，中子星表面的一些热核(I型)X射线爆发发生在系统的硬持久状态。当爆发光度足够高时，被动冷却中子星的大气模型很好地描述了这些爆发的光谱演化。当爆发光度降到最大光度的20%-70%的临界值以下时，观测到的光谱演化与模型预测的结果有偏差。偏差的幅度和临界光度与持续光度相关，这使得我们认为这些偏差是由吸积粒子的额外加热引起的。我们提出了一种计算被吸积粒子加热的中子星大气模型的方法，该模型假定吸积粒子的能量是通过与电子的库仑相互作用来释放的。我们计算了不同化学组成的大气的温度结构和出射光谱，并研究了结果与吸积粒子的速度、温度和穿透角的关系。我们发现，加热的大气形成了两个不同的区域。上层是由康普顿散射冷却的高温(20-100keV)的日冕状表层，以及更深的、几乎等温的光学厚层，温度为几keV。相应地，出射光谱有两个分量：温度接近等温区的黑体和硬康普顿分量(具有指数衰减的幂定律)。它们的相对贡献取决于吸积粒子的能量耗散率与中子星表面的本征通量之比。这些光谱与未受干扰的、被动冷却的中子星大气的光谱严重背离，主要的区别是光谱中存在高能尾巴和低能部分的强烈过剩。它们也缺乏铁吸收边，这是在具有太阳化学成分的未受干扰的低光度大气的光谱中可见的。利用计算的光谱，我们得到了纯氦大气和太阳丰度大气的稀释度和颜色校正因子随相对光度的依赖关系。结果表明，由吸积加热的氦模型大气相当于爱丁顿光度的5%，很好地描述了4U1820−30 X射线爆发的后期阶段。
Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20–70% of the maximum luminosity. The amplitude of the deviations and the critical luminosity correlate with the persistent luminosity, which leads us to suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We computed the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the velocity of accreted particles, their temperature and the penetration angle. We show that the heated atmosphere develops two different regions. The upper one is the hot (20–100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra correspondingly have two components: a blackbody with the temperature close to that of the isothermal region and a hard Comptonized component (a power law with an exponential decay). Their relative contribution depends on the ratio of the energy dissipation rate of the accreted particles to the intrinsic flux from the neutron star surface. These spectra deviate strongly from those of undisturbed, passively cooling neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820−30.