Recently, it has been discovered that the transition of X-ray pulsars to the low luminosity state ($L\lesssim 10^{35}\, {\rm erg\ \rm s^{-1}}$) is accompanied by a dramatic spectral change. That is, the typical power-law-like spectrum with high-energy cut-off transforms into a two-component structure with a possible cyclotron absorption feature on top of it. It was proposed that these spectral characteristics can be explained qualitatively by the emission of cyclotron photons in the atmosphere of the neutron star caused by collisional excitation of electrons to upper Landau levels and further Comptonization of the photons by electron gas. The electron gas are expected to be overheated in a thin top layer of the atmosphere. In this paper, we perform Monte Carlo simulations of the radiative transfer in the atmosphere of an accreting neutron star while accounting for a resonant scattering of polarized X-ray photons by thermally distributed electrons. The spectral shape is shown to be strongly polarization-dependent in soft X-rays ($\lesssim 10\, {\rm keV}$) and near the cyclotron scattering feature. The results of our numerical simulations are tested against the observational data of the X-ray pulsar A 0535+262 in the low-luminosity state. We show that the spectral shape of the pulsar can be reproduced by the proposed theoretical model. We discuss applications of the discovery to the observational studies of accreting neutron stars.

中文翻译：

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质量吸积率极低的 X 射线脉冲星中的光谱形成：蒙特卡罗方法

最近发现，X射线脉冲星向低光度态的跃迁（$L\lesssim 10^{35}\, {\rm erg\ \rm s^{-1}}$）伴随着剧烈的光谱变化。也就是说，典型的具有高能截止的类幂律光谱转变为双分量结构，其顶部可能具有回旋加速吸收特征。有人提出，这些光谱特征可以通过中子星大气中的回旋光子发射来定性地解释，这是由电子碰撞激发到上朗道能级和电子气对光子的进一步康普顿化引起的。预计电子气在大气层的薄顶层中会过热。在本文中，我们对吸积的中子星大气中的辐射转移进行了蒙特卡罗模拟，同时考虑了热分布电子对偏振 X 射线光子的共振散射。光谱形状在软 X 射线（$\lesssim 10\, {\rm keV}$）中和回旋加速器散射特征附近显示出强烈的偏振依赖性。我们的数值模拟结果与低光度状态下的 X 射线脉冲星 A 0535+262 的观测数据进行了测试。我们表明，脉冲星的光谱形状可以通过所提出的理论模型来再现。我们讨论了这一发现在吸积中子星观测研究中的应用。光谱形状在软 X 射线（$\lesssim 10\, {\rm keV}$）中和回旋加速器散射特征附近显示出强烈的偏振依赖性。我们的数值模拟结果与低光度状态下的 X 射线脉冲星 A 0535+262 的观测数据进行了测试。我们表明，脉冲星的光谱形状可以通过所提出的理论模型来再现。我们讨论了这一发现在吸积中子星观测研究中的应用。光谱形状在软 X 射线（$\lesssim 10\, {\rm keV}$）中和回旋加速器散射特征附近显示出强烈的偏振依赖性。我们的数值模拟结果与低光度状态下的 X 射线脉冲星 A 0535+262 的观测数据进行了测试。我们表明，脉冲星的光谱形状可以通过所提出的理论模型来再现。我们讨论了这一发现在吸积中子星观测研究中的应用。