On the Nature of the X-ray Emission of Accreting Millisecond Pulsar SAX J1808.4-3658
On the Nature of the X-ray Emission of Accreting Millisecond Pulsar SAX J1808.4-3658
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DOI：
10.1046/j.1365-8711.2003.06773.x
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发表时间：
2002-04
期刊：
影响因子：
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通讯作者：
J. Poutanen;M. Gierliński
中科院分区：
文献类型：
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作者：
J. Poutanen;M. Gierliński
我们研究了吸积 X 射线毫秒脉冲星 SAX J1808.4-3658 在不同能量下的脉冲轮廓。在时间平均光谱中清楚地识别出两个主要发射成分，即黑体和康普顿化尾部，它们表现出很强的可变性，第一个成分滞后于第二个成分。如果发射是由中子星表面汤姆逊光学深度∼0.3-1的热板片（辐射激波）中的康普顿化产生的，则可以解释观察到的变化。黑体辐射沿平板法线强烈发射（“刀”状或“铅笔”状发射图案），而康普顿化发射具有更宽的角度分布，峰值位于距平板法线约 50°-60° 处（“扇”状图案）。我们构建了一个详细的 X 射线产生模型，考虑了史瓦西时空中的多普勒增强、相对论性像差和引力光弯曲。我们还提出了精确的解析公式，用于使用别洛博罗多夫最近开发的形式来计算快速旋转中子星的光变曲线。我们的模型很好地同时再现了不同能量下的脉冲轮廓、相应的相位滞后以及时间平均频谱。我们将致密星的质量限制在 1.2 和 1.6 M O 之间。通过拟合观测到的轮廓，我们确定致密天体的半径为 R ∼11 km（如果 M = 1.6 M O ），而当 M = 1.2 M O 时。最佳拟合半径为∼6.5 km，表明SAX J1808.4-3658中的致密天体可能是一颗奇怪的恒星。我们得到的系统倾角下限为 65°。
We study the pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658 at different energies. The two main emission components, the blackbody and the Comptonized tail, which are clearly identified in the time-averaged spectrum, show strong variability with the first component lagging the second one. The observed variability can be explained if the emission is produced by Comptonization in a hot slab (radiative shock) of Thomson optical depth ∼0.3-1 at the neutron star surface. The blackbody radiation is strongly beamed along the normal to the slab (a 'knife'-like or 'pencil'-like emission pattern), while the Comptonized emission has a broader angular distribution peaking at about 50°-60° from the slab normal (a 'fan'-like pattern). We construct a detailed model of the X-ray production accounting for the Doppler boosting, relativistic aberration and gravitational light bending in the Schwarzschild space-time. We present also accurate analytical formulae for computations of the light curves from rapidly rotating neutron stars using the formalism recently developed by Beloborodov. Our model reproduces well the pulse profiles at different energies simultaneously, corresponding phase lags, as well as the time-averaged spectrum. We constrain the compact star mass to be bounded between 1.2 and 1.6 M O .. By fitting the observed profiles, we determine the radius of the compact object to be R ∼11 km if M = 1.6 M O ., while for M = 1.2 M O . the best-fitting radius is ∼6.5 km, indicating that the compact object in SAX J1808.4-3658 can be a strange star. We obtain a lower limit on the inclination of the system of 65°.