从黑洞吸积盘的偏振连续辐射推断吸积盘的倾角
Inferring the Inclination of a Black Hole Accretion Disk from Observations of its Polarized Continuum Radiation
摘要：
目前正在通过拟合X射线双星中恒星质量黑洞的吸积盘发射的X射线连续谱来估计它们的自旋参数。对于这种方法，有必要知道产生X射线的磁盘内部区域的倾斜度。由于内盘的方向应该垂直于黑洞的自转轴，通常的做法是假设黑洞的自转与双星的轨道角动量矢量一致，并根据第二颗恒星的光变曲线中的椭球调制来估计后者的倾角。我们表明，磁盘的倾斜度可以直接推断，如果我们有光谱和偏振信息的磁盘辐射。当星盘倾角从正面到侧面变化时，偏振度的预测值在0%到5%之间变化。用现有的X射线偏振测量技术，对一个典型的亮X射线双星进行10天左右的观测，偏振度的测量精度可达0.1%。这样的测量将把盘的倾角限制在1度或2度以内，并将显著提高黑洞自旋估计的可靠性。此外，它还将提供关于黑洞自旋轴和双星轨道旋转轴之间倾斜的新信息，这将限制恒星质量黑洞在形成过程中所经历的任何速度反冲。
英文摘要：
Spin parameters of stellar-mass black holes in X-ray binaries are currently being estimated by fitting the X-ray continuum spectra of their accretion disk emission. For this method, it is necessary to know the inclination of the X-ray-producing inner region of the disk. Since the inner disk is expected to be oriented perpendicular to the spin axis of the hole, the usual practice is to assume that the black hole spin is aligned with the orbital angular momentum vector of the binary, and to estimate the inclination of the latter from ellipsoidal modulations in the light curve of the secondary star. We show that the inclination of the disk can be inferred directly if we have both spectral and polarization information on the disk radiation. The predicted degree of polarization varies from 0% to 5% as the disk inclination changes from face-on to edge-on. With current X-ray polarimetric techniques the polarization degree of a typical bright X-ray binary could be measured to an accuracy of 0.1% by observing the source for about 10 days. Such a measurement would constrain the disk inclination to within a degree or two and would significantly improve the reliability of black hole spin estimates. In addition, it would provide new information on the tilt between the black hole spin axis and the orbital rotation axis of the binary, which would constrain any velocity kicks experienced by stellar-mass black holes during their formation.
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