利用X射线观测数据探测天文学黑洞候选体时空

In the last 60 years, precise experiments in the Solar System and accurate radio observations of binary pulsars have confirmed the predictions of Einstein's theory of general relativity in weak gravitational fields. Today the challenge is to test the theory in more extreme situations. One of the most fascinating predictions is surely the existence of black holes. According to Einstein's gravity, a black hole is a relatively simple object, which is completely specified by its mass M, spin angular momentum J, and a possible non-vanishing electric charge Q. It is thought that astrophysical black holes can form from the gravitational collapse of massive stars and that the spacetime geometry around them is described by the Kerr solution. Astronomical observations have discovered a number of black hole candidates. These objects are too compact and heavy to be relativistic stars or clusters of relativistic stars, and for this reason they are thought to be black holes. However, except this indirect evidence, there is not yet observational confirmation that black hole candidates are the Kerr black holes of general relativity..The Kerr paradigm can be tested by studying specific features in the electromagnetic spectrum of black hole candidates. These features are produced by relativistic effects occurring in the vicinity of these objects and their analysis can thus provide important information on the spacetime geometry. Current techniques to test black hole candidates are the analysis of the thermal spectrum of thin accretion disks and the study of the profile of the iron K-alpha line. However, there is a fundamental degeneracy among the estimate of the spin, possible deviations from the Kerr solution, and the viewing angle, and with current measurements it is impossible to test the Kerr hypothesis: non-Kerr objects may be interpreted as Kerr black holes with a different spin and observed from a different viewing angle. The nature of black hole candidates can be tested with the help of observations not available today, to be combined together and with the already available data to break this degeneracy. The aim of this project is to investigate how future X-ray measurements can be used to test the Kerr black hole hypothesis and, in particular, to figure out the possibilities offered by the proposed XTP (X-ray Timing and Polarization) mission.

迄今为止，除了间接证据，并无直接观测证明天文学中的黑洞候选体就是由广义相对论预言的克尔黑洞。可以通过观测黑洞候选体的电磁辐射谱的一些特征，来验证上述的克尔黑洞猜想。这些辐射谱特征来自于黑洞候选体附近的相对论效应。普通辐射谱分析方法中，如下几个黑洞观测量很可能出现简并：黑洞角动量，黑洞候选体时空与克尔时空之间可能的偏离量，观察者观测角度。这导致依据目前的观测数据，我们无法确切弄清楚黑洞候选体是否为克尔黑洞。非克尔黑洞的辐射谱，可以解释为一个不同角动量或不同观测角度的克尔黑洞的辐射谱。不远将来的观测数据，联合目前已知的数据，有可能能够解除上述几个估计量的简并，从而能够探测黑洞候选体周围准确的时空结构。本课题的目标是，探讨如何利用未来的X射线观测数据，尤其是X射线时变与偏振探测卫星任务（XTP）的数据，来探测黑洞候选体的本质。

爱因斯坦的广义相对论理论在弱引力场中得到了广泛的检验，主要是通过太阳系内的实验和脉冲双星的射电观测进行检验，理论预测与现有数据吻合较好。与之相反，强引力区域在很大程度上仍未被探索，许多广义相对论的拓展在弱引力场中给出了相同的预测，它们只有在引力变强时才会出现偏差。天体物理学黑洞是在强场区域检验爱因斯坦广义相对论理论的理想实验室。.我们课题组已经开发了一个相对论性的反射谱模型来探测吸积黑洞周围的强引力场。这个模型称作relxill_nk，它是relxill包的延拓。我们已经使用我们的新模型对一些恒星质量的和超大质量黑洞的X射线观测，得到了一些对于可能偏离克尔度规的初步约束。我们的结果证实了克尔解，并表明X射线反射光谱可能是一个检验爱因斯坦引力在强场区域下的有力工具。