Black hole candidates in X-ray binary systems and at the centers of galaxies are expected to be the Kerr black holes of General Relativity, but the actual nature of these objects has yet to be verified. In this paper, we consider the possibility that they are exotic compact objects and we describe their exterior gravitational field with a subclass of the Manko-Novikov metrics, which are exact solutions of the vacuum Einstein's equations and can describe the spacetime geometry around bodies with arbitrary mass-multipole moments. We point out that around a Manko-Novikov object there may exist many disconnected non-plunging regions at small radii, with no counterpart in the Kerr background, and that their existence may be tested. For instance, in the presence of an accretion disk, they may be filled by the accreting gas, forming a ring structure that might remind the rings of Saturn. We suggest that the existence of these regions may have a clear observational signature in the waveform of the gravitational radiation emitted by an EMRI: in the last stage of the inspiral, the waveform would be the combination of "regular chirps", produced when the small object orbits in one of the non-plunging regions, and "bursts", released when the small object jumps from a non-plunging region to another one at smaller radii. Our conclusions are supported by some numerical calculations of trajectories in the geodesic approximation, in which a particle plunges from the ISCO and then seems to get trapped in the potential well at smaller radii.
Testing for gravitationally preferred directions using the lunar orbit
由于引力是一种长程力，{ \it a priori}可以想象，宇宙的全球物质分布为局部引力物理学选择了一个优选的静止坐标系。在后牛顿近似下，偏好框架效应的现象学由两个参数描述，$α_1$和$α_2$，其中第二个参数已经受到非常严格的约束。重申以前的建议，我们通过一个详细的Hill-Brown型计算的扰动月球轨道，月球激光测距数据有可能约束$α_1$在$10^{-4}$水平。据发现，某些逆行平面轨道表现出共振敏感性的外部扰动链接到一个固定的方向在空间中。月球轨道离这样的共振很远，由于太阳潮汐，月球轨道没有明显的增强。我们的希尔-布朗分析扩展到与可能的差分加速度向银河系中心的扰动。然而，有人认为，有很强的{ \it先验}的理论约束，这种效果的可想象的大小。
