We continue the study of spherically symmetric black hole solutions in torsion bigravity, a class of Einstein-Cartan-type gravity theories involving, besides a metric, a massive propagating torsion field. In the infinite-range limit, these theories admit asymptotically flat black hole solutions related to the presence of attractive fixed points in the asymptotic radial evolution of the metric and the torsion. We discuss these fixed points, and the way they are approached at large radii. Several phenomenological aspects of asymptotically flat torsion-hairy black holes are discussed: (i) location of the light ring and of the shadow; (ii) correction to the redshift of orbiting stars; and (iii) modification of the periastron precession of orbiting stars. By comparing the observable properties of torsion-hairy black holes to existing observational data on supermassive black holes obtained by the Event Horizon Telescope collaboration, and by the GRAVITY collaboration, we derive constraints on the theory parameters of torsion bigravity. The strongest constraint is found to come from the recent measurement of the periastron precession of the star S2 orbiting the Galactic-center massive black hole [Astron. Astrophys. 636, L5 (2020)], and to be a thousand times more stringent than solar-system gravitational tests.

中文翻译：

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扭转大引力的远程极限中的黑洞

我们继续研究扭转大引力中的球对称黑洞解，这是一类爱因斯坦-嘉当型引力理论，除了度量外，还涉及一个巨大的传播扭转场。在无限范围的极限内，这些理论承认渐近平坦的黑洞解与在度量和扭转的渐近径向演化中有吸引力的不动点的存在有关。我们讨论这些固定点，以及它们在大半径下的接近方式。讨论了渐近扁平扭转毛状黑洞的几个现象学方面：（i）光环和阴影的位置；(ii) 对轨道恒星红移的修正；(iii) 轨道恒星周围星体进动的修正。通过将扭转毛状黑洞的可观测性质与由事件视界望远镜合作获得的超大质量黑洞的现有观测数据进行比较，我们推导出了对扭转大引力理论参数的约束。发现最强的约束来自最近对围绕银河系中心大质量黑洞运行的恒星 S2 的近天体进动的测量 [Astron. 天体。636, L5 (2020)]，并且比太阳系引力测试严格一千倍。发现最强的约束来自最近对围绕银河系中心大质量黑洞运行的恒星 S2 的近天体进动的测量 [Astron. 天体。636, L5 (2020)]，并且比太阳系引力测试严格一千倍。发现最强的约束来自最近对围绕银河系中心大质量黑洞 [Astron. 天体。636, L5 (2020)]，并且比太阳系引力测试严格一千倍。