Ultralight bosonic fields around a rotating black hole can extract energy and angular momentum through the superradiant instability and form a dense cloud. We investigate the scenario involving two scalar fields, $ϕ$ and $χ$, with a coupling term $\frac{1}{2}λϕχ^2$, which is motivated by the multiple-axion framework. The ultralight scalar $ϕ$ forms a cloud that efficiently produces $χ$ particles nonperturbatively via parametric resonance, with a large mass hierarchy, $μ_χ\gg μ_ϕ$. Rather than escaping the system as investigated by previous studies, these $χ$ particles remain bound, orbiting the black hole. Moreover, the particle production occurs primarily at the peak of the cloud's profile, allowing $χ$ particles in quasicircular orbits to pass repeatedly through resonant regions, leading to cumulative amplification. This selective process naturally forms a dense ring of $χ$ particles, with a mass ratio to the cloud fixed by $(μ_ϕ/μ_χ)^2$. Our findings reveal a novel mechanism for generating bound-state particles via parametric resonance, which also impacts the evolution of the cloud. This process can be probed through its imprint on binary dynamics and its gravitational-wave signatures.
Stellar Superradiance and Low-Energy Absorption in Dense Nuclear Media
Weizmann Institute of Science（魏茨曼科学研究所）；Niels Bohr Institute（尼尔斯·玻尔研究所）；CENTRA, Departamento de Física, Instituto Superior Técnico (IST), Universidade de Lisboa (UL)（里斯本大学高级技术学院物理系中心引力研究中心）；Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交通大学李政道研究所）；Shanghai Jiao Tong University（上海交通大学）；University of Manchester（曼彻斯特大学）；The Hong Kong University of Science and Technology（香港科技大学）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器）
Superradiant evolution of the shadow and photon ring of Sgr A$^\star$
超轻玻色子可以通过超辐射不稳定性影响自旋黑洞（BH）的动力学，从而导致超大质量BH阴影的时间演化。我们研究的前景见证超辐射诱导BH阴影的演变，考虑超轻矢量和张量场。我们引入了两个观测敏感的阴影时间演化：阴影漂移，并在与光子环自相关的方位角的变化。这两个观测量是高度互补的，这取决于观察者的倾角。专注于超大质量物体Sgr A $^\star$，我们表明，这两个观测值可以在几年的时间尺度上明显变化，在超辐射不稳定的存在下，导致签名，这是在现实的观测时间内的事件视界望远镜的范围内（但受益显着延长观测期），并铺平了道路，探测超轻玻色子在$\sim 10^{-17}\,{\rm eV}$质量范围。
Distorting Kerr Images with Parity-Odd Scalar Hair
South China University of Technology（华南理工大学）；Shanghai Jiao-Tong University（上海交通大学）；University of Jinan（济南大学）；Beijing Normal University（北京师范大学）；School of Physical Science and Technology, Ningbo University（宁波大学物理科学与技术学院）；Peking University（北京大学）
