The effect of accretion on scalar superradiant instability (2501.09280v1)

Abstract: Superradiance can lead to the formation of a black hole (BH) condensate system. We thoroughly investigate the accretion effect on the evolution of this system, and the gravitational wave signals it emits in the presence of multiple superradiance modes. Assuming the multiplication of the BH mass and scalar mass as a small number, we obtain the analytical approximations of all important quantities, which can be directly applied to phenomenological studies. In addition, we confirm that accretion could significantly enhance the gravitational wave (GW) emission and reduce its duration, and show that the GW beat signature is similarly modified.

• The paper investigates how accretion influences scalar superradiant instability around rotating black holes, affecting gravitational wave emission and duration.
• The study uses an analytical model showing accretion enhances gravitational wave emission but shortens its duration, impacting detection strategies.
• Findings imply accretion-rich systems like AGN and X-ray binaries are potential labs to test superradiance, probing ultralight bosons and new physics.

An Analysis of the Effects of Accretion on Scalar Superradiant Instability

The paper entitled "The effect of accretion on scalar superradiant instability" authored by Yin-Da Guo, Shou-Shan Bao, Tianjun Li, and Hong Zhang provides a comprehensive investigation of the impact of accretion on black hole (BH) scalar superradiant instability, a phenomenon of significant interest for its potential implications in astrophysics and fundamental physics. This paper focuses explicitly on ultra-light bosons interacting with rotating BHs and examines the interplay between accretion dynamics and superradiant effects.

Scalar superradiance pertains to the amplification of scalar fields surrounding a rotating BH, leading to the extraction of rotational energy from the BH. This can result in a BH-condensate system characterized by the trapping of these fields in bound states. The presence of such a condensate could generate detectable gravitational wave (GW) signals under certain conditions. This research extends the analysis by incorporating the variables of accretion, which has traditionally been neglected in such discussions.

Key Findings and Analytical Approaches

1. Analytical Framework and Assumptions:
   • The authors construct an analytical model assuming the product of BH mass and scalar mass is a small number, thereby facilitating analytics feasible for phenomenological applications.
   • They employ the simplistic setup of a free scalar field around a Kerr black hole, ignoring backreaction on the metric and scalar self-interaction due to the low energy density of the condensate.
2. Accretion and Its Influence:
   • The accretion flow significantly affects the superradiance process by contributing to both the mass and angular momentum of the BH.
   • Accretion tends to enhance the GW emission from the BH-condensate system while simultaneously reducing the duration of these emissions, which implicates potential observational strategies.
3. Time Scales and Evolution Dynamics:
   • Key time scales, including the superradiant timescale, GW emission timescale, and accretion timescale, interplay to determine the evolution path.
   • Various scenarios arise from the relative size of these time scales, affecting the BH physics, and can be systematically predicted using analytical estimations devised in this paper.
4. Multi-Mode Scalar Interactions:
   • In exploring the presence of multiple superradiant modes, it reveals how different modes could impact the overall dynamics and lead to intricate signal profiles, particularly with gravitational wave signatures.

Implications for Observational Astrophysics and Theoretical Physics

The paper presents notable implications in both theoretical and observational contexts:

• Gravitational Wave Astronomy: The enhancement of GW emission due to accretion positions scalar superradiance as a viable target for future GW detectors like LISA. This opens opportunities for using GWs to probe ultralight bosons and assess their contribution to dark matter.
• Astrophysical BH Phenomenology: The findings suggest that active galactic nuclei or BHs in accretion-rich environments (e.g., X-ray binaries) could serve as natural laboratories to test superradiance scenarios. Thus, superradiance signals could be an indirect measure of accretion rates and dynamics in these systems.
• Fundamental Physics: The paper provides foundational insights into how classical gravitational systems can exhibit complex quantum-like behavior. Analyzing hypothetical ultralight particles like axions may contribute to understanding beyond the Standard Model physics, aligning with theoretical pursuits such as string theory.

Conclusions and Future Directions

The paper delivers a robust analytical approach to elucidating the impact of accretion on scalar superradiant instability at black holes. By extending beyond isolated systems to consider the realistic environments where accretion plays an essential role, this work paves the way for enhanced phenomenological models that can be cross-referenced with astronomical data. Future research may expand on coupling with additional particle physics models and in-depth numerical simulations to explore rich parameter spaces, including overtone mixing and non-linear effects, thus potentially signaling new physics.