Natural Quenching of Inner Viscous Torques in Accretion Disks Around Mini-Boson Stars

Abstract

The standard Novikov-Thorne model for geometrically thin accretion disks assumes a zero-torque inner boundary condition at the innermost stable circular orbit (ISCO). Relaxing this assumption to allow a finite viscous torque---driven by magnetic stresses bridging the plunging region---can significantly enhance the radiative efficiency and high-frequency spectral emission of black hole accretion flows. While this mechanism is well-established for Kerr spacetimes, its thermodynamic viability around exotic, horizonless compact objects remains largely unexplored. In this article, we investigate the application of the inner viscous torque boundary condition to a thin disk embedded in a regular mini-boson star spacetime. By numerically integrating the coupled Einstein-Klein-Gordon field equations and employing the Lense-Thirring slow-rotation approximation, we generate an exact, horizonless background that strictly satisfies the weak energy condition. We demonstrate that the topological absence of a coordinate ISCO fundamentally alters the accretion dynamics. Without a sudden geodesic plunge, stable circular orbits penetrate deep into the dense scalar core, causing the physical inner boundary of the disk to be determined dynamically by the peak of the Keplerian angular velocity. Because the specific angular momentum of the fluid approaches zero at this deep microscopic boundary, the standard viscous torque is mathematically quenched. Consequently, the enhanced radiative flux divergence typically associated with boundary torques in black holes is naturally suppressed around regular boson stars. This torque quenching offers a robust phenomenological discriminator between standard black holes and horizonless ultralight scalar structures.