The impact of superradiance on the spin evolution of variably accreting massive black holes

TL;DR

This study investigates how variable accretion rates influence the spin evolution of rapidly spinning black holes affected by superradiance, revealing that early accretion boosts significantly alter the superradiance region and long-term black hole spin states.

Contribution

It introduces simulations of variable accretion effects on superradiance-driven black hole spin evolution, highlighting the impact of early accretion boosts on the superradiance exclusion region.

Findings

Early accretion boosts can delay or reshape the superradiance drop.

Super-Eddington accretion can reduce the superradiance exclusion region by 40%.

Lighter axion clouds are more sensitive to early accretion boosts.

Abstract

This paper explores how time-varying increases in mass accretion onto rapidly spinning black holes influence their long-term spin evolution when affected by superradiance - a process where energy is extracted from the black hole by a surrounding axion field. Using simulations the study tracks how sudden accretion boosts affect a critical spin-down phase (the superradiance drop) during which the black hole's spin rapidly decreases while its mass remains nearly constant. The black hole spin evolution is controlled by the competition between two processes: how fast angular momentum is added through accretion, and how fast it is removed by the axion cloud. One major conclusion is that boosts to the accretion rate before the superradiance drop have the strongest effect, as they can delay or reshape the drop and significantly shrink the region of the mass-spin plane depopulated due to the superradiance. In particular, a super-Eddington accretion rate of 5 times Eddington accretion, lasting for 4 Myr and occurring 30 Myr before the superradiance drop can reduce the superradiance exclusion region in the mass-spin plane by 40 percent. In contrast, boosts to the accretion rate after the superradiance drop only cause temporary changes in the black hole spin. The study also shows that black holes with lighter axion clouds are more sensitive to these early boosts and can show observable spin changes lasting tens to hundreds of millions of years. Heavier axion clouds, however, require much stronger or longer-lasting boosts to produce similar effects, making them more stable under variable accretion.