Superradiant instabilities by accretion disks in scalar-tensor theories

TL;DR

This paper investigates how accretion disks around black holes can trigger superradiant instabilities in scalar-tensor theories, revealing potential constraints on alternative gravity models from astrophysical observations.

Contribution

It demonstrates that superradiant instabilities can occur in scalar-tensor theories with realistic accretion disks, offering new constraints on these theories from black hole spin observations.

Findings

Superradiant instabilities are possible in scalar-tensor theories with accretion disks.

Highly spinning black holes may challenge certain scalar-tensor dark energy models.

Obstacles in plasma-driven superradiance in GR can be overcome in scalar-tensor frameworks.

Abstract

We study the superradiant instability in scalar-tensor theories of gravitation, where matter outside a black hole provides an effective mass to the scalar degree of freedom of the gravitational sector. We discuss this effect for arbitrarily spinning black holes and for realistic models of truncated thin and thick accretion disks (where the perturbation equations are nonseparable), paying particular attention to the role of hot coronal flows in the vicinity of the black hole. The system qualitatively resembles the phenomenology of plasma-driven superradiant instabilities in General Relativity. Nevertheless, we show that the obstacles hampering the efficiency of plasma-driven superradiant instabilities in General Relativity can be circumvented in scalar-tensor theories. We find a wide range of parameter space where superradiant instabilities can be triggered in realistic scenarios, and discuss the constraints on scalar-tensor theories imposed by this effect. In particular, we argue that the existence of highly spinning accreting black holes is in tension with some scalar-tensor alternatives to the dark energy, e.g. symmetron models with screening.