arXiv subjects

Enrico Cannizzaro

Publications and source records attributed to Enrico Cannizzaro.

21 records · Page 2Linked to original sources

arXiv2022

Superradiant instabilities by accretion disks in scalar-tensor theories

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.

gr-qc↗

arXiv2021

Plasma-photon interaction in curved spacetime II: collisions, thermal corrections, and superradiant instabilities

Motivated by electromagnetic-field confinement due to plasma near accreting black holes, we continue our exploration of the linear dynamics of an electromagnetic field propagating in curved spacetime in the presence of plasma by including three effects that were neglected in our previous analysis: collisions in the plasma, thermal corrections, and the angular momentum of the background black-hole spacetime. We show that: (i) the plasma-driven long-lived modes survive in a collisional plasma except when the collision timescale is unrealistically small; (ii) thermal effects, which might be relevant for accretion disks around black holes, do not affect the axial long-lived modes; (iii) in the case of a spinning black hole the plasma-driven modes become superradiantly unstable at the linear level; (iv) the polar sector in the small-frequency regime admits a reflection point due to the resonant properties of the plasma. Dissipative effects such as absorption, formation of plasma waves, and nonlinear dynamics play a crucial role in the vicinity of this resonant point.

gr-qc↗

arXiv2021

Plasma-photon interaction in curved spacetime I: formalism and quasibound states around nonspinning black holes

We investigate the linear dynamics of an electromagnetic field propagating in curved spacetime in the presence of plasma. The dynamical equations are generically more involved and richer than the effective Proca equation adopted as a model in previous work. We discuss the general equations and focus on the case of a cold plasma in the background of a spherically-symmetric black hole, showing that the system admits plasma-driven, quasibound electromagnetic states that are prone to become superradiantly unstable when the black hole rotates. The quasibound states are different from those of the Proca equation and have some similarities with the case of a massive scalar field, suggesting that the linear instability can be strongly suppressed compared to previous estimates. Our framework provides the first step towards a full understanding of the plasma-photon interactions around astrophysical black holes.

gr-qc↗