APS : Blasts of Light from Axions

Ikeda, Taishi; Cardoso, Vitor; Brito, Richard

doi:10.1103/PhysRevLett.122.081101

Article
Report number	arXiv:1811.04950
Title	Blasts of Light from Axions
Related title	Electromagnetic emission from axionic clouds and the quenching of superradiant instabilities
Author(s)	Ikeda, Taishi (Lisbon, CENTRA) ; Brito, Richard (Potsdam, Max Planck Inst. ; INFN, Rome ; Rome U.) ; Cardoso, Vitor (Lisbon, CENTRA ; CERN)
Publication	2019-03-01
Imprint	2018-11-12
Number of pages	6
Note	6 pages, RevTex4. v2: Overall improvement. Accepted for publication in Physical Review Letters
In:	Phys. Rev. Lett. 122 (2019) 081101
DOI	10.1103/PhysRevLett.122.081101
Subject category	hep-th ; Particle Physics - Theory ; hep-ph ; Particle Physics - Phenomenology ; astro-ph.HE ; Astrophysics and Astronomy ; gr-qc ; General Relativity and Cosmology
Abstract	The nature of dark matter is one of the longest-standing puzzles in science. Axions or axion-like particles are a key possibility, and arise in mechanisms to solve the strong CP problem but also in low-energy limits of string theory. Extensive experimental and observational efforts are actively looking for `axionic' imprints. Independently on their nature, their abundance, and on their contribution to the dark matter problem, axions form dense clouds around spinning black holes, grown by superradiant mechanisms. It was recently suggested that once couplings to photons are considered, an exponential (quantum) stimulated emission of photons ensues at large enough axion number. Here we solve numerically the classical problem in different setups. We show that laser-like emission from clouds exists at the classical level, and we provide the first quantitative description of the problem.
Copyright/License	preprint: (License: arXiv nonexclusive-distrib 1.0) publication: © 2019-2025 authors (License: CC-BY-4.0)

$Time evolution of $\Phi_{1}$ (upper panel), $FF_{0}$ (middle panel) at $r=20M$ and the energy flux (bottom panel) for an axion with mass $M\mu=0.2$ around a BH with $a=0.5M$. The coupling constant is super-critical with $k_{\rm axion}A_0=0.3$. The initial EM profile is described by $(E_0/A_0,w/M)=(10^{-3},5),(10^{-3},20),(10^{-4},5)$ for run $7, 8, 9$ respectively, and $r_0=40$. The overall behavior and growth rate of the instability at large timescales are insensitive to the initial conditions.$ $Time evolution of $\Phi_{1}$ (upper panel), $FF_{0}$ (middle panel) at $r=20M$ and the energy flux (bottom panel) for an axion with mass $M\mu=0.2$ around a BH with $a=0.5M$. The coupling constant is super-critical with $k_{\rm axion}A_0=0.3$. The initial EM profile is described by $(E_0/A_0,w/M)=(10^{-3},5),(10^{-3},20),(10^{-4},5)$ for run $7, 8, 9$ respectively, and $r_0=40$. The overall behavior and growth rate of the instability at large timescales are insensitive to the initial conditions.$ $Time evolution of the monopole part of the EM scalar $F_{\mu\nu}F^{\mu\nu}$ for a coupling $M\mu=0.2$ at $r=20M$ when $k_{\rm axion}A_{0}=0.01$ (upper panel) and $k_{\rm axion}A_{0}=0.4$ (lower panel), in a Minskowski background. The scalar field is kept fixed and described by \eqref{Eq.axion cloud initial data}. The initial profile is described by $(E_{0}/A_{0},w/M,r_{0}/M)=(0.001,5.0,40.0)$, but the qualitative features of the evolution are independent on these. For small couplings $k_{\rm axion}A_0$ there is no instability and the initial EM fluctuation decays exponentially. For large couplings, on the other hand, an exponential growth ensues.$ Show more plots

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