Axion Cloud and BH

Summary Table

Category	Progenitor	Type	Energy Mechanism	Emission Mechanism	Counterparts									References	Brief Comments
					LF Radio	HF Radio	Microwave	Terahertz	Optical/IR	X-rays	Gamma-rays	Gravitational Waves	Neutrinos
Collision / Interaction	Superradiant Axion Cloud and BH	Repeat	Laser	Synch.	Yes	--	--	--	--	--	--	Yes	--	http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1709.06581	Observational counterparts could be associated with electron-positron annihilation and/or positronium.

Definitions: LF Radio (3 MHz to 3 GHz); HF Radio (3 GHz to 30 GHz); Microwave (30 to 300 GHz)

Model Description

Spinning BHs have superradiant instabilities, and thus may be surrounded by a dense superradiant axion cloud. Similarly to how a laser can be generated by stimulated axion decay in dense axion clusters, a laser can be triggered in superradiant axion clusters. Such is known as a black hole laser powered by axion superradiant instabilities (BLAST). For a BLAST’s emission to be consistent with FRB observations, the required mass dictates that the BHs be primordial. However, because PBHs form when over densities of gas collapse, they do not have spin, and are unlikely to spin up via accretion. The merging of two PBHs is thus considered for FRBs, where the required spin and resultant superradiant instabilities can be induced. Repeating bursts could occur: the BLAST will form a photon plasma that blocks axion decay and thus halts lasing until e+/e− annihilation reduces the plasma density, and the process can restart.

Observational Constraints

Observational counterparts could be associated with e+e− annihilation and/or positronium (a bound particle of an e+ and e−), though these are not specified. GWs are also expected.