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Abstract

The coupling between scalar and vector fields has a long and interesting
history. Axions are one key possibility to solve the strong CP problem and
axion-like particles could be one solution to the dark matter puzzle. Given the
nature of the coupling, and the universality of free fall, nontrivial important
effects are expected in regions where gravity is strong. Here, we show that
i. A background EM field induces an axionic instability in flat space, for
large enough electric fields. Conversely, a homogeneous harmonic axion field
induces an instability in the Maxwell sector. When carried over to curved
spacetime, this phenomena translates into generic instabilities of charged
black holes (BHs).
ii. In the presence of charge, BH uniqueness results are lost. We find
solutions which are small deformations of the Kerr-Newman geometry and hairy
stationary solutions without angular momentum, which are `dragged' by the
axion. Axion fields must exist around spinning BHs if these are immersed in
external magnetic fields. The axion profile can be obtained perturbatively from
the electro-vacuum solution derived by Wald.
iii. Ultralight axions trigger superradiant instabilities of spinning BHs and
form an axionic cloud in the exterior geometry. The superradiant growth can be
interrupted or suppressed through axionic or scalar couplings to EM. These
couplings lead to periodic bursts of light, which occur throughout the history
of energy extraction from the BH. We provide numerical and simple analytical
estimates for the rates of these processes.
iv. Finally, we discuss how plasma effects can affect the evolution of
superradiant instabilities.