Gravitational Waves and Possible Fast Radio Bursts from Axion Clumps
  The axion objects such as axion mini-clusters and axion clouds around
spinning black holes induce parametric resonances of electromagnetic waves
through the axion-photon interaction. In particular, it has been known that the
resonances from the axion with the mass around $10^{-6}$eV may explain the
observed fast radio bursts (FRBs). Here we argue that similar bursts of high
frequency gravitational waves, which we call the fast gravitational wave bursts
(FGBs), are generated from axion clumps with the presence of gravitational
Chern-Simons (CS) coupling. The typical frequency is half of the axion mass,
which in general can range from kHz to GHz. We also discuss the secondary
gravitational wave production associated with FRB, as well as the possible host
objects of the axion clouds, such as primordial black holes with typical masses
around $10^{-5}M_{\odot}$. Future detections of FGBs together with the observed
FRBs are expected to provide more evidence for the axion.
