Resonant instability of axionic dark matter clumps
  Axion is a popular candidate for dark matter particles. Axionic dark matter
may form Bose-Einstein condensate and may be gravitationally bound to form
axion clumps. Under the presence of electromagnetic waves with frequency
$\omega=m_{a}/2$, where $m_{a}$ is the axion mass, a resonant enhancement may
occur, causing an instability of the axion clumps. With analytical and
numerical approaches, we study the resonant instability of axionic dark matter
clumps with infinite homogeneous mass distribution, as well as distribution
with a finite boundary. After taking realistic astrophysical environments into
consideration, including gravitational redshift and plasma effects, we obtain
an instability region in the axion density-clump size parameter space with
given mass and coupling of axions. In particular, we show that, for axion
clumps formed by the QCD axions in equilibrium, no resonant instability will
occur.
