  A bosonic field impinging on a rotating black hole can be amplified as it
scatters off the hole, a phenomena known as superradiant scattering. If in
addition the field has a non-zero rest mass then the mass term effectively
works as a mirror, reflecting the scattered wave back towards the black hole.
In this physical system, known as a black-hole bomb, the wave may bounce back
and forth between the black hole and some turning point amplifying itself each
time. Consequently, the massive field grows exponentially over time and is
unstable. Former analytical estimations of the timescale associated with the
instability were restricted to the regimes $M\mu>>1$ and $M\mu<<1$, where $M$
and $\mu$ are the masses of the black hole and the field, respectively. In
these two limits the growth rate of the field was found to be extremely weak.
However, subsequent numerical investigations have indicated that the
instability is actually greatest in the regime $M\mu=O(1)$, where the previous
analytical approximations break down. Thus, a new analytical study of the
instability timescale for the case $M\mu=O(1)$ is physically well motivated. In
this Letter we study analytically for the first time the phenomena of
superradiant instability (the black-hole bomb mechanism) in this physically
interesting regime -- the regime of greatest instability. We find an
instability growth rate of $\tau^{-1}=\omega_I=1.7\times 10^{-3}M^{-1}$ for the
fastest growing mode. This instability is four orders of magnitude stronger
than has been previously estimated.
