  With mass ratio larger than $\sim 5$ (which depends on the black hole spin
and the star radius), star disruption is not expected for a black hole merging
with a neutron star during the final plunge phase. In the late inspiral stage,
the black hole is likely charged as it cuts through the magnetic field carried
by the neutron star, leaving a temporarily charged black hole after merger. The
unstable charged state of the remnant black hole rapidly neutralizes by
interacting with the surrounding plasma and photons, which we investigate in
first principle by numerically solving a coupled set of Boltzmann equations of
1+1 form for non-spinning BH background. The resulting basic picture is as
follows. Electrons and positrons are accelerated in the BH electric field,
which then lose energy to surrounding soft photons via Compton scattering; more
electrons and positrons will be created from pair production as the hard
photons colliding with soft photons, or through the Schwinger process in strong
electromagnetic fields. The cascade stops when the charged black hole accretes
enough opposite charges and becomes neutralized. We find that $\sim 10\%$
(which depends on the soft photon energy and number density) of the total
electric energy is carried away to infinity in a time interval $\sim 1$ ms by
very-high-energy ($>50$ GeV, the low energy detection threshold of the MAGIC
telescope) gamma rays whose spectrum is approximately a power law with spectral
index $\sim -2.3$. We expect the discharge picture to be true for spinning
charged BHs as well.
