  Magnetospheres of neutron stars can be perturbed by star quakes, interaction
in a binary system, or sudden collapse of the star. The perturbations are
typically in the kHz band and excite magnetohydrodynamic waves. We show that
compressive magnetospheric waves steepen into monster shocks, possibly the
strongest shocks in the universe. The shocks are radiative, i.e. the plasma
energy is radiated before it crosses the shock. As the kHz wave with the
radiative shock expands through the magnetosphere, it produces a bright X-ray
burst. Then, it launches an approximately adiabatic blast wave, which will
expand far from the neutron star. These results suggest a new mechanism for
X-ray bursts from magnetars and support the connection of magnetar X-ray
activity with fast radio bursts. Similar shocks may persistently occur in
magnetized neutron-star binaries, generating an X-ray precursor of the binary
merger. Powerful radiative shocks are also predicted in the magnetosphere of a
neutron star when it collapses into a black hole, producing a bright X-ray
transient.
