  Short timescale variability is often associated with a black hole system. The
consequence of an electromagnetic outflow suddenly generated near a Kerr black
hole is considered assuming that it is described by a solution of a force-free
field with a null electric current. We compute charged particle acceleration
induced by the burst field. The interaction between the particle and the field
is characterized by a large dimensionless number. We show that the particle is
instantaneously accelerated to the relativistic regime by the field with a very
large amplitude. The typical maximum energy attained by a proton for an event
near a super massive black hole is $E_{\rm max} \sim 100$ TeV, which is enough
observed high-energy flares.
