  Cosmic sources of gamma-ray radiation in the GeV range are often
characterized by violent variability, in particular this concerns blazars,
gamma-ray bursts, and the pulsar wind nebula Crab. Such gamma-ray emission
requires a very efficient particle acceleration mechanism. If the environment,
in which such emission is produced, is relativistically magnetized (i.e., that
magnetic energy density dominates even the rest-mass energy density of matter),
then the most natural mechanism of energy dissipation and particle acceleration
is relativistic magnetic reconnection. Basic research into this mechanism is
performed by means of kinetic numerical simulations of various configurations
of collisionless relativistic plasma with the use of the particle-in-cell
algorithm. Such technique allows to investigate the details of particle
acceleration mechanism, including radiative energy losses, and to calculate the
temporal, spatial, spectral and angular distributions of synchrotron and
inverse Compton radiation. The results of these simulations indicate that the
effective variability time scale of the observed radiation can be much shorter
than the light-crossing time scale of the simulated domain.
