  An accelerated particle sees the Minkowski vacuum as thermally excited, which
is called the Unruh effect. Due to an interaction with the thermal bath, the
particle moves stochastically like the Brownian motion in a heat bath. It has
been discussed that the accelerated charged particle may emit extra radiation
(the Unruh radiation) besides the Larmor radiation, and experiments are under
planning to detect such radiation by using ultrahigh intensity lasers. There
are, however, counterarguments that the radiation is canceled by an
interference effect between the vacuum fluctuation and the radiation from the
fluctuating motion. In this reports, we review our recent analysis on the issue
of the Unruh radiation. In this report, we particularly consider the
thermalization of an accelerated particle in the scalar QED, and derive the
relaxation time of the thermalization.
