  The relativistic form of the Zubarev density operator can be used to study
quantum effects associated with acceleration of the medium. In particular, it
was recently shown that the calculation of perturbative corrections in
acceleration based on the Zubarev density operator makes it possible to show
the existence of Unruh effect. In this paper, we present the details of the
calculation of quantum correlators arising in the fourth order of the
perturbation theory needed to demonstrate the Unruh effect. Expressions for the
quantum corrections for massive fermions are also obtained.
