  Conversion of gravitational waves into electromagnetic radiation is
discussed. The probability of transformations of gravitons into photons in
presence of cosmological background magnetic field is calculated at the
recombination epoch and during subsequent cosmological stages. The produced
electromagnetic radiation is concentrated in the X-ray part of the spectrum. It
is shown that if the early Universe was dominated by primordial black holes
(PBHs) prior to Big Bang Nucleosynthesis (BBN), the relic gravitons emitted by
PBHs would transform to an almost isotropic background of electromagnetic
radiation due to conversion of gravitons into photons in cosmological magnetic
fields. Such extragalactic radiation could be noticeable or even dominant
component of Cosmic X-ray Background.
