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Resonant conversion of high energy gravitons into photons in large scalecosmological magnetic fields at the post recombination epoch is considered. Itis shown that the probability of the resonance photon production is much higherthan the non-resonant one. As a result an observable isotropic background ofcosmic gamma rays might be created. As shown in our previous paper, an earlypopulation of primordial black holes (PBHs) prior to big bang nucleosynthesis(BBN) could be an efficient source of high frequency gravitational waves. Forthe primordial black hole mass about $10^8$ g the produced photons would be thedominant component of the soft to hard Cosmic X-ray Background (CXB) and forlower masses the spectrum is shifted down to the ultraviolet and optic.

Resonant high energy graviton to photon conversion at post recombination epoch