Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Conversion of relic gravitational waves into photons in cosmological
  magnetic fields   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.

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The categories of this paper are: hep-ph
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys
30
The categories of this paper are: q-alg
26
The categories of this paper are: bayes-an
36
The categories of this paper are: cmp-lg
22
The categories of this paper are: chao-dyn
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con
11
The categories of this paper are: hep-lat
12
The categories of this paper are: nucl-th
17
The categories of this paper are: q-fin
27
The categories of this paper are: comp-gas