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Parametrization of the nucleus-nucleus gamma-ray production cross sections below 100 GeV/nucleon: Subthreshold pions and Hard photons

"Subthreshold pions" and so-called "hard photons" are two important channels for producing less than 1 GeV $γ$-rays and $e^\pm$ pairs from nuclear collisions with energy per nucleon below the $π$-meson production threshold. I use publicly available experimental data to parametrize these two channels' $γ$-ray and $e^\pm$ production cross sections and extend the pion contribution to these particles spectra at higher energies using their corresponding spectra from $pp$ interactions. These parametrizations are valid for collision energy $T_p\leq 100$ A GeV and agree reasonably well with the available experimental data. The new parametrizations allow, for the first time, accurate studies of astrophysical $γ$-rays below 1 GeV.