2004ApJ...600..960S

The consequences of the finite effective mass of transverse photons in plasmas due to Coulomb effects on the value of threshold energies of inelastic photon-induced cosmic interaction processes such as photodisintegration and photopair and photomeson production are investigated. In all cases, a reduction of the value of the threshold energy as compared to the vacuum case occurs. Because the effective photon mass scales with the plasma density, the strongest reduction effect is found for stellar plasmas; in particular, the photon threshold energy for photopair production in stellar cores is 46% smaller than in the vacuum case. For the dilute intergalactic space plasma, we demonstrate that the threshold energy reductions for intergalactic photopair absorption and for the Greisen-Zatsepin-Kuzmin (GZK) effect on ultra-high-energy cosmic radiation are negligibly small.