Neutrino Radiative Decay in External Field and Medium

The current state of the theory of massive neutrino radiative decay is reviewed. By extending our previous studies, we thoroughly analyze the process of radiative decay of a massive Dirac neutrino in the strong external magnetic field H >> H-0 = m(e)(2)c(3)/e (h) over bar = 4.41 x 10(13) G in the presence of medium (degenerate electron gas), and calculate its probability. It is shown that, in the presence of dense medium, the latter quantity becomes much larger than that in the magnetic field in the case of both relativistic and nonrelativistic neutrinos. Possible astrophysical applications of the obtained results are considered.