Quantum mechanical calculations of synchro-curvature radiations: maser possibility

We calculate the radiative transition rates for synchro-curvature radiation to explore the possibility of maser in the environment that may occur in the magnetosphere of neutron stars (NSs). Unlike previous studies, we employ relativistic quantum mechanics, solving the Dirac equation for an electron in helical magnetic fields. Following Voisin et al. [,], we utilize adiabatic spinor rotations, under the assumption that the curvature of magnetic-field lines is much larger than the Larmor radius, to obtain the wave functions of an electron. We classify the electron states either by the spin operator projected on the magnetic field or by the helicity operator. To demonstrate that there is a regime where the true absorption rate becomes negative, we numerically evaluate the obtained formulae for some parameter values that may be encountered in the outer gaps of different types of NSs. We show that there is indeed a range of parameters for the negative true absorption rate to occur. We will also study the dependence on those parameters systematically and discuss the classical limit of our formulae. We finally give a crude estimate of the amplification factor in the same environment.