Photon absorption in a magnetized vacuum and formation of cyclotron-annihilation lines in γ-emission of neutron stars

Abstract

We study cyclotron and polarization spectral features formed by a magnetized vacuum in γ-radiation of neutron stars at frequencies just above the one-photon annihilation (absorption) resonance. We propose a new method for determination of the neutron-star magnetic field based on observation of the fundamental and first cyclotron-annihilation resonances due to pair creation in the ground and first excited states (Landau levels). For the dipole model of the stellar magnetic field, we show that detection of the neighboring lines in absorption and in emission at MeV energies may provide direct evidence of a magnetic field ∼2.5–4×10 8 T within the source which can radiate at large angles to the local magnetic field. This method can be available, perhaps, already in the era of INTEGRAL and SWIFT missions.