Fermi Gamma-ray Space Telescope

Constraints on Magnetar's Maximum Visible Energies from Opacities for Photon Splitting and Pair Creation

Kun Hu
(Matthew G. Baring)


Over the last decade, the GBM experiment on the Fermi Gamma-Ray Space Telescope has detected hundreds of magnetar flares. It has provided us with a suite of spectra with no emission extending above around 1 MeV. Magnetic pair creation is expected to attenuate high-energy photons emitted in the magnetosphere of magnetars. Magnetic photon splitting, a process where a single photon splits into two photons, is also important for the attenuation of the radiation below the pair threshold, especially for magnetars. We explore the opacities of these two processes in the inner magnetospheres of magnetars by calculating the attenuation lengths, and determining escape energies, which are the maximum energies for transparency in the magnetosphere. These computations provide upper bounds of a few MeV or less to the visible energies for magnetars for locales within 1-10 stellar radii of the surface. These constraints apply not only to Fermi-GBM flares but also to their quiescent hard X-ray tail emission. Any spectral cutoffs due to attenuation should be strongly polarized, providing interesting signatures for future MeV-band missions such as the AMEGO concept.