Pair Cascades in Magnetar Magnetospheres

Many magnetars show hard quiescent X-ray emission components extending to a few hundred keV. This emission is likely due to resonant inverse Compton scattering (RICS) of surface thermal photons by relativistic electrons and positrons above the neutron star surface. We have developed a Monte Carlo code to compute the RICS emission from relativistic electrons with starting energies above pair threshold, traveling along closed field lines and undergoing rapid RICS energy losses. The emission is attenuated by both pair production and photon splitting in the very strong magnetic fields. The electron-positron pairs then emit both synchrotron radiation and RICS on adjacent field lines and the code can follow arbitrary generations of pairs and photons. Due to the extreme magnetic fields of magnetar magnetospheres, quantum effects are incorporated in all processes. We find that, in the absence of continued particle acceleration around the field loops, the RICS energy loss and beaming of the radiation along the local field produces a strong dependence of the spectrum and attenuation on viewing angle. The synchrotron radiation and RICS from pairs and the lower-energy photons from splitting steepen the spectra below the cutoffs. At most viewing angles, the spectra are cut off around several MeV and can show polarization signatures of photon splitting that could be detected by future telescopes with sensitivity in the MeV band.