Fermi Gamma-ray Space Telescope

Hard Tails in Magnetars: The Resonant Compton Upscattering Model

Zorawar Wadiasingh
(Matthew G. Baring, Alice K. Harding, Peter L. Gonthier & Kun Hu)


Pulsed non-thermal quiescent X-ray emission extending between 10 keV to >150 keV has been observed in about 10 magnetars by RXTE, INTEGRAL, Suzaku, NuSTAR and Fermi-GBM. In particular, Fermi-GBM has determined the spectrum for tails in four magnetars, with those for 1E 1841-045 and 4U 0142+614 indicating significant spectral curvature at around 100 keV. For inner magnetospheric models of such hard X-ray signals, resonant Compton upscattering of soft thermal photons from the neutron star surface is the most efficient process for their generation. We present new angle-dependent hard X-ray upscattering model spectra for uncooled monoenergetic relativistic electrons injected in inner regions of pulsar and magnetar magnetospheres. These spectra are integrated over closed field bundles and volumes and obtained for different observing perspectives. Moreover, we explore opacities due to magnetic pair and photon splitting in the radiative transfer of the emissivity. The spectral cut-off energies are critically dependent on the observer viewing angles and electron Lorentz factor. We find that electrons with energies less than around 15 MeV will emit most of their radiation below 250 keV, consistent with the inferred turnovers in magnetar hard X-ray tails. Moreover, electrons of higher energy still emit most of the radiation below around 1 MeV, except for quasi-equatorial emission locales for select pulses phases. Phase-resolved Fermi-GBM data in the 30-250 keV window will prove helpful in constraining the model geometry. The emission exhibits strong phase-dependent polarization above around 30 keV. Polarimetric diagnostics on emission geometry thereby define science agendas for future hard X-ray polarimeters and Compton telescopes.