Hard X-ray Emission by Resonant Compton Upscattering in Magnetars
Abstract
Flat spectrum, non-thermal X-ray quiescent emission extending between 10 keV and around 150 keV has been seen in a number of magnetars by RXTE, INTEGRAL, Suzaku and Fermi-GBM. For inner magnetospheric models of such hard X-ray emission, resonant Compton upscattering is anticipated to be the most efficient process for generating continuum radiation. This is because the scattering becomes resonant at the cyclotron frequency, and the effective cross section exceeds the classical Thomson value by over two orders of magnitude, thereby enhancing the efficiency of continuum production and the cooling of relativistic electrons. We present angle-dependent hard X-ray upscattering model spectra for uncooled monoenergetic relativistic electrons injected in inner regions of pulsar magnetospheres. These spectra are integrated over closed field lines and obtained for different observing perspectives. 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 observed turnovers in magnetar hard X-ray tails. Moreover, electrons of higher energy still emit most of the radiation below 1 MeV, except for very select viewing perspectives that sample tangents to field lines, thereby making it difficult to observe signals extending into the Fermi-LAT band. Our spectral computations use, for the first time, a new Sokolov and Ternov (ST) formulation of the QED Compton scattering cross section in strong magnetic fields. Such an ST formalism is formally correct for treating spin-dependent effects that are important in the cyclotron resonance.
- Publication:
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AAS/High Energy Astrophysics Division #13
- Pub Date:
- April 2013
- Bibcode:
- 2013HEAD...1312609W