Journal
ASTROPHYSICAL JOURNAL
Volume 854, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/aaa460
Keywords
pulsars: general; radiation mechanisms: nonthermal; scattering; stars: magnetars; stars: neutron; X-rays: general
Categories
Funding
- South African National Research Foundation
- National Science Foundation [AST-1009725, AST-1009731]
- NASA Astrophysics Theory and Fermi Guest Investigator Programs [NNX13AQ82G, NNX13AP08G]
- NASA Astrophysics Theory Program [NNX13AO12G]
- Michigan Space Grant Consortium
- NASA Astrophysics Theory Program
- NASA [NNX13AQ82G, 464861, NNX13AP08G, 464864] Funding Source: Federal RePORTER
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Hard, nonthermal, persistent pulsed X-ray emission extending between 10 and similar to 150 keV has been observed in nearly 10 magnetars. For inner-magnetospheric models of such emission, resonant inverse Compton scattering of soft thermal photons by ultrarelativistic charges is the most efficient production mechanism. We present angle-dependent upscattering spectra and pulsed intensity maps for uncooled, relativistic electrons injected in inner regions of magnetar magnetospheres, calculated using collisional integrals over field loops. Our computations employ a new formulation of the QED Compton scattering cross section in strong magnetic fields that is physically correct for treating important spin-dependent effects in the cyclotron resonance, thereby producing correct photon spectra. The spectral cutoff energies are sensitive to the choices of observer viewing geometry, electron Lorentz factor, and scattering kinematics. We find that electrons with energies less than or similar to 15 MeV will emit most of their radiation below 250 keV, consistent with inferred turnovers for magnetar hard X-ray tails. More energetic electrons still emit mostly below 1 MeV, except for viewing perspectives sampling field-line tangents. Pulse profiles may be singly or doubly peaked dependent on viewing geometry, emission locale, and observed energy band. Magnetic pair production and photon splitting will attenuate spectra to hard X-ray energies, suppressing signals in the Fermi-LAT band. The resonant Compton spectra are strongly polarized, suggesting that hard X-ray polarimetry instruments such as X-Calibur, or a future Compton telescope, can prove central to constraining model geometry and physics.
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