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A. K. Harding, M. G. Baring (NASA/GSFC), P. L. Gonthier (Hope College), M. Knecht (University of Illinois - Urbana/Champaign)
Growing evidence for the existence of magnetars, neutron stars with ultra-strong magnetic fields above 1013 Gauss, has sparked interest in exploring the physics of this new type of star. It was suggested (Baring & Harding 1998, ApJ, 507) that high-field pulsars and magnetars should be radio-quiet, because photon splitting, in which gamma-ray photons split into two lower energy photons, competes with and suppresses pair creation. The creation of electron-positron pairs in electromagnetic cascades is copious in pulsars with fields below 1013 Gauss, and is believed to be an essential requirement for coherent radio emission. We have modeled pair cascades in pulsars with ultra-strong fields via a Monte Carlo simulation that includes photon splitting, pair production and synchrotron radiation of created pairs. The calculation produces pair yields and output photon spectra from cascades as a function of surface magnetic field strength. We find that pair yields decrease at high field strengths, not only because of competition between pair creation and photon splitting, but because pairs are created near threshold in very low Landau states, suppressing synchrotron emission. The suppression of synchrotron emission of created pairs decreases the cascade generations and dramatically lowers the pair yields. We will discuss the importance of these results for radio-quiescence of high-field pulsars and for the high energy observability of magnetars.
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