Magnetic pair creation transparency in gamma-ray pulsars
Story, Sarah A.
Baring, Matthew G.
Master of Science
Magnetic pair creation, Î³ [arrow right] e + e - , is a key component in polar cap models of gamma-ray pulsars, and has informed assumptions about the still poorly understood radio emission. The Fermi Gamma-Ray Space Telescope has now detected more than 100 Î³-ray pulsars, providing rich information for the interpretation of young energetic pulsars and old millisecond pulsars. Fermi observations have established that the high-energy spectra of most of these pulsars have exponential turnovers in the 1-10 GeV range. These turnovers are too gradual to arise from magnetic pair creation in the strong magnetic fields of pulsar inner magnetospheres, so their energy can be used to provide a physically motivated lower bound to the typical altitude of GeV band emission. This work computes pair creation opacities for photon propagation in neutron star magnetospheres. It explores the constraints that can be placed on the emission location of Fermi Î³-rays due to single-photon pair creation transparency below the turnover energy, as well as the limitations of this technique. These altitude bounds are typically in the range of 2-6 neutron star radii for the Fermi pulsar sample, and provide one of the few possible constraints on the emission altitude in radio quiet pulsars that do not possess double-peaked pulse profiles.
Pure sciences; Astrophysics