Fermi Gamma-ray Space Telescope

Two-Photon Pair Creation Opacities in Gamma-Ray Pulsars

M. G. Baring
J. A. Barchas, S. A. Story

Abstract:

A major portion of the Fermi mission's legacy is centered on its enormous increase in the gamma-ray pulsar database. This has included the identification of exponential maximum energy turnovers in the 1-10 GeV window in the majority of Fermi pulsars. This regulation is remarkable given the wide disparities in light cylinder radii and field strengths there, and as well as disparate spin-down luminosities among the pulsar population. It is distinctly possible that magnetospheric attenuation of GeV gamma-rays via two-photon pair creation with surface X-rays can cause or contribute to the formation of these turnovers. This paper computes two-photon pair opacities for a comprehensive range of altitudes, colatitudes and surface X-ray temperatures. It is found that for the major portion of dipole magnetospheres, except near the stellar surface and in the equatorial light cylinder zones, the pair opacity is insufficient to generate the observed turnovers for typical surface temperatures. This suggests that the accelerating gap potentials do not vary substantially from pulsar to pulsar. The introduction of field line sweepback in the computations increases pair opacity substantially at altitudes near the light cylinder radius.