Steady, periodic gamma-ray emission from accreting X-ray pulsars

Abstract

We study the production mechanisms of medium-energy γ-rays from accreting X-ray pulsars. The models concern electrons accelerated in strongly shielded accelerators above the inner part of the accretion disk whose energies are limited by curvature radiation reaction. The curvature photons are sufficiently energetic to produce secondary e ± pairs in collisions with the hard X-rays from the accretion shock front above the neutron star surface outside the acceleration regions, which are shielded from X-rays. A part of the synchrotron photons radiated by these secondary pairs is still energetic enough to produce a third generation of pairs which also emit synchrotron photons. These processes will continue until the energies of the synchrotron photons are low enough to avoid the pair production process in collision with the stellar X-rays. Summing over all generations of the surviving synchrotron photon spectra gives the model predicted γ-ray spectrum which can be expressed as 55 β -9/2 B -4/7 12(M/M ⊙) -1/7 R -3/7 6 L 9/7 37(D/10 kpc) -2/ln(E max/E min)(E γ/eV) -2 photon s -1 cm -2 eV -1. We suggest that those accreting X-ray pulsars with reported detection of transient very high energy γ-rays (e.g., Her X-1, Cen X-3, and Cyg X-3) should be detectable sources for the Gamma Ray Observatory in the energy range of 10 5-10 7 eV.