Origin of ultra-high-energy γ-rays from Cygnus X-3 and related sources

Abstract

Ultra-high-energy (UHE) γ-ray emission from Cygnus X-3 (refs 1,2) and the several other binary X-ray sources has been observed at γ-ray energies ≥1015 eV (refs 3–5). Also, collisionless shocks are expected to form in accretion flows onto neutron stars or black holes6. Here, therefore, we consider the diffusive shock acceleration of ions as the mechanism responsible for the γ-ray emission observed. The shock acceleration time can under reasonable assumptions be sufficiently short to allow acceleration of ions to energies near 1016 eV. We propose that the subsequent proton–proton (p–p) collisions and photodissociation of 4He can produce a flux of neutrons that escapes from the acceleration site despite the high magnetic fields (∼108 G). These neutrons, by interacting with the binary companion7,8, produce the observed UHE radiation. Radiation at such energies might be a common property of accreting binaries.