Inverse Compton γ rays from Cyg X-3?

THE detection of ∼ 100-MeV γ rays from Cyg X-3 has been reported from recently published SAS-2 data (R. C. Lamb et al.17). The γ-ray luminosity is ∼ 1037 erg s−1 assuming the source is ∼ 10 kpc distant. Cyg X-3 is well known as an X-ray and infrared source, and a source of non-thermal radio outbursts1 (Lx ∼ 1038, LIR ∼ 1037, Lradio ∼ 1033–1035 erg s−1). The infrared, X-ray and γ-ray fluxes all display a 4.8-h modulation in phase with one another1, which we shall presume to be attributable to binary motion2,3. We note here that inverse Compton scattering of ∼ 100 MeV relativistic electrons, in a region between the X-ray source and its companion, by the plentiful X rays, provides a mechanism for the γ-ray emission and its observed modulation.