Search for y Rays above 10¹⁴ eV from Cygnus X-3 during the June and July 1989 Radio Outbursts
Cassiday, G.L. et al.1989, Phys. Rev., 63,2329.

Evidence for 10¹⁸ eV Neutral Particles from the Direction of Cygnus X-3
Cassiday, G.L. et al.1989, Phys. Rev., 62,383.