A 12.6-ms pulsar in Cygnus X-3

The X-ray binary Cygnus X-3 has long been suspected of containing a fast, young pulsar, but previous observations at various wavelengths have failed to provide any evidence of a signal with a periodicity of less than 4.8 h (the well-known periodicity in the X-ray and infrared wavebands which is ascribed to orbital motion of a binary system). Here we report measurements of 1,000-GeV γ-rays which suggest a pulsar period of 12.5908±0.0003 ms, for data recorded in 1983 at the times of X-ray maximum in the 4.8-h cycle. The conservative probability of such a periodicity occurring by chance is 4 × 10 −7 and, moreover, supportive evidence is provided by some earlier data.