NEW MEASUREMENTS OF THE 12.6 MILLISECOND PULSAR IN CYGNUS-X-3

We presented evidence (Chadwick et al.) for a 12.59 ms pulsar in Cygnus X-3 on the basis of TeV gamma-ray observations. Cygnus X-3 has been observed in 1988 using the Durham University Mark IV gamma-ray telescope, and new data show further evidence for pulsed emission at a phase in the 4.8 hr cycle and with a pulsar period and secular period derivative which are compatible with our earlier measurements. The conservative overall Rayleigh probability of uniformity of phase for this new result is 1.7 × 10-6. We also report here a reanalysis of the data from our observations of Cygnus X-3 from 1981 to 1985 using a new X-ray ephemeris of the 4.8 hr X-ray cycle. This suggests that Cygnus X-3 is producing sporadic very high energy gamma rays at a fixed time in the 4.8 hr X-ray cycle.