Further Search for High Energy Gamma Rays from CP 1133

Abstract

Last year we reported results of observations in April 1968 on the pulsar CP 1133 (ref. 1) which could have been compatible with the emission of a pulsed flux of γ-rays of ~ 2 × 10^(−8) photons m^(−2) s^(−1) at an energy of about 7 × 10^(13) eV. A similar, apparently positive effect from this source was reported by O'Mongain et al. at an energy 3 × 10^(12) eV; their observations could not distinguish between a pulsed or a continuous flux. The observations of Fazio et al., however, at a similar energy, gave no evidence for either a pulsed or a continuous flux. All these experiments used broadly similar night-sky Čerenkov light receivers. Any possibility that fast light pulses might originate in the pulsar itself rather than by Čerenkov radiation in the Earth's atmosphere has been eliminated by telescope observations using coincident photo-multipliers and small angular fields. Apparao has pointed out the importance that the observation of any γ-ray flux, in conjunction with the low limits for any optical emission, has in determining the magnetic field at the source. In view of the conflicting evidence for the existence of a γ-ray flux from CP 1133 we have repeated and extended our earlier observations when this source was again favourably placed in the sky.