Further Search for High Energy Gamma Rays from CP 1133

Abstract

LAST year¹ 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 × 10¹³ eV. A similar, apparently positive effect from this source was reported by O'Mongain et al.² at an energy 3 × 10¹² eV; their observations could not distinguish between a pulsed or a continuous flux. The observations of Fazio et al.³, 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 receivers⁴. 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 fields⁵. Apparao⁶ has pointed out the importance that the observation of any γ-ray flux, in conjunction with the low limits for any optical emission⁷, 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.