Transient emission of ultra-high energy pulsed γ rays from Crab pulsar PSR0531

Abstract

A new experiment has been established to measure the energy spectrum of ultra-high energy (UHE) γ rays (E> 2,000 GeV) from a number of celestial objects. During the commissioning of the equipment several potential sources were studied, including the Crab pulsar PSR0531. UHE γ rays from this object have been observed in previous experiments to be practically all pulsed1–4, but no single observation has been of high statistical significance, and the flux values have been difficult to reproduce5. The current observations are based on 34 h of exposure between 25 September and 2 November 1981, and show overall an integral pulsed flux in agreement with our estimate of the average of previous data; most important, they also show strong evidence for a burst of pulsed emission of ∼15 min duration. There had been previous suggestions that the Crab pulsar is variable at the energy we used and at slightly lower energies1,3, but at time scales of months or days. The present work is the first to demonstrate short duration (15 min) emission and to offer an explanation of previous apparently discordant results.