A BURST OF PULSED VHE GAMMA-RAYS FROM AE AQUARII

We report the results of observations of AE Aquarii made on 25 occasio ns between July 1992 and October 1993 with the University of Durham ve ry high energy (VHE) gamma-ray telescopes at Narrabri, N.S.W., Austral ia. The observations lasted for a total of 78 h and no persistent low- level pulsed emission was detected above an energy threshold of 350 Ge V. However, an excess of pulsed VHE gamma-rays was observed on 1993 Oc tober 11. The burst lasted for approximately 4200 s and occurred at ph ase 0.62-0.74 in the 9.88 h orbital cycle. The count rate excess was s ignificant at the 0.99997 level and the periodicity was at the second harmonic of the 33 s period. The hadron-induced background in the burs t data has been suppressed by applying simple imaging techniques to th ose events recorded by an individual telescope and using trigonometric height of maximum analysis for the events recorded by two telescopes. These two independent results together provide evidence at a probabil ity of chance occurrence of 10(-7) after allowing for degrees of freed om, for the emission of a burst of pulsed gamma-rays from AE Aquarii w ith threshold energy > 350 GeV, which show the characteristic features of gamma-ray induced cascades. There is also some evidence to suggest that the brightness spectrum of Cerenkov events recorded during the b urst is harder than that recorded in a sample of the background. The f lux during the burst was approximately 4% of the total cosmic ray back ground at E > 350 GeV, corresponding to a peak pulsed luminosity of 10 (32) erg s(-1).