J. E. Dickinson

350 GeV gamma rays from AE Aqr

Abstract We report here the results of VHE γ-ray observations made during 1990 and 1991 using the Durham University Mark III and IV VHE γ-ray telescopes at Narrabri, N.S.W., Australia. There are 4 separate datasets, each obtained from observations over typically 10 days during a lunar dark period. The dataset from October 1990 exhibits periodicity at a chance probability of 1 × 10 -4 , and a similar effect is noted in the dataset from August 1991 at 1.6 × 10 −2 . In both cases, most of the power is in the second harmonic; the period is constant over at least 10 days and similar to that seen at optical wavelengths. No significant signal was detected in the September 1991 and October 1991 datasets. In addition, a strong and highly significant outburst lasting about 1 minute on 13 October 1990 and detected by two independent telescopes is reported. This outburst shows the 33 s periodic pulse structure and the pulses are in phase with those observed during the strongest sustained periodic signal in our data, from an observation lasting 3 h and taken 48 h previously. A second, weaker burst was also observed during August 1991.

A burst of pulsed VHE gamma rays from AE Aquarii

Abstract We report the results of observations of AE Aquarii made on 25 occasions between July 1992 and October 1993 with the University of Durham very high energy (VHE) gamma-ray telescopes at Narrabri, N.S.W., Australia. The observations lasted for a total of 78 h and no persistent low-level pulsed emission was detected above an energy threshold of 350 GeV. However, an excess of pulsed VHE γ-rays was observed on 1993 October 11. The burst lasted for approximately 4200 s and occurred at phase 0.62–0.74 in the 9.88 h orbital cycle. The count rate excess was significant at the 0.99997 level and the periodicity was at the second harmonic of the 33 s period The hadron-induced background in the burst data has been suppressed by applying simple imaging techniques to those 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 probability of chance occurrence of 10−7, after allowing for degrees of freedom, for the emission of a burst of pulsed γ-rays from AE Aquarii with threshold energy > 350 GeV, which show the characteristic features of γ-ray induced cascades. There is also some evidence to suggest that the brightness spectrum of Cerenkov events recorded during the burst is harder than that recorded in a sample of the background. The flux during the burst was approximately 4% of the total cosmic ray background at E> 350 GeV, corresponding to a peak pulsed luminosity of 1032 erg s−1

TeV gamma rays from Geminga

The bright gamma -ray source. Geminga has been identified as an X-ray and gamma -ray pulsar with a pulse period of 237 ms. Geminga was observed with the University of Durham very high-energy (VHE) gamma -ray telescopes at Dugway, Utah, in 1983. The analysis of these data has revealed pulsations at the X-ray/ gamma -ray pulse period, with the VHE gamma -ray pulses being in phase with the lower-energy gamma -ray pulses. The VHE and HE gamma -ray light curves are connected with a significance for positive correlation of 6*10-4. The VHE gamma -ray signal corresponds to a flux of about 3*10-11 cm-2 s-1 at energies greater than 1000 GeV, and a luminosity of 6*1030 erg s-1.

The University of Durham Mark 6 Gamma Ray Telescope

The design, construction and operation of the University of Durham ground-based gamma ray telescope is discussed. The telescope has been designed to detect gamma rays in the ≤ 200 GeV region and to achieve good discrimination between gamma ray and hadron initiated showers in the higher energy region (≳ 300 GeV). The telescope was commissioned in 1995 and a description of its design and operation is presented, together with a verification of the telescopes performance.

The effect of the geomagnetic field on TeV gamma -ray detection

Very high energy (VHE) gamma -ray astronomy using the ground based atmospheric Cerenkov technique is an important new field of high energy astrophysics and a range of VHE gamma -ray sources has been detected at various levels of significance by a number of experimental groups. Outstanding evidence for gamma -rays from the Crab Nebula was obtained by the Whipple collaboration. This involved the use of a sophisticated technique based on computer simulations to select gamma -ray-like events from the proton induced background When this same technique was applied to data from other sources detected using an earlier version of the Whipple telescope, the evidence for a gamma -ray signal disappeared. The inability to reconcile the Whipple result from the Crab Nebula with both the failure of the Whipple camera to detect other objects and the detections made with other non-imaging telescopes has caused some concern at a critical time in the evolution of this branch of high energy astrophysics. In an attempt to reconcile these results, the authors have made a detailed study of the effects of the geomagnetic field on the electron-photon cascades. They have simulated the Cerenkov light produced by individual protons and gamma -rays at different orientations in the atmosphere, and made measurements of the sensitivity of the observations to the geomagnetic field. Indications are that these geomagnetic effects may be a cause of the differing results.

A ground based installation for the detection of gamma rays with energy 50–2000 GeV

This paper describes the University of Durham ground‐based gamma ray programme for the 1990s, and improvements being made to the Cerenkov Observatory at Narrabri, N. S. W.