A. G. Gregory

Prospects for 1019eV cosmic ray studies in South Australia

Abstract We discuss some of the important elements necessary for the successful operation of a very large cosmic ray array. Some of these points are illustrated using a detector Monte Carlo. The importance of composition measurement is stressed, particularly in regard to anisotropy interpretation. We review some of the issues connected with the other uncertainty in anisotropy studies, the galactic magnetic field, and show that with reasonable assumptions about its large scale structure, an experimental site in the southern hemisphere would have a good view of both the Virgo cluster region and the Magellanic Clouds. The features of a possible site at Woomera in South Australia are discussed. Finally, some technical aspects of solar power and communication technology are reviewed. Some options for the array control, data collection procedures and system timing are outlined.

Measurement of the cosmic-ray energy spectrum between 3×1015 and 3×1016 eV using a photon-density spectrum technique

The authors describe a new experiment to measure the cosmic-ray energy spectrum in the astrophysically interesting energy range from 3*1015 eV, to 3*1016 eV, close to the knee in the spectrum. The technique used is a novel variation of the photon-density spectrum technique in which the use of two separated detectors enables the energy spectrum to be investigated independently of any assumptions about the primary mass composition. The results they present are in reasonable agreement with earlier measurements in this energy range and, in conjunction with measurements at lower energies, may indicate the presence of a bump or kneecap in the spectrum. The experiment may also be used to infer gross features of the primary mass composition, and preliminary results indicate the potential of the technique.

Very High Energy Gamma Ray Astronomy at White Cliffs Solar Power Station

Three mirrors of the White Cliffs Solar Power Station are currently being used for very high energy γ -ray Astronomy while the University of Adelaide very high energy γ -ray telescope is being designed. Use is made of fast-timing to obtain γ -ray arrival directions to an accuracy approaching 1 °. The experimental arrangement and operation of the telescope is described and our current observing program is outlined.

A Search for Pulsed Gamma Emission from Two Pulsars

We have recently used the atmospheric air-shower Cerenkov technique in an attempt to observe pulsed gamma radiation from two southern pulsars, PSR 0833—45 and MP 0959. Northern hemisphere observers do not agree whether the pulsars CP 0950, CP 1133 and CP 1919 are sources of gamma emission, either pulsed or uniform in time.

The CANGAROO Project: Very High Energy Gamma-ray Astronomy at Woomera

In this paper the Very High Energy (VHE) gamma-ray astronomy program at the University of Adelaide is described. VHE gamma rays with energies above ~5 × 10 11 eV are observed using the atmospheric Cerenkov technique. Results from the first three years observations at Woomera and the current upgrading of the telecope are described. The CANGAROO project, a collaboration between the University of Adelaide and a number of Japanese institutions, is also introduced.

A search for prompt ultra-high-energy gamma rays from supernova 1987a

A search has been made using the Buckland Park air shower array for evidence of any excess of events from the direction of the recent supernova in the Large Magellanic Cloud. Upper limits resulting from this search and their significance are discussed in this paper. Introduction Supernovae play a particularly important role in high energy astrophysics since they represent a phenomenon having clear associations with high energy particles. They have long been considered to be potentially significant sources of very high energy cosmic rays (Colgate and Johnson 1960). It is natural therefore to look for very energetic phenomena associated with supernova 1987A and we have searched for evidence for high energy phenomena at about the time of its discovery. Phenomena such as prompt electromagnetic pulses including X-rays and gamma-rays have been proposed for supernova outbursts. It is believed that such phenomena are more likely to manifest themselves more strongly some months after the outburst (Gaisser and Stanev 1987). It is nonetheless important to determine what limits can be set for prompt radiation in the outburst period so that the models may be properly tested. We have used the Buckland Park air shower array to search for evidence for ultra-high energy (UHE) gamma-ray events at or about the time of the outburst as indicated by the neutrino observations. Any observed events would presumably be originated by UHE gamma-rays since charged cosmic rays would be scattered and delayed by galactic and intergalactic magnetic fields. We have searched with a burst detector for evidence of any burst of events in coincidence with the neutrino burst and have searched our routinely recorded directional data for any possible directional excess over a period ~3 weeks before and after the outburst. Detectors The Buckland Park air shower array consists of 31 scintillator particle detectors. It detects primary particles with typical energies of —10 eV within an area of ~ 3 x 10 m. The array, which is situated at sea level at a latitude of 35°S, has been described elsewhere (Crouch et al. 1981, Clay et al. 1985) and its use as a gamma-ray telescope discussed by Ciampa et al. (1986). It is designed for continuous operation but is susceptible to occasional power failures and requires to be reset after such occurrences. A power failure occurred on the day prior to the observation of the neutrino bursts. In normal operation the array triggers on air showers, with a particle size threshold of — 3 x 10 particles, at a rate of ~ 1 event/10 s. In this mode it has a dead time after each event of — 0.5 s. A simple data collection system also operates to search for any possible bursts (Bruce et al. 1985) with essentially zero (4 /AS) dead time. Due to the power failure, normal data collection was not operating at the time of the neutrino bursts but the burst detector was operational. Burst Detector Results The burst detector records the occurrence of any two array events within an interval of 0.5 s. It measures the time spacing of the events (to ± 4 /is) and also records the absolute time (to ± 5 s). It is constructed to avoid significant dead time effects for bursts containing up to 16 events by employing temporary fast data storage for event information. We have examined our burst data for the day of 23 February 1987 (Central Australian Summer Time) and find no evidence for any excess of potential bursts. There were 89 potential burst events recorded on 23 February compared to an expectation of 95 burst events by chance and, on that day, there were no unexpected sequences of events constituting a burst. There are two times on the day of 23rd February which are of particular interest. They are 2h 52m UT when a neutrino burst was observed at Mt. Blanc by Aglietta et al. (1987) and 7h 35m UT when a neutrino burst was observed independently by the 1MB collaboration (Bionta et al. 1987) and at Kamioka (Hirata et al. 1987). We have no record of any potential UHE gamma ray bursts at either of these times. The closest observed records we have are at 2h 48m 25s UT and at 7h 23m 34s UT respectively. At the two times of interest, the supernova was at zenith angles of 65 ° and 42° and the array had corresponding energy thresholds of 5 x 10 eV and ~2x 10 eV. If there had been a gamma ray burst associated with a neutrino burst and of a similar (—10 s) duration we would have expected to have detected a potential burst signal if more than four gamma-rays had been detected by the array. With a knowledge of the array collecting area characteristics, we can place limits on the flux of UHE gamma-rays in such a burst coincident with a neutrino burst of ~5 particles (> 10 eV) over an area of 5000 m for the Mt. Blanc time and ~ 5 particles (> 3 x 10 eV) over an area of 8000 m for the IMB/Kamioka time. Assuming a distance of 50 kpc to the supernova, these give limits of total radiated energy in any prompt gamma-ray burst of 2 x 10 erg or 4 x 10 erg above the respective observational energy thresholds. Directional analysis results The array burst recorder merely observes the time of occurrence of any bursts, it does not discriminate in terms of direction, except by atmospheric collimation. Apart from the period 22nd February to 24th February, the full array with directional analysis operated normally over the period of discovery of the supernova. We have analysed our recorded data over seven weeks (3 February—27 March) covering the supernova discovery https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1323358000021883 Downloaded from https://www.cambridge.org/core. IP address: 54.70.40.11, on 21 Apr 2019 at 06:38:46, subject to the Cambridge Core terms of use, available at 74 Bird et al. Proc. ASA T{]) mi AZIMUTHAL ANGLE FROM SOURCE Figure 1 —The distribution of air shower events observed around the apparent direction (marked by the cross) of Supernova 1987A from 3 February to 27 March 1987. The array angular uncertainty differs somewhat between zenith and azimuthal directions and depends on the zenith angle of the source at any given time. The characteristic uncertainties in angle for the data in this figure would be ~3° . in order to determine whether or not any UHE signal was present in the early supernova stages. Figure 1 shows directional data for the seven weeks of observation from an area around the supernova in order to determine whether or not there is any clear excess in the direction of the supernova. There is no simple clustering observable with the data presented in this way. At the declination of the Large Magellanic Cloud, the array, which has an angular resolution of about 1° under ideal conditions, has a foreshortened appearance to the incoming beam and its angular resolution is thus somewhat degraded in a roughly north-south declination direction. Also, the angular resolution depends on the particle size of each observed shower since this determines the number of independent detector timing measurements which are available. As a result, the angular resolution is complicated, in principle depending on unknowns such as the energy spectrum of any observable photons and the way in which photon initiated showers develop in the atmosphere. Using conventional cosmic ray showers as a guide we have derived approximations to the angular resolution as a function of zenith angle and we have used this information to follow, with the array, the diurnal path of the supernova, accepting only showers within the (varying) array angular resolution about the source. A similar procedure was followed for 23 background sources spaced at intervals of one hour in right ascension to give a background for comparison. We find that, for the period of three weeks before the supernova, we observed 55 on-source events compared to a background of 49.2 + 1.5 (an upper limit of 20 excess events at the 95% confidence level) and in the four weeks following the supernova we observed 62 on-source events compared to a background of 60.5 ±1.6 (an upper limit of 18 excess events). These data were obtained above a photon energy threshold of -10 1 5 eV and, assuming a distance of 50 kpc to the supernova, correspond to an upper limit of 10 erg s to the flux at source above this threshold.

Absolute calibration and intercalibration of photomultipliers for the measurement of Cherenkov light intensities

Abstract We describe a technique for the absolute calibration of photomultipliers for the detection of lighthaving a Cherenkov spectrum. This technique utilizes the Chernekov light generated by single vertical cosmic ray muons in a horizontal ultraviolet transmitting glass plate. For use at a field station, a similar, but less time consuming, technique employing the ungated spectrum from a perspex (acrylic) cylinder is described. We discuss the use of temperature-stabilized LEDs for continuous monitoring of gain changes due to factors such as photomultiplier fatigue or temperature variations.

First Test Data from the CANGAROO Project for Stereo Čerenkov Imaging

The CANGAROO project incorporates two Cerenkov imaging telescopes at Woomera to obtain stereo images of very high-energy gamma-ray (and cosmic-ray) showers. The first stereo observations, with one imaging system, were made in March 1992, and preliminary stereo imaging observations began in July 1992. This paper describes the stereo imaging technique, the sources under investigation, and the indications from the first data sets.

The Adelaide Very High Energy Gamma-Ray Astronomy Programme

The University of Adelaide group is in the final stages of designing a new Y.H.E. gamma-ray telescope. The telescope will be located at Woomera, South Australia, and should be operational in 1987. The main features of the proposed telescope are described. Before completion of the new telescope, V.H.E. observations have already started using three mirrors of the White Cliffs Solar Power Station. The operation and performance of this system as a gamma-ray telescope is described.

The Development of UAT Rocket Instrumentation for X-ray Astronomy

The Universities of Adelaide and Tasmania (UAT) have now collaborated in the preparation of four experiments on British Skylark rockets. Two independent X-ray detectors of total sensitive area 40 cm 2 were flown on each of two rocket flights launched in April, 1967. The most significant result of these measurements was the discovery of Cen XR-2 and the measurement of the variation in its intensity and spectrum. The third flight, launched in December 1967, carried three X-ray detectors of total area 140 cm 2 . One of the main results from this flight, evidence for a new X-ray source at high galactic latitude, will be presented in the following paper.