High-energy cosmic ray anisotropies

The cosmic-ray group at the University of Tasmania has operated a number of muon telescopes at the Poatina underground cosmic ray station (latitude 41.8¬∞S, 357 hg cm-2 below the top of the atmosphere) since 1972. The most recently constructed telescopes comprise four trays of 2 metre long proportional counters arranged to give a total effective collecting area of approximately 4 m2. The muon threshold energy for vertical incidence at the depth of the station is 100 GeV corresponding to a median primary energy of 1.2 TeV. In 1983 the recording system of one telescope was upgraded to register the arrival direction of incident particles. This system utilizes the telescope configuration which provides an angular resolution for vertically incident particles of approximately 50 x 50‚ For the current project, the same recording system was further upgraded with the construction of a high-precision temperature controlled clock which provides the arrival time of incident particles to the nearest millisecond. Data collected over the period August 1987 to January 1989 have been used to search for possible narrow-angle anisotropies in the declination band 20¬∞N to 90¬∞S. No significant deviation from an isotropic distribution of arrival directions has been found. In particular, the observations fail to confirm the existence of a previously reported narrow-angle excess of particles observed at the same station; providing further evidence of the apparent limited duration of such anisotropies. An upper limit on the particle flux from possible point sources has been calculated as 6.1 x 10-9 cm-20 at the 95% confidence level. In addition, the particle arrival time data has been used to search for a time modulated underground muon flux from the direction of the Vela pulsar. The observations reveal no evidence for the existence of any steady periodic emission from the pulsar, but two marginally significant bursts of pulsed emission of duration several hours have been observed. Perhaps more significantly, these episodes of periodic enhancement were observed just before a glitch in the pulsar period; the first burst occurred 20 days before the glitch and the second just 2 hours before the initial frequency jump. If the enhancements are genuine signals, the measured underground muon flux is substantially higher than would be expected based on extrapolations from ground-based gamma-ray observations to the underground observing site using standard high-energy particle interaction models.