Ultra high energy gamma ray point sources and cosmic ray anisotropy

Abstract

The experimental set-up at the Baksan Air Shower Array, used to detect air showers above ~0.2xl0(^14)eV, is described. An estimation of the angular resolution using the cosmic ray shadow of the Sun and the Moon gives a value of ~2.5˚ which is consistent with previous estimates from Monte-Carlo simulations. Using data from this array covering 1985-1992, a search is made for 7-ray emission from 18 candidate sources. Upper limits to the flux from these sources are stated in all cases. A periodicity search is made on data for which the excess for a single transit of a particular source is above 3(7. The results of this periodicity analysis on such days points to 4 possible observations of pulsed emission at the 95% confidence level. These are 4U0115+63 on 19.03.89, PSR19534-29 on 12.02.85, 1E2259+586 on 01.08.91 and PSR0655+64 on 12.08.89. Without confirmation from other groups however the findings are not significant enough to stand alone. A harmonic analysis has been performed on the 8 years of data and after pressure corrections and a Farley & Storey analysis to eradicate any spurious sidereal variations we find negligible evidence of 2nd or 3rd harmonic but a 1st harmonic amplitude and phase of (12.7 ±1.2) x 10(^-4) at 23.1 ± 0.3hr right ascension. When one takes into account the cosϐ effect on the sidereal anisotropy this value becomes 17.4±1.6xl0(^-4).Future developments and improvements to be undertaken at BASA, including the building of a muon detector, are outlined.