OBSERVATION OF A SIDEREAL ANISOTROPY NEAR 1500 GeV

The underground muon flux produced by primary cosmic rays with a median rigidity of 1.5 x 10 V has been monitored to search for periodic and transient rate variations. The detector has operated at an average rate of 4.47 Hz for most of the 1.2 yr interval starting 1978 Jan 1. A peak with relative amplitude (3.8 ± 0.9) x10 -4 is observed in the Fourier transform of data at the sidereal 0.9 frequency, where the errors are 68% confidence limits based upon an off-center Rayleigh distribution. After correction for solar motion the projected anisotropy ( î cosë cosä ) is (4.9 ± 0.9) x 10 -4 at right ascension 2.8 ± 1.2, where cosë = 0.75. There are no discernible peaks at the solar or antisidereal frequencies. We observe a second sidereal harmonic of amplitude (3.8 ± 0.9) x 10 -4 (the same size as the first harmonic) at right ascension 0.99.8 (or 21.8) ± 1.2.