UNEXPECTED VARIATIONS IN PULSAR FLUX-DENSITIES AT MM-WAVELENGTHS

Monitoring of the flux-densities near 30 GHz are reported for four pul sars. Statistically significant variations apparently random in nature are found over 10 to 20 minutes, with rms amplitudes 20% to 100% of t he mean flux. These fluctuations are substantially above the predictio ns for interstellar scintillation at these high frequencies. The obser vations presented here comprise the first attempt towards understandin g the effects of the ISM on pulsar signals at such high frequencies, a spectral region where very little is known about the behaviour of pul sars or the ISM. When extrapolated from observations at lower frequenc ies, the four pulsars should exhibit very weak interstellar scintillat ion, with rms modulation indices from 2% to 14%. The fluctuations ther efore are attributed rather to intrinsic variation of the pulsar emiss ion at these frequencies.