Long Term Variations of Pulsar Intensities

PULSAR intensity variations may be divided roughly into three classes. There are rapid variations on a time-scale of seconds to minutes which occur simultaneously over a wide range of frequency and must therefore be intrinsic to the source 1 . On a somewhat longer time-scale there are variations typically over a few minutes to a few hours which correlate only over limited bandwidths. There is strong evidence that they are caused by irregular diffraction in the interstellar medium 2,3 . Finally, there are variations on a time-scale of days to months 4 about which little is known because extended regular observations are required. In this report we present some new results obtained during timing observations carried out on a routine basis over 11 months.