Secular changes in the flux density of PSR 0833-45

The pulsar 0833-45 associated with the Vela supernova remnant has been known as a strong source with fairly constant pulse amplitude since its discovery by Large et al. (1968). The flux density of PSR 0833-45, however, is not stable. A large increase from 2 to 8 Jy was noted in November 1972, and Vela was added to the list of low frequency variable sources which were monitored in 15 sessions at 4-week intervals during 1975/76. Seven sessions which were analyzed for the pulsar position also showed that the flux dropped 50% between October 1975 and March 1976. This change is now confirmed by the results of all 15 sessions. Variations in the flux density of PSR 0833-45 over a 12 1/2 year interval are shown in a graph. The presence of a slow periodic component in Vela and the absence of significant flux changes with periods up to 180 days, contrasts completely with the behavior of other pulsars. Thus Vela's behavior is unusual and it seems to be the only pulsar with a possible periodic change in flux density.