Pulsar timing .III. Timing noise of 50 pulsars.

An examination of the timing behavior of 50 pulsars has shown that timing activity is more than 50% correlated with pulsar period derivative, weakly correlated with period, and uncorrelated with radio luminosity, galactic altitude, and other source parameters. An upper limit is placed on the timing noise for the binary pulsar PSR 1913 + 16 that is consistent with its small period derivative. A detailed analysis of 11 pulsars indicates that random walks in the rotational phase (2 objects), frequency (4-7 objects), and frequency derivative (2 objects) are consistent with the data. Timing activity in general (apart from pulse shape changes and the glitches of the Crab and Vela pulsars and PSR 1641-45) appears to be consistent with a random walk origin. It is shown explicitly that the frequency second derivatives (apart from that of the Crab pulsar) and apparent steps in frequency that have been reported for some pulsars are probable manifestations of random walk processes.