The effect of nulls upon subpulse drift in PSRs 0809 + 74 and 0818 − 13

An investigation of nulling and subpulse drifting in the two pulsars PSRs 0809 + 74 and 0818 − 13 is presented from data recorded at 408 MHz. Upper limits are placed, in the case of PSR 0809 + 74, on the time constants of the decay and rise of pulse energy at nulls of 0.065 and 0.26 s respectively. Significant drifting is found to occur during nulls in this pulsar. Its behavior is best described by a sudden change of drift rate at the start of the null, followed by an exponential relaxation to the normal drift rate, with emission restarting when a fixed fraction of the recovery has taken place. A similar phenomenon is observed for PSR 0818 − 13. For both pulsars the magnitude of these events appears to be proportional to the null strength. These observations are taken as evidence that the entity that is drifting survives in some form during nulls. An attempt is made to interpret this in terms of current models.