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 behaviour is best described by a sudden change of drift rate at the start of 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 length. (author)