On the long nulls of PSRs J1738-2330 and J1752+2359

In this paper, we compare and contrast the emissions of two high nulling fraction pulsars, PSR J1738-2330 and PSR J1752+2359. In both pulsars, the emission bursts appear in a quasi-periodic fashion with typical separations of several hundred pulses. In J1738-2330, there is evidence of two underlying periodicities with memory persisting for at least 11 bursts. In contrast, in J1752+2359, the pattern coherence is rapidly lost and the burst/null lengths appear to be selected randomly from their respective quasi-normal distributions. The typical emission bursts of J1738-2330 exhibit a steady exponential decay of on-pulse energy accompanied by a flickering emission characterized by short frequent nulls towards their end. In the bursts of J1752+2359, the flickering is absent, the decay more pronounced and the energy released during each bright phase is approximately constant. Unlike J1738-2330, the average profiles for the first and last pulses of the bursts of J1752+2359 differ slightly from the pulsar's overall profile, hinting at differences between the two pulsars in their transitions from null to burst state (and vice versa). During its long null phases, J1752+2359 is found to emit random weak inter-burst pulses whose profile peak is somewhat offset with respect to the overall average profile. Such pulses have no equivalent in J1738-2330, or in any known pulsar hitherto. These can pervade the entire emission of this pulsar and have a separate physical origin to normal pulses. On the basis of our comparison, we conclude that a pulsar's nulling fraction, even when high, remains a poor guide to its detailed subpulse behaviour, as previously found for pulsars with small nulling fractions.