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

Abstract: This paper compares and contrasts the emission of two high nulling fraction pulsars, PSRs J1738-2330 and J1752+2359. In both pulsars the emission bursts appear in a quasi-periodic fashion with typical separations of several hundred pulses, and in J1738-2330 there is evidence of two underlying periodicities with memory persisting for at least 11 bursts. By 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 the last pulses of J1752+2359 bursts 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 (IBPs) 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. They may 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.