The Null Patterns of Pulsars J1649+2533 and B2310+42

We summarise an ongoing study of the nulling behaviour of the radio pulsars PSR J1649+2533 and PSR B2310+42 based on extensive observations at Arecibo Observatory and the Westerbork Synthesis Radio Telescope. The pulsars have null fractions around 20% and 9% respectively and the nulls generate prominent low-frequency features in the power spectra. For both pulsars the histograms of both null and burst lengths have a bimodal distribution. One distribution clusters at the single pulse duration and represents "pseudo-nulls": noisy or weak pulses within the respective pulsar's high-frequency cycle. However, all histograms have a strong secondary peak, and those of the null lengths of both pulsars are approximately lognormal in form, differing only in peak values and shape parameter. By contrast, the burst length histograms cannot be fitted by lognormal distributions. Their steep peaks imply an apparent 'repulsion' between nulls so that following a null sequence there is a minimum time lapse before another null sequence is likely to start. Quasi-periodicity arises when the null occurrence increases so that clusters in the null-burst cycles appear. This suggests that nulling occurs as part of a thermostat-like or hysteresis process.