Long-term observations of three nulling pulsars

We present an analysis of approximately 200 h of observations of the pulsars J1634-5107, J1717-4054 and J1853+0505, taken over the course of 14.7 yr. We show that all of these objects exhibit long-term nulls and radio-emitting phases (i.e. minutes to many hours), as well as considerable nulling fractions (NFs) in the range ˜67-90 per cent. PSR J1717-4054 is also found to exhibit short time-scale nulls (1-40P) and burst phases (≲200P) during its radio-emitting phases. This behaviour acts to modulate the NF, and therefore the detection rate of the source, over time-scales of minutes. Furthermore, PSR J1853+0505 is shown to exhibit a weak emission state, in addition to its strong and null states, after sufficient pulse integration. This further indicates that nulls may often only represent transitions to weaker emission states which are below the sensitivity thresholds of particular observing systems. In addition, we detected a peak-to-peak variation of 33 ± 1 per cent in the spin-down rate of PSR J1717-4054, over time-scales of hundreds of days. However, no long-term correlation with emission variation was found.