Investigation of the unique nulling properties of PSR B0818-41

Abstract

We report on the unique nulling properties of PSR B0818-41, using the Giant Metrewave Radio Telescope (GMRT) at 325 and 610 MHz. This pulsar shows well-defined nulls, with lengths ranging from a few tens of pulses to a few hundreds of pulses. We estimate a nulling fraction of about 30 per cent at 325 MHz. Furthermore, we find the following interesting behaviour of the pulse intensities, pulse shapes, pulse widths and the drift rate, just before and after the nulls. (i) There is a clear difference between the transitions from bursts to nulls to that from the nulls to bursts. The pulsar's intensity does not switch off abruptly at the null, but fades gradually, taking ~10P₁. On the other hand, just after the nulls the intensity rises to a maximum over a short (less than one period) time-scale. (ii) While the last active pulses before nulls are dimmer, the first few active pulses just after the nulls outshine the normal ones. This effect is very clear for the inner region of the pulsar profile, where the mean intensity of the last few active pulses just after the nulls is ~2.8 times more than that for the last active pulses just before the nulls. (iii) There is a significant evolution of the shape of the pulsar's profile, around the nulls, especially at the beginning of the bursts: an enhanced bump of intensity in the inner region, a change in the ratio of the strengths of the leading and trailing peaks towards a more symmetric profile, an increase in profile width of about 10 per cent and a shift of the profile centre towards later longitudes. Some of these can be explained by a (temporary?) shift of the emission regions to different heights and/or slightly outer field lines in the magnetosphere. (iv) Just before the onset of the nulls, for about 60 per cent of the occasions, the apparent drift rate becomes slower (correlated with the gradual decrease of pulse intensity), transitioning to an almost phase stationary drift pattern. Further, when the pulsar comes out of the null, the increased intensity is very often accompanied by what looks like a disturbed drift rate behaviour, which settles down to the regular drift pattern as the pulsar intensity returns to normal. Thus, we find some very specific and well-correlated changes in the radio emission properties of PSR B0818-41 when the emission restarts after a null. These could imply that the phenomenon of nulling is associated with some kind of a 'reset' of the pulsar radio emission engine. We also present plausible explanations for some of the observed behaviour, using the Partially Screened Gap model of the inner pulsar accelerator.