Bistable Profile Illumination in Pulsars B0919+06 & B1859+07

Abstract

A new single pulse behaviour has been identified in two pulsars, B0919+06 and B1859+07. Normally both starts emit bright subpulses in a region near the trailing edge of heir profile. However, occasionally these stars exhibit "events" wherein the emission longitude gradually decreases by about their profile width, remains in this position for typically tens of pulses and then gradually returns over a few pulses to the usual longitude. The effect bears some resemblance to a profile "mode change", but here the effect is gradual and episodic. When the separated profiles of the normal and "event" emission, they reveal a broad and complex profile structure in each pulsar - but one which can probably be understood correctly in terms of a single conical beam. Possibly the effect entails an extreme example of "absorption"-induced profile asymmetry, as suspected in other pulsars. Alternatively, shifting sources of illumination within the pulsar beam may be responsible.