Bistable Profile Illumination in Pulsars B 0919 + 06 & B 1859 + 07

A new single-pulse behaviour has been identified in two pulsars, B0919+06 and B1859+07. Normally both stars emit bright subpulses in a region near the trailing edge of their profile. However, occasionally these stars exhibit “events” wherein the emission longitude gradually decreases, by about their profile width, remains in this position for typically several 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 separate profiles of the normal and “event” emission are studied, they reveal a broad and complex profile structure in each pulsar—but one which can probably be understood correctly in terms of the geometry 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’s beam may be responsible.