The beam topology and dynamic emission properties of pulsar B0943+10 -- VI. Discovery of a 'Q'-mode precursor and comparison with pulsar B1822-09

This paper reports new observations of pulsars B0943+10 and B1822--09 carried out with the Arecibo Observatory (AO) and the Giant Metrewave Radio Telescope (GMRT), respectively. Both stars exhibit two stable emission modes. We report the discovery in B0943+10 of a highly linearly polarized precursor component that occurs primarily in only one mode. This emission feature closely resembles B1822-09's precursor which also occurs brightly in only one mode. B0943+10's other mode is well known for its highly regular drifting subpulses that are apparently produced by a rotating carousel system of 20 beamlets. Similary, B1822-09 exhibits subpulse-modulation behavior only in the mode where its precursor is absent. We survey our 18 hours of B0943+10 observations and find that the sideband-modulation features, from which the carousel-rotation time can be directly determined, occur rarely--less than 5% of the time--but always indicating 20 beamlets. We present an analysis of B1822-09's modal modulation characteristics at 325-MHz and compare them in detail with B0943+10. The pulsar never seems to null, and we find a 43-rotation-period feature in the star's Q mode that modulates the interpulse as well as the conal features in the main pulse. We conclude that B1822-09 must have a nearly orthogonal geometry and that its carousel circulation time is long compared to the modal sub-sequences available in our observations, and the mainpulse/interpulse separation is almost exactly 180 degrees. We conclude the precursors for both stars are incompatible with core-cone emission. We assess the interesting suggestion by Dyks et al. that downward-going radiation produces B1822-09's precursor emission.