Pulse-sequence cartography of pulsar polar-cap emission

Abstract

One nearly coherent sequence of ``drifting" pulses from PSR B0943+10 has prompted us to develop a technique for ``mapping" the pattern of polar-cap ``storms" or ``sparks" responsible for its emission. Using several different fluctuation-spectral analysis methods, we have been able both to determine the phase-modulation frequency of the ``drift" (and several of its harmonics) to unprecedented precision and to identify a superposed amplitude modulation corresponding to a stable set of ``sparks". If the individual-pulse sequence is produced by successive sight-line traverses through this stable, rotating ``spark" pattern, then a transform relationship exists between them. We have developed procedures for computing both this ``cartographic" transform and its inverse, in order first to study the character of the polar-cap emission ``map" and then to confirm that this ``map" in turn produces the observed pulse sequence. The ``cartographic" transform depends critically on the geometry---not only on the magnetic latitude alpha and sight-line impact angle beta , but on the longitude of the magnetic axis phi_0 and the sign of beta . When combined with the spacing P_2 and the polarization-angle rotation chi_2 between subpulses, the sense of rotation of the star and of the ``spark" pattern can be determined. The ``inverse" transform provides a powerful ``closure" tool to verify and refine the "input" geometry. However, the most significant uses may come through studying what features of the observed sequence are lost in the process of recreating the sequence from the polar-cap ``map" of a sequence. Note, for instance, that a rotating pattern of ``sparks" will almost always produce a sequence which will average to a symmetrical profile. For those pulsars, like 0943+10, with both prominent ``drifting" subpulses and highly asymmetic average profiles, the "cartographic" transform provides the technical basis for important new insights into the physics of the pulsar emission region.