Absolute broad-band polarization behaviour of PSR B0329+54: a glimpse of the core emission process

In this paper we report multifrequency single-pulse polarization observations of the PSR B0329+54 normal mode using the Giant Metre-wave Radio Telescope at 325 and 610 MHz and the Effelsberg Observatory at 2695 MHz. Our observations show that towards the central part of the polarization position angle traverse there is an unusual 'arc'-like structure, which comprises a broad-band 'kink' and a frequency-dependent 'spot'. The features are intimately connected with the intensity dependence of the core component: the stronger emission arrives earlier and its linear polarization is displaced farther along the 'kink'. Moreover, at high intensities, the circular polarization is ?/+ antisymmetric; the nearly complete positive circular is characteristic of the weaker, later core subpulses. We find that the 'kink' emission is associated with the extraordinary (X) propagation mode, and hence propagation effects do not appear to be capable of producing the core component's broad-band, intensity-dependent emission. Rather, the overall evidence points to a largely geometric interpretation in which the 'kink' provides a rare glimpse of the accelerating cascade or height-dependent amplifier responsible for the core radiation.