On the polarisation and emission geometry of pulsar 1929+10: Does its emission come from a single pole or two poles?

Abstract

Pulsar El 1929+10 is remarkable on a number of grounds. Its narrow primary components exhibit virtually complete and highly stable linear polarisation, which can be detected over most of its rotation cycle. Various workers have been lured by the unprecedented range over which its linear polarisation angle can be determined, and more attempts have been made to model its emission geometry than perhaps for any other pulsar. Paradoxically, there is compelling evidence to interpret the pulsar's emission geometry both in terms of an aligned configuration whereby its observed radiation comes from a single magnetic-polar emission region and in terms of a nearly orthogonal configuration in which we receive emission from regions near each of its two poles. Pulsar 1929+10 thus provides a fascinating context in which to probe the conflict between these lines of interpretation in an effort to deepen our understanding of pulsar radio emission. Least-squares fits to the polarisation-angle traverse fit poorly near the main pulse and interpulse and have an inflection point far from the centre of the main pulse. This and a number of other circumstances suggest that the position-angle traverse is an unreliable indicator of the geometry in this pulsar, possibly in part because its low level 'pedestal' emission makes it impossible to properly calibrate a polarimeter which correlates orthogonal circular polarisations. Taking the interpulse and main-pulse comp. I1 widths as indicators of the magnetic latitude, it appears that pulsar 1929+10 has an α value near 90˚ and thus has a two-pole interpulse geometry. 'This line of interpretation leads to interesting and consistent results regarding the geometry of the conal components. Features corresponding to both an inner and outer cone are identified. In addition, it appears that pulsar 1929+10-and a few other stars-have what we are forced to identify as a 'further-in' cone, with a conal emission radius of about 2.3˚/P½ Secondarily, 1929+ 10's nearly complete linear polarisation provides an ideal opportunity to study how mechanisms of depolarisation action on a pulse-to-pulse basis. Secondary-polarisation-mode emission appears in significant proportion only in some limited ranges of longitude, and the subsequent depolarization is studied using different mode-separation techniques. The characteristics of the two polarisation modes are particularly interesting, both because the primary mode usually dominates the secondary so completely and because the structure seen in the secondary mode appears to bear importantly on the question of the pulsar's basic emission geometry. New secondary-mode features are detected in the average profile of this pulsar which appears independent of the main-pulse component structure and which apparently constitute displaced modal emission. Individual pulses during which the secondary-mode dominates the primary one are found to be considerably more intense than the others and largely depolarised. Monte-Carlo modeling of the mode mixing in this region, near the boundary of comps. 11 and 111, indicates that the incoherent interference of two fully and orthogonally polarised modes can adequately account for the observed depolarisation. The amplitude distributions of the two polarisation modes are both quite steady: the primary polarisation mode is well fitted by a X2 distribution with about nine degrees of freedom; whereas the secondary mode requires a more intense distribution which is constant, but sporadic.