Topology and Polarisation of Subbeams Associated With Pulsar 0943+10's ``Drifting''-Subpulse Emission: I. Analysis of Arecibo 430- and 111-MHz Observations

The ``drifting'' subpulses exhibited by some radio pulsars have fascinated both observers and theorists for 30 years, and have been widely regarded as one of the most critical and potentially insightful aspects of their emission. Here, we report on detailed studies of pulsar B0943+10, whose nearly coherent sequences of ``drifting'' subpulses have permitted us to identify their origin as a system of subbeams that appear to circulate around the star's magnetic axis. We introduce several new techniques of analysis, and we find that both the primary and secondary features in the star's fluctuation spectra are aliases of their actual values. We have also developed a method of tracing the underlying pattern responsible for the observed sequences, using a ``cartographic'' transform and its inverse, permitting us to study the characteristics of the polar-cap emission ``map'' and to confirm that such a ``map'' in turn represents the observed sequence. We apply these techniques to the study of three different Arecibo observations. The ``B''-mode sequences are consistent in revealing that the emission pattern consists of 20 subbeams, which rotate around the magnetic axis in about 37 periods or 41 seconds. Even in the ``Q'' mode sequence, we find evidence of a compatible circulation time. The similarity of the subbeam patterns at different radio frequencies strongly suggests that the radiation is produced within a set of columns, which extend from close to the stellar surface up though the emission region and reflect some manner of a ``seeding''phenomenon at their base. The subbeam emission is then tied neither to the stellar surface nor to the field.Comment: 25 pages with 26 figures; in press in MNRA

Interface mediated interactions between particles -- a geometrical approach

Particles bound to an interface interact because they deform its shape. The stresses that result are fully encoded in the geometry and described by a divergence-free surface stress tensor. This stress tensor can be used to express the force on a particle as a line integral along any conveniently chosen closed contour that surrounds the particle. The resulting expression is exact (i.e., free of any "smallness" assumptions) and independent of the chosen surface parametrization. Additional surface degrees of freedom, such as vector fields describing lipid tilt, are readily included in this formalism. As an illustration, we derive the exact force for several important surface Hamiltonians in various symmetric two-particle configurations in terms of the midplane geometry; its sign is evident in certain interesting limits. Specializing to the linear regime, where the shape can be analytically determined, these general expressions yield force-distance relations, several of which have originally been derived by using an energy based approach.Comment: 18 pages, 7 figures, REVTeX4 style; final version, as appeared in Phys. Rev. E. Compared to v2 several minor mistakes, as well as one important minus sign in Eqn. (18a) have been cured. Compared to v1, this version is significantly extended: Lipid tilt degrees of freedom for membranes are included in the stress framework, more technical details are given, estimates for the magnitude of forces are mad

Growth Velocities of Branched Actin Networks

The growth of an actin network against an obstacle that stimulates branching locally is studied using several variants of a kinetic rate model based on the orientation-dependent number density of filaments. The model emphasizes the effects of branching and capping on the density of free filament ends. The variants differ in their treatment of side vs. end branching and dimensionality, and assume that new branches are generated by existing branches (autocatalytic behavior) or independently of existing branches (nucleation behavior). In autocatalytic models, the network growth velocity is rigorously independent of the opposing force exerted by the obstacle, and the network density is proportional to the force. The dependence of the growth velocity on the branching and capping rates is evaluated by a numerical solution of the rate equations. In side-branching models, the growth velocity drops gradually to zero with decreasing branching rate, while in end-branching models the drop is abrupt. As the capping rate goes to zero, it is found that the behavior of the velocity is sensitive to the thickness of the branching region. Experiments are proposed for using these results to shed light on the nature of the branching process.Comment: 6 figure