The topology and polarization of sub-beams associated with the 'drifting' sub-pulse emission of pulsar B0943+10-I. Analysis of Arecibo 430- and 111-MHz observations

Abstract

The ‘drifting’ sub‐pulses 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. Moreover, Ruderman & Sutherland, in their classic model, suggested that such regular modulation was produced by a system of sub‐beams, rotating around the magnetic axis under the action of E×B drift. Such ‘drift’ sequences have thus been thoroughly studied in a number of pulsars, but it has proven difficult to verify the rotating sub‐beam hypothesis, and thus to establish an illuminating connection between the phenomenon and the actual physics of the emission. Here, we report on detailed studies of pulsar B0943+10, whose nearly coherent sequences of ‘drifting’ sub‐pulses have permitted us to identify their origin as a system of sub‐beams that appear to circulate around the magnetic axis of the star. We introduce several new techniques of analysis, and we find that both the primary and secondary features in the fluctuation spectra of the star 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 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: a 1992 430‐MHz sequence which includes a transition from the highly organized ‘B’ profile mode of the star to its disorganized ‘Q’ mode; a 1972 430‐MHz ‘B’‐mode sequence; and a 1990 111‐MHz ‘B’‐mode sequence. The ‘B’‐mode sequences are consistent in revealing that the emission pattern consists of 20 sub‐beams, which rotate around the magnetic axis in about 37 periods or 41 s. Even in the ‘Q’‐mode sequence, we find evidence of a compatible circulation time. The similarity of the sub‐beam 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 through the emission region and reflect some manner of ‘seeding’ phenomenon at their base. The sub‐beam emission is then tied neither to the stellar surface nor to the field. While the origin of the ‘memory’ responsible for the stability of the pattern over several circulation times is unknown, the hollow conical form of the average pattern is almost certainly the origin of the conal beam forms observed in most pulsars.

A Deep Campaign to Characterize the Synchronous Radio/X-Ray Mode Switching of PSR B0943+10

“…• ) seen nearly poleon (Deshpande & Rankin 2001). Its distance, based on the dispersion measure, and the Galactic electron density distribution of Cordes & Lazio (2002), is ∼630 pc.…”

Differential frequency-dependent delay from the pulsar magnetosphere

“…This rotation (caused by E × B drift) moves the sparks slightly between pulses, so the subpulses appear to drift. The rate of rotation has been measured for a few pulsars, A&A 552, A61 (2013) allowing detailed maps of the polar cap to be made (see for example, Deshpande & Rankin 2001), and modified versions of this model have been used to try to explain all of the features listed above (see e.g. Filippenko & Radhakrishnan 1982;Gil & Sendyk 2000;Gil et al 2003Gil et al , 2008.…”

The Radio and X-ray Mode-Switching Pulsar PSR B0943+10

“…One expects to see the phenomenon of drifting subpulses when the line of sight crosses nearly tangentially the hollow cone of emission which characterizes the radio beam of a pulsar. In this framework, a determination of the geometry and orientation of PSR B0943+10 was derived thanks to a detailed modeling of the average pulse profile and of the drifting subpulses [13]. These authors estimated α based on different plausible assumptions on the width of the emission cone and its frequency dependence.…”

“…• before the main pulse [13,19]. There is no consensus in the pulsar community on what causes the phenomenon of drifting subpulses, which is observed in a large number of objects.…”

“…A variety of models have been proposed based on very different scenarios including oscillating standing waves, emission regions rotating around the magnetic or the spin axis, plasma instabilities, neutron star oscillations (see, e.g., [20] and references therein). In the frame of the "rotating carousel" model [13,21], the drifting subpulses are associated with emission from 20 polar cap sparking regions that rotate around the magnetic axis due to the E × B drift effect [22,21]. A complete rotation of the carousel takes P 4 = 20/(1 − f p ) = 40.7 s (i.e.…”