Pulsar microstructure and its quasi-periodicities with the S2 VLBI system at a resolution of 62.5 nanoseconds

We report a study of microstructure and its quasi-periodicities of three pulsars at 1.65 GHz with the S2 VLBI system at a resolution of 62.5 ns, by far the highest for any such statistical study yet. For PSR B1929+10 we found in the average cross-correlation function (CCF) broad microstructure with a characteristic timescale of 95 ± 10 µs and confirmed microstructure with characteristic timescales between 100 and 450 µs for PSRs B0950+08 and B1133+16. On a finer scale PSRs B0950+08, B1133+16 (component II) and B1929+10 show narrow microstructure with a characteristic timescale in the CCFs of ∼10 µs, the shortest found in the average CCF or autocorrelation function (ACF) for any pulsar, apart perhaps for the Crab pulsar. Histograms of microstructure widths are skewed heavily toward shorter timescales but display a sharp cutoff. The shortest micropulses have widths between 2 and 7 µs. There is some indication that the timescales of the broad, narrow, and shortest micropulses are, at least partly, related to the widths of the components of the integrated profiles and the subpulse widths. If the shortest micropulses observed are indeed due to beaming then the ratio, γ, of the relativistic energy of the emitting particles to the rest energy is about 20000, independent of the pulsar period. We predict an observable lower limit for the width of micropulses from these pulsars at 1.65 GHz of 0.5 µs. If the short micropulses are instead interpreted as a radial modulation of the radiation pattern, then the associated emitting sources have dimensions of about 3 km in the observer's frame. For PSRs B0950+08 and B1133+16 (both components) the micropulses had a residual dispersion delay over a 16-MHz frequency difference of ∼2 µs when compared to that of average pulse profiles over a much larger relative and absolute frequency range. This residual delay is likely the result of propagation effects in the pulsar magnetosphere that contribute to limiting the width of micropulses. No nanopulses or unresolved pulse spikes were detected. Cross-power spectra of single pulses show a large range of complexity with single spectral features representing classic quasi-periodicities and broad and overlapping features with essentially no periodicities at all. Significant differences were found for the two components of PSR B1133+16 in every aspect of our statistical analysis of micropulses and their quasi-periodicities. Asymmetries in the magnetosphere and the hollow cone of emission above the polar cap of the neutron star may be responsible for these differences. the observed intensity fluctuations can be either caused by a longitudinal modulation of the radiation pattern over the cross section of the polar magnetic field lines or by a radial modula- tion of the radiation pattern along the opening polar magnetic field lines, or, again, by a combination of both. The longitudi- nal modulation is most likely related to the stationary geome- try of the emission beam fixed to each of the poles of the ro- tating neutron star. The radial modulation is likely related to plasma bunching and linked to the elementary emission mech- anism. In this model the spectrum of the radio emission is a function of the radial distance from the neutron star, and the beam width is frequency dependent. High frequency radiation is emitted closer to the neutron star and the beam is narrower, low frequency radiation is emitted further out and the beam is broader, reflecting the opening of the polar magnetic field