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

Abstract

Abstract. 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 20 000, 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.

On the origin of giant pulses in radio pulsars

“…This interpretation is strongly supported by observations: the timescales of microstructure are proportional to the pulsar period (Hankins 1996;Popov et al 2002), testifying to the same characteristic angular scale of the emission pattern. According to the most straightforward considerations, the angular width of the micro/nano-pulses is related to the opening angle of the relativistically beamed radiation.…”

“…Firstly, they can be attributed to the spatial variations in relativistically streaming pulsar plasma, which are likely to affect the emission process and further propagation of radio waves. Then, in the case of microstructure, the characteristic scale of the plasma inhomogeneities is ∼3 km (Popov et al 2002), whereas the giant nanopulses result from the plasma clumps ∼3 m in size . These plasma inhomogeneities are usually associated with the soliton-like structures which indeed may arise in the strongly turbulent plasma of pulsar magnetosphere (e.g.…”

Polarized Quasiperiodic Structures in Pulsar Radio Emission Reflect Temporal Modulations of Non-Stationary Plasma Flow

“…The average pulse profiles for many pulsars are often seen to be composed of multiple emitting components. As was pointed out in some earlier studies (e.g., Soglasnov et al 1981;Soglasnov et al 1983;Popov et al 2002), the individual components can have different microstructure timescales. Hence, we isolate individual pulse components wherever possible before estimating timescales.…”

Self-modulation of nonlinear waves in a weakly magnetized relativistic electron-positron plasma with temperature

“…Our results may contribute to improve the understanding of physical processes in pair plasmas such as the formation and evolution of pulsar microstructures [1,[32][33][34][35][36]. …”