EDP Sciences
Free access
Volume 396, Number 1, December II 2002
Page(s) 171 - 187
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021402

A&A 396, 171-187 (2002)
DOI: 10.1051/0004-6361:20021402

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

M. V. Popov1, N. Bartel2, W. H. Cannon2, 3, A. Yu. Novikov3, V. I. Kondratiev1 and V. I. Altunin4

1  Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya 84/32, Moscow 117997, Russia
2  York University, Department of Physics and Astronomy, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada
3  Space Geodynamics Laboratory/CRESTech, 4850 Keele Street, Toronto, Ontario M3J 3K1, Canada
4  Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

(Received 5 April 2001 / Accepted 19 September 2002)

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 $95\pm 10~\mathrm{\mu s}$ and confirmed microstructure with characteristic timescales between 100 and $450~\mathrm{\mu s}$ 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 $\sim $10  $\mathrm{\mu s}$, 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 $7~\mathrm{\mu s}$. 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, $\gamma$, 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 $0.5~\mathrm{\mu s}$. 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 $\sim $ $\mathrm{\mu s}$ 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.

Key words: stars: pulsars: general -- radio continuum: stars -- methods: data analysis -- methods: observational

Offprint request: M. V. Popov, mpopov@asc.rssi.ru

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