Interstellar Seeing. II. The Case of the Vela Pulsar: Source Unresolved

Abstract

I use a method based on interstellar scintillations for discerning information about source sizes on scales less than one micro-arc sec. I use a comprehensive model for a pulsar signal, scintillated amplitude modulated noise, that includes source fluctuations and noise statistics. The method takes into account time-frequency averaging in the signal processing as well as effects due to source structure. The method is applied to interferometric visibility data on the Vela pulsar which show slightly less scintillation modulation than expected for a point source in the strong scattering regime. The decreased scintillation modulation is likely to be due exclusively to time-frequency averaging rather than from any source size effects. The implied upper limit on source extent, derived through Bayesian inference, is compared to predictions that take into account beaming from the relativistic plasma flow in neutron star magnetospheres. The upper limit for the transverse source size ($\lesssim 400$ km at 95% confidence for a pulsar distance of 0.5 kpc) is easily consistent with conventional models for radio emission regions in pulsar magnetospheres that place them well inside the light cylinder at only a few neutron-star radii from the star's surface.