Resolving pulsar magnetospheres with picoarcsecond resolution scintellometry
The radio emission of pulsars remains, more than 40 years since their discovery, a poorly understood phenomenon. The single most important unconstrained component of pulsar emission theories is the site of the emission in relation to the surface of the neutron star, with the three major classes of models predicting particle acceleration at wildly different locations: the polar gap, the slot gap or the outer gap in the pulsar magnetosphere. The predicted emission height is tiny, ranging from ~10km (for inner gap models; 13 pico-arcseconds at a distance of 500pc) to ~5000km (for outer gap models; the size of the light cylinder for a 0.1s period pulsar). The angular resolution necessary to resolve such structure remains well beyond the capabilities of modern instrumentation.
Fortunately, the scattering of pulsar radiation by the interstellar medium (ISM) presents a means to directly measure the emission height. Pulsar radiation is subject to strong interference effects as it propagates through the turbulent ISM; interstellar scattering causes the point-like image of the pulsar to break up into thousands of sub-images or speckles that all interfere with each other. The ISM effectively operates as a giant interferometer — albeit with imperfect optics — whose diameter is comparable to the ~10AU transverse distance of speckles across the scattering region.
The technique of pulsar scintellometry uses the scintillation as an interferometer to detect a relative astrometric shift in the location of the emission region as the pulsar beam rotates through our line of sight. At its most basic, the principle behind the astrometric measurement is shown in the Figure below (see Pen et al. 2014 & Brisken et al. 2010 for full details). An angular displacement in the emission site causes a lateral displacement in the scintillation pattern at Earth. An angular difference in the emission site between different regions of the pulse would therefore cause a time lag between the scintillation patterns associated with different parts of the pulse profile. This technique (see Pen et al. 2014) improves on this principle by applying holographic phase-retrieval techniques to partially descatter the pulsar radiation in order to boost the pulsar S/N by a factor of several thousand over the raw scattered signal, and thus attain an additional ~103 increase in astrometric precision. This enables us to achieve pico-arcsecond relative astrometry on the location of the pulsar emission between time channels spread across the pulse profile.
In this project you would undertake low-frequency VLBI observations with FAST and the MWA to make speckle images of scintillating pulsars and apply the techniques of scintellometry to resolve the structure of pulsar emission regions