Resolving Pulsar Emission Regions Using Interstellar Scintillation
Pulsars are some of nature's most remarkable phenomena. Although they have about the mass of the sun, they are only the size of a small city and spin dizzyingly fast - up to a thousand revolutions per second. These remarkable objects feature physics puzzles in almost every facet, from their ultra-dense interiors to their incredibly bright emission. In addition to their intrinsic physics, they act as astrophysical clocks with phenomenal stability, and they can be used as tools to study a vast array of physics. My research has generally focused on observational studies of pulsar emission regions and of the interstellar plasma, which scatters the pulsar emission.
Pulsars scintillate at radio wavelengths as a result of multipath propagation in the interstellar medium. This scattering can actually improve the resolution achievable from Earth. A familiar example is that stars twinkle, but planets don't (see here for a great demonstration of this property). Actually, the human eye couldn't distinguish a planet from a star without the scattering! For pulsars, the interstellar scattering material acts like an enormous, random lens, with a diameter that can be greater than the distance to the sun. I developed statistical techniques that image the radio emission from the Vela pulsar (which is about 1000 light-years away) at a scale of about 4 km. This gives an angular resolution of about 100 picoarcseconds - about the same angular size as a virus on the moon or the width of a human hair on the sun. These techniques can even estimate the size of the emission region for individual pulses. Because the site of the radio emission is still not well understood, these techniques provide a valuable window into the enigmatic radio emission from pulsars.
I gave a 45-minute talk at the Anacapa School (grades 7-12) that introduces some of these concepts and can be viewed here.
The Vela Pulsar's Emission Size. PDF of intensity (black) for averaged pairs of dynamic spectra separated by a fixed number (Δτ) of pulses. The upper model (green) required no fitted parameters. The lower models (red) for the residuals were fit with two parameters: the decorrelation timescale of the scintillation pattern, and the emission size. The estimated transverse emission size is 4 km. From Johnson, Gwinn, and Demorest (2012; http://adsabs.harvard.edu/abs/2012ApJ...758....8J).