Resolving Pulsar Emission Regions Using Interstellar Scintillation


For the Air through which we look upon the Stars, is in a perpetual Tremor; as may be seen by the tremulous Motion of Shadows cast from high Towers, and by the twinkling of the fix’d Stars. But these Stars do not twinkle when viewed through Telescopes which have large apertures. For the Rays of Light which pass through divers parts of the aperture, tremble each of them apart, and by means of their various and sometimes contrary Tremors, fall at one and the same time upon different points in the bottom of the Eye, and their trembling Motions are too quick and confused to be perceived severally. And all these illuminated Points constitute one broad lucid Point, composed of those many trembling Points confusedly and insensibly mixed with one another by very short and swift Tremors, and thereby cause the Star to appear broader than it is, and without any trembling of the whole. Long Telescopes may cause Objects to appear brighter and larger than short ones can do, but they cannot be so formed as to take away that confusion of the Rays which arises from the Tremors of the Atmosphere.
— Opticks, Isaac Newton

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.

Reference: ADS or arXiv; Mathematical Supplement: ADS or arXiv

 From Johnson, Gwinn, and Demorest (2012;  http://adsabs.harvard.edu/abs/2012ApJ...758....8J ).

From Johnson, Gwinn, and Demorest (2012; http://adsabs.harvard.edu/abs/2012ApJ...758....8J).

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).