Research

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.

A few of my research interests are listed below; check ADS or arXiv for a listing of papers.

Resolving Pulsar Emission Regions

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. Thus, even though the human eye has insufficient aperture to distinguish the angular size of a planet from a star, by using the optical scintillation in the atmosphere, we can tell them apart. In effect, the interstellar scattering material acts like an enormous, random lens, with a diameter that can be greater than the distance to the sun. We have 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. We can also 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 underlying physics.

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

VLBI with RadioAstron

RadioAstron is an international collaboration for space VLBI. The space radio telescope (Spektr-R) was launched into orbit on July 18, 2011. This orbit is highly eccentric, and has an apogee of 390,000 km -- about the distance to the moon. We are using joint observations with many ground telescopes to study the nature of the nearby scattering material and the details of the pulsar emission. The Early Science Phase is wrapping up now, so stay tuned for results!

Optimal Correlation Estimators for Quantized Signals

In astronomy, we're always observing sources that emit noise. Remarkably, all the information in this noise is contained in its correlations between different frequencies, times, and polarizations. Thus, estimating these correlations from a finite number of samples is of fundamental importance. It turns out that nearly all of this information is preserved if you reduce each measured voltage to a single bit of data characterizing the sign (called one-bit quantization). This quantization distorts the correlations, but a simple prescription, the Van-Vleck Correction, can remove most of the distortion. With more bits, the efficiency continues to improve. However, rather aggressive quantization is necessary to control the data rates for modern telescopes.

This raises the natural question: "What is the optimal method for estimating correlation of a quantized signal?" The best scheme will eliminate the bias and minimize the noise in the estimation. We derived a general framework to answer this question, based on a maximum-likelihood criterion, which resolves many paradoxes in the behavior of quantization noise and improves current schemes when the correlation is strong.

Reference: ADS or arXiv