NANOGrav Physics Frontiers Center Postdoctoral Fellow at Oregon State University
My research spans two complementary regions of the gravitational wave spectrum: the nanohertz band where pulsar timing arrays like NANOGrav operate, and the millihertz band that the upcoming space-based observatory LISA will probe. Click on a detector below to jump to the related research section.
Pulsar timing arrays (PTAs) detect gravitational waves at nanohertz frequencies (10-10–10-7 Hz) by monitoring tiny variations in the arrival times of pulses from millisecond pulsars. With strong evidence now in hand for a stochastic gravitational wave background, most likely sourced by inspiraling supermassive black hole binaries, one of the next big questions is how that background is distributed across the sky. A non-uniform population of binaries should imprint anisotropy on the background, and detecting it would be a strong hint of the underlying astrophysics.
My current main thrust is developing an analytical framework for the sensitivity of a PTA to anisotropic backgrounds, pulsar-by-pulsar, direction-by-direction. The figure below shows an effective sensitivity skymap Seff for a simulated array; the array is, as expected, most sensitive where pulsars are concentrated. The forthcoming paper will describe the formalism in full.
The frequency-dependent (chromatic) delays imprinted on pulse arrival times by the interstellar and interplanetary medium can be up to two orders of magnitude louder than the gravitational wave background itself. How those delays are modeled directly affects the inferred spectrum of the background, so getting the noise models right is essential. I work on building customized chromatic noise models for each pulsar in NANOGrav’s datasets, testing the significance of competing noise prescriptions on a per-pulsar basis.
LISA, the planned space-based gravitational wave detector, will be sensitive between roughly 10-5 and 10-1 Hz. A prime LISA source is the extreme mass ratio inspiral (EMRI): a stellar-mass compact object (~10 M☉) spiraling into a massive black hole (105–108 M☉) at the center of a galaxy. These orbits typically start out highly eccentric and only emit detectable radiation in brief bursts as the compact object swings past periapsis.
Each one of these bursts is a gravitational wave “peep” — firmly in the LISA band even when the long-period orbit itself is not. A single peep is unlikely to be individually resolvable, but a cosmological population of them out to redshift z = 3 contributes to LISA’s signal confusion noise. My dissertation and follow-up work focuses on modeling that background and quantifying how much it can obscure otherwise-detectable LISA sources.
