Detecting Gravitational Waves with Pulsars: Correcting for Interstellar Scattering

Science Center, Bent Corridor

Detecting gravitational waves, ripples in space-time created by the movement of massive objects, is one of the great challenges of modern physics. Measuring these waves will open a new window on the universe. Pulsars, neutron stars with very short and regular rotation periods, can be used to detect gravitational waves. Pulsar timing arrays—worldwide, coordinated observations of rapidly spinning ‘millisecond pulsars’—can be used to measure the minute variations in space-time created by these waves, with wavelengths between 1 and 50-lightyears. Proposed sources in this range include supermassive black hole binaries, cosmic strings, and early-universe exotica. These observations require very precise measurement of the pulse arrival times to the level of 10 nanoseconds or so. Timing must correct for the slight variations created by the interstellar medium (ISM), the plasma between stars, which can cause variations on the order of several microseconds and are time-variable because of the rapid motion of pulsars through our Galaxy. Taking a two-dimensional fourier transform of the dynamic spectra, which plots pulsar intensity as a function of both time and observing frequency, we get a secondary spectrum, that shows a parabolic arc comprised of smaller ‘arclets.' By studying the curvature of these arcs we can model the ISM as a screen and find the distance of that screen. This ultimately will assist us and colleagues in correcting the arrival times to the required precision.