RUI: Detecting Gravitational Waves with Pulsars: Mitigating the ISM Delay

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Pulsar timing can be used to explore many aspects of gravitation and stellar physics. In particular, pulsars can be used to search for gravitational waves - ripples in space-time caused by the acceleration of large masses such as black holes - by monitoring the arrival time of the pulses. This long-term, collaborative effort is one of the great quests of modern physics and astrophysics. The technique employed is sensitive to gravitational waves with light-year wavelengths produced, for example, by orbiting supermassive black holes at the cores of merging galaxies. This wavelength regime cannot be probed by other techniques, for example those being explored by LIGO and the proposed LISA mission. Multi-path propagation through the inhomogeneous interstellar gas - and subsequent interference effects - causes the radio signal to vary in strength or scintillate. Scintillation effects hamper high precision timing of pulsars by introducing random noise into the timing signal. The required timing precision is extraordinary: about 100 ns maintained for years.The PI and his Oberlin College undergraduate students are developing a technique that allows time delays smaller than a microsecond to be estimated from the time-variable spectrum of the pulsar. Although there is much development work needed on this technique, initial results are promising and underscore the importance of understanding and correcting for this delay. The proposed research will develop this technique in detail using high-sensitivity observations from the Arecibo radio telescope and other large telescopes such as the NRAO/Green Bank Telescope and the LEAP (virtual array) telescope being organized in Europe. The main goal is to focus on this important source of timing noise and develop a practical technique for correcting for it in the timing signal of relatively weak millisecond pulsars.This research will benefit the scientific community by improving our ability to detect gravitational waves with an array of millisecond pulsars. In addition, what we learn about scattering of radio waves by the ionized gas in the Milky Way will improve our understanding of how the Milky Way "works," for example the degree of turbulence of the interstellar gas is how it is distributed in space. Both of these topics are of interest not only to specialists, but to a much wider audience as well. As a professor at a liberal arts college, the PI shares the excitement of this work with his research students and with the wide range of students he teaches. His teaching of Introductory Astronomy to about 100 undergraduates a year, for example, benefits directly from his involvement in scientific research. This research program will involve undergraduate students in the excitement of exploring a new phenomenon and piecing together parts of a bigger puzzle. By working closely with the PI and his collaborators, the students will gain a broad range of technical skills as well as further develop their ability to independently explore problems. Using state-of-the-art electronics and computers at the largest radio telescopes in the world, the students will gain confidence in their ability to tackle large and complex problems. Opportunities to collaborate with scientists in the U.S. and abroad and to report their work at conferences and in publications will enhance students' research experience and prepare them for graduate training or other roles in the technical workforce.