Outreach-focused research using radio telescope uncovers surprising results about galactic structure
The U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO) 20-meter Telescope at Green Bank Observatory is often used for educational outreach projects with undergraduate and high school students. The 20-meter’s flexible observing setup enabled frequent, long-duration observations of eight pulsars, spanning two and a half years for a student-driven study carried out by students in the Pulsar Science Collaboratory program, led by NSF NRAO postdoc Jacob E. Turner. The collaboration to probe structural change in the interstellar medium led to striking observations tracing changes on timescales far shorter than expected.
Just as the Earth’s atmosphere causes starlight to twinkle in the night sky, the interstellar medium—the dust and gas between stars—reveals insights about its ever-changing nature through similar effects on light from pulsars. A pulsar’s rhythmic flash pierces through the material between the stars and interacts with free electrons in the medium. Pulsars emit light in many directions, and the signal scatters at a range of angles on its way to a telescope; when combined, these multiple lines of sight cause patterns of interference in the pulsars’ spectra, leading to a “twinkling” phenomenon called scintillation. When a pulsar’s signal through the interstellar medium is observed with sufficient resolution, more detailed data analysis yields characteristic larger-scale features called scintillation arcs.
By studying these arcs, Turner and the team were able to characterize the localized “screens” of interstellar clouds scattering the pulsars’ light. They then used this data to begin constructing a map of our galaxy’s structure, using the pulsars like lighthouses to anchor the ebb and flow of features within the interstellar medium. “Basically, the way we measure this is through the interference patterns of the pulsars’ radio light, and we expect them to be fairly consistent across decades,” Turner explains. “But there are some parts of the galaxy that are way more dynamic than we would expect it to be on these timescales.”
Since their discovery in the late 1960s, pulsars have yielded clues to a wide range of cosmic questions; in this study, Turner compared the data to similar measurements from decades ago. The findings, however, were surprising. Some of the scintillation properties of the pulsars remained consistent throughout the team’s monthly observations, while PSR J2022+5154, for instance, displayed changes indicating over a factor of 30 change in its scintillation pattern as measured just twenty years ago, and a factor of 10 change compared to those from two years ago.
Furthermore, the team reports evidence for a previously undetected structure between the Earth and one pulsar, and indications that a similar structure for another pulsar lies within the Orion-Cygnus arm of the Milky Way, thus providing greater clarity as to its orientation within the galaxy. Although this study includes data from only eight well-known pulsars, the stunning results suggest that updates may be needed for current models of electron density in the interstellar medium.
“Thanks to the Pulsar Science Collaboratory, this project gave undergraduates and high school students, and high-school teachers, the chance to acquire and analyze high-quality data that led to these surprising findings,” Turner acknowledges. “It begs the question, how many other parts of the galaxy which were thought to be fairly static might actually be undergoing noticeable structural changes,”
This research was published in the Astrophysical Journal.
About NRAO
The National Radio Astronomy Observatory (NRAO) and the Green Bank Observatory (GBO) are major facilities of the U.S. National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
About the Pulsar Search Collaboratory
The Pulsar Science Collaboratory (PSC), a partnership between West Virginia University, the Green Bank Observatory, and the NANOGrav collaboration is funded by the National Science Foundation (awards: #1516512, #1516269, #2020265). The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Arecibo Observatory was operated by the University of Central Florida, Ana G. Mendez-Universidad Metropolitana, and Yang Enterprises under a cooperative agreement with the National Science Foundation.