Exploring an Elusive Pulsar

  • Fronefield Crawford, assistant professor of astronomy at Franklin & Marshall College
  • Crawford uses the Robert C. Byrd Green Bank Telescope in West Virginia, the world's largest fully steerable radio telescope, to conduct his research.

Ten years ago, Fronefield Crawford was part of a team of astronomers that discovered a binary pulsar approximately 2,500 light years away from Earth. Then, the assistant professor of astronomy at Franklin & Marshall College says something strange happened—the pulsar, a rapidly rotating neutron star the size of New York City, vanished.

The pulsar's disappearing act meant that Crawford and his colleagues could not regularly observe and study their new discovery. Several years later, they got another surprise. The pulsar was detected again by a radio telescope, this time at a higher observing frequency. The most likely explanation was that the pulsar had been eclipsed by its companion star.

"It's an elusive pulsar," Crawford says. "Observations prior to 2006 were done at a lower frequency than they are now. Radio waves are blocked more easily by a companion star at a lower frequency, and are more transparent at a higher frequency. It's a bit like turning your radio dial up to a higher frequency to hear a broadcast on a different channel."

Crawford is leading the effort to learn more about the pulsar system, initially through observing runs at the Robert C. Byrd Green Bank Telescope in West Virginia, the world's largest fully steerable radio telescope. His work was recently featured as part of "365 Days of Astronomy," part of the International Year of Astronomy.

Discovering pulsars is nothing out of the ordinary for Crawford's team, which includes astronomers from Canada, England, Germany, Italy and the United States. Using radio telescopes, astronomers record signals from space and, using sophisticated signal processing algorithms and powerful computers, look for the hallmarks of a pulsar.

"In a sense, it's almost like a fishing expedition," says Crawford, who describes pulsars as rapidly spinning, super-dense magnetized spheres that emit steady radio pulses. "We're looking at certain parts of the sky for signals. Sometimes, if you process the data in a certain way, you get faint signatures that are indicative of a pulsar."

There are two goals in fishing for pulsars, Crawford says. The first is to make new discoveries—adding to the pulsar population and extending our understanding of the formation, evolution and properties of pulsars in general—while the second is to identify individual pulsar systems that might be used as laboratories for high-precision tests of physical theories, such as the theory of relativity. Crawford calls those pulsars "jewels in the rough." The pulsar Crawford is studying might be one of those jewels. "The jury is still out on this one, but it may end up being a very important system," he says.

The pulsar could have recently transitioned from an X-ray binary (XRB) system into a millisecond pulsar (MSP), which is a pulsar spinning hundreds of times per second owing to the accretion of material over time from its companion star.

"We think an MSP is being born here," Crawford says. "If that's true, it supports the idea that an XRB becomes an MSP, like a caterpillar becomes a moth. You see the caterpillar, and you see the moth, but if you never saw a moth break out of a cocoon you might not know about the transition. The same can be said here. You have to be really lucky to catch the system in the act."

Joining Crawford in the deep-space exploration are two F&M students, Claire Gilpin '12 and Debbie Schmidt '12, both astrophysics majors. The students traveled to West Virginia in February to collect and analyze data from the Green Bank Telescope. A grant from the National Radio Astronomy Observatory, which operates the Green Bank Telescope, helped with the students' travel expenses.

The investigation continues, and Crawford says it will take at least a year of regular observations to more fully understand the pulsar's physical properties. Further observations and data analysis will surely keep Crawford, his team and students busy—as long as the pulsar does not pull another disappearing act.

Crawford uses the Robert C. Byrd Green Bank Telescope in West Virginia, the world's largest fully steerable radio telescope, to conduct his research.

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