Gamma-ray sources guide astronomers to pulsars

WASHINGTON — Energetic gamma rays are providing astronomers with a new way to hunt those hard-to-find whirling dervishes known as pulsars.

“We usually have to look over the whole sky” to find pulsars, said Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Va. “Now we can use the gamma-ray point sources as guides, telling us exactly where to look.”

Ultradense, collapsed remnants of massive stars, pulsars rotate up to hundreds of times a second and emit beacons of light that sweep across the sky like lighthouse beams. Pulsars are not only predicted to be key sources of gravitational waves — the subtle ripples in spacetime predicted by Einstein’s theory of general relativity — but their clockwork pulses can be used to detect those waves in surrounding space.

Although researchers have calculated that the Milky Way galaxy contains tens of thousands of the fastest spinning pulsars, known as millisecond pulsars, radio telescopes have found only about 100. Because these celestial bodies can reside anywhere in the galaxy, searching for millisecond pulsars was like looking for the proverbial needle in a haystack, requiring radio surveys of the entire sky, noted Ransom, who presented his new work November 2 during the 2009 Fermi Symposium.

Last year, pulsar hunters got some encouraging news. Researchers using the Fermi Gamma-ray Space Telescope demonstrated that many millisecond pulsars emit gamma rays. However, many pulsars emit so few high-energy gamma rays — only 300 to 1,000 over an entire year — that it has been difficult to use the gamma rays to identify the period of many of these pulsars (or even to clearly identify if any of these rapid rotators are the source of the gamma rays).

But Ransom and colleagues calculated that a significant number of the unidentified gamma-ray sources found by Fermi might turn out to be pulsars. So the researchers recently aimed a giant radio dish, the Green Bank Telescope in West Virginia, at four of these sources and found that three were indeed radio-emitting millisecond pulsars, Ransom reported.

“For the first time, gamma-ray observations are guiding radio observations,” commented Fermi researcher Peter Michelson of Stanford University in Palo Alto, Calif.

The gamma-ray technique may soon reveal dozens of previously unknown pulsars, Ransom estimated, including radio-bright pulsars whose radio emissions could be clocked to reveal the presence of gravitational waves (SN: 6/6/09, p. 14).