Gamma Ray Telescope Finds First "Invisible" Pulsar

Anne Minard
for National Geographic News
October 16, 2008
A pulsar that had previously been invisible to orbiting and ground-based observatories has been discovered thanks to one of astronomy's newest pairs of "glasses," the Fermi Gamma-ray Space Telescope.

A pulsar is a type of neutron star, the small, dense remnant of a massive star that exploded as a supernova.

Unlike ordinary neutron stars, pulsars send out jets of radiation from their magnetic poles that sweep across Earth's line of sight as the star spins on its axis.

The newfound pulsar, which sits 4,600 light-years away in the constellation Cepheus, rotates at about a million miles an hour, and its beam of gamma rays reaches Earth about three times a second.

Fermi, a collaboration between NASA, the U.S. Department of Energy, and international partners, was launched in June to scan the skies for gamma rays, the most energetic wavelengths in the electromagnetic spectrum.

Pulsars have been spotted before based on radio waves and x-rays, but the new pulsar is the first object ever found solely based on gamma rays, according to Fermi scientists.

"We're learning that the Fermi telescope is the perfect instrument for finding young pulsars that were hidden from us before," said Alice Harding, a co-author on the study and a scientist at NASA's Goddard Space Flight Center in Maryland.

Harding said the mission could discover a new class of previously invisible pulsars, identify the mysterious sources of so-called gamma ray bursts, and expand estimates of the number of supernovae in our galaxy.

Higher Sensitivity

Radio beams from the first pulsar were discovered in 1968, and astronomers have counted 1,800 pulsars since then.

From 1991 through 2000, NASA's Energetic Gamma Ray Experiment Telescope (EGRET) cataloged hundreds of gamma ray sources, some of which turned out to be pulsars.

But many of EGRET's gamma ray sources remain unidentified, Harding said.

"We now think they're these gamma ray-only pulsars," she said. "We have many other sources that EGRET didn't detect because the sensitivity wasn't as good."

The newly discovered pulsar is positioned within the remains of a young supernova called CTA 1, which exploded around 10,000 years ago.

The Large Area Telescope aboard Fermi, which scans the entire sky every three hours in its search for gamma ray sources, saw roughly a gamma ray a minute coming from CTA 1.

(Related: "New Probe's First Gamma Ray Sky Map Unveiled" [August 26, 2008].)

Based on this rate of gamma ray pulses, scientists were able to piece together the neutron star's pulsing behavior and rotation period.

Pulsars slow down and stop emitting radiation beams within a hundred million years or less after their creation, Harding added. In the case of the CTA 1 pulsar, the spin is slowing by about a second every 87,000 years.

Harding and colleagues describe the new pulsar in a paper appearing online in the October 16 edition of Science Express.

Unlocking the Vault

Patrick Slane, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who is not a study co-author, called the new work an "exciting paper."

He and others had detected the CTA 1 supernova remnant and a whisper of the neutron star using x-rays but hadn't been able to find the telltale pulsations.

"A measurement of the pulse period unlocks the vault containing all of the information about the energetics of the system," Slane wrote in an email. "So this is hugely important."

Study co-author Harding pointed out that scientists don't yet understand the basic physics of pulsars, including the way they send out beams in the first place.

Using Fermi, she said, "we will really revolutionize that."

© 1996-2008 National Geographic Society. All rights reserved.