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Radio quiet gamma-ray pulsars solve mystery

Posted: JULY 3, 2009

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A new class of radio quiet pulsar detected by NASA's Fermi Gamma-ray Space Telescope solves the mystery of previously unidentified gamma-ray sources.

A pulsar is a highly magnetized, rapidly spinning neutron star, the dense core remaining after a supernova explosion. Most of the 1,800 known pulsars were found through their periodic radio emissions, however, Fermi has now uncovered 16 pulsars based on their high energy gamma rays alone.

Click here for larger image of all sky map of the positions and names of 16 new pulsars (yellow) and eight millisecond pulsars (magenta) studied using Fermi's LAT. The famous Vela, Crab, and Geminga pulsars (right) are the brightest ones Fermi sees. The pulsars Taz, Eel, and Rabbit have taken the nicknames of nebulae they are now known to power. The Gamma Cygni pulsar resides within a supernova remnant of the same name. Image: NASA/DOE/Fermi LAT Collaboration.

"These are the first pulsars ever detected by gamma rays alone, and already weÕve found 16," says Robert Johnson, professor of physics at the University of California. "The existence of a large population of radio-quiet pulsars was suspected prior to this, but until Fermi was launched, only one radio-quiet pulsar was known, and it was first detected in X-rays."

Thirteen of Fermi's pulsars are associated with previously unidentified gamma-ray sources already detected by the EGRET instrument on the Compton Gamma-ray Observatory. "It's been a longstanding question what could be powering those unidentified sources, and the new Fermi results tell us that a lot of them are pulsars," says Saz Parkinson. "These findings are also giving us important clues about the mechanism of pulsar emissions."

The science team used computational techniques to comb through Fermi's Large Area Telescope (LAT) data to detect the pulsars. "From the faintest pulsar we studied, the LAT sees only two gamma-ray photons a day," says Marcus Ziegler, who comments that detection of gamma-ray pulsations from a typical source requires weeks or months of data from the LAT.

Click here for movie that shows one cycle of pulsed gamma rays from the Vela pulsar as constructed from photons detected by Fermi's Large Area Telescope. Image: NASA/DOE/Fermi LAT Collaboration.

The fact that Fermi can 'feel' the gamma-ray pulsations gives important information about the mechanism behind the emissions. Radio pulsars emit narrow beams of radio waves that sweep around like a lighthouse beacon, bathing the Earth in radiation to enable detections. If the radio beam misses the Earth, the pulsar cannot be detected by radio telescopes, but FermiÕs ability to detect so many radio-quiet gamma-ray pulsars indicates that the gamma-rays are emitted in a beam that is wider and more fan-like than the radio beam.

"This favors models in which the gamma rays are emitted from the outer magnetosphere of the pulsar, as opposed to the polar cap much closer to the surface of the star," says Parkinson.

Johnson tells Astronomy Now that it is very likely that all pulsars have strong radio emission. "Note that not all of the 16 pulsars discovered by way of their gamma-ray emission are necessarily radio quiet (in the sense that the radio beam doesn't intercept Earth)," he says. "In some cases they simply may not yet have been found in radio, since the radio surveys have not been carried out uniformly over the entire sky. Radio astronomers are already actively engaged in following up on the Fermi detections to see if radio emissions can be detected."

The results from the Fermi survey are presented in the 2 July edition of Science Express. In the same issue, scientists also report on newly confirmed millisecond pulsars. These pulsars exist alongside a companion star, feeding off stellar material and spinning its rotation rate up to 1,000 times a second. By contrast, isolated pulsars slow down as they age and lose energy. Fermi scientists have detected gamma-ray pulsations from eight millisecond pulsars that were previously discovered at radio wavelengths.

"Fermi has truly unprecedented power for discovering and studying gamma-ray pulsars," says Paul Ray of the Naval Research Laboratory in Washington. "Since the demise of the Compton Gamma Ray Observatory a decade ago, weÕve wondered about the nature of unidentified gamma-ray sources it detected in our Galaxy. These studies from Fermi lift the veil on many of them."

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