October 22, 2008
Space telescope finds a pulsar with a gamma-ray wink
About three times a second, the rotating corpse of a 10,000-year-old star sweeps a beam of gamma rays toward Earth. This object, known as a pulsar, is the first one known to "blink" at Earth only in gamma rays, and was discovered by an orbiting observatory launched in June with significant involvement from researchers at Stanford and the SLAC National Accelerator Laboratory.
"This is the first example of a new class of pulsars that will give us fundamental insights into how stars work," said Stanford astrophysicist Peter Michelson, the principal investigator for the Large Area Telescope (LAT), which is carried aboard NASA's orbiting observatory, the Fermi Gamma-ray Space Telescope.
Researchers at SLAC get the first peek at the celestial data beamed down from LAT before sending it on to an international collaboration of scientists for analysis.
The gamma-ray-only pulsar lies within a supernova remnant known as CTA 1, about 4,600 light-years away from Earth in the constellation Cepheus. Its lighthouse-like beam of gamma rays sweeps across Earth every 316.86 milliseconds and emits 1,000 times the energy of our sun. Details of the discovery appear in the Oct. 16 edition of Science Express.
A pulsar is a rapidly spinning neutron star, the crushed core left behind when a massive star explodes. Astronomers have cataloged nearly 1,800 pulsars. Although most were found through their pulses of radio waves, some of these objects also beam energy in other forms, including visible light, X-rays and gamma rays, each of which occupy their own spot on the electromagnetic spectrum.
Unlike previously discovered pulsars, the source in CTA 1 appears to blink only in gamma-ray energies and offers researchers a new way to study the stars in our universe. Scientists think CTA 1 is only the first of a large population of similar objects. "The LAT provides us with a unique probe of the galaxy's pulsar population, revealing objects we would not otherwise even know exist," said Steve Ritz, NASA's project scientist for the Fermi observatory. He is stationed at NASA's Goddard Space Flight Center in Greenbelt, Md.
The pulsar in CTA 1 is not located at the center of the supernova remnant's expanding gaseous shell. Supernova explosions can be asymmetrical, often imparting a "kick" that sends the neutron star careening through space. Based on the remnant's age and the pulsar's distance from its center, astronomers believe the neutron star is moving at about a million miles per hour.
It is possible that the pulsar is emitting radio wavesthus far unseenin addition to gamma rays. "The radio beam probably never swings toward Earth, so we never see it. But the wider gamma-ray beam does sweep our way," explained NASA's Alice Harding.
The LAT scans the entire sky every three hours and detects photons with energies ranging from 20 million to more than 300 billion times the energy of visible light. The instrument sees about one gamma ray each minute from CTA 1. That's enough for scientists to piece together the neutron star's pulsing behavior, its rotation period and the rate at which it is slowing down.
A pulsar's beams arise because neutron stars possess intense magnetic fields and rotate rapidly. Charged particles stream outward from the star's magnetic poles at nearly the speed of light to create the gamma-ray beams the telescope sees. Because the beams are powered by the neutron star's rotation, they gradually slow the pulsar's spin. In the case of CTA 1, the rotation period is increasing by about one second every 87,000 years.
This measurement is also vital to understanding the dynamics of the pulsar's behavior and can be used to estimate the pulsar's age. From the slowing period, researchers have determined that the pulsar is actually powering all the activity in the nebula where it resides.
"This observation shows the power of the LAT," Michelson says. "It is so sensitive that we can now discover new types of objects just by observing their gamma-ray emissions."
The list of authors for the article in Science Express demonstrates the collaborative nature of the gamma-ray research. The list contains scientists from 48 research groups in the United States, Italy, France, Sweden, Japan and Germany.