Strange Pulsing Star Puzzles Astronomers
Odd flashing pulses coming from a super-magnetic star called a magnetar have astronomers glued to telescopes across the globe.
Magnetars are particularly energetic versions of neutron stars, which are the burned out remnants of regular stars.
In March, astronomers detected a magnetar approximately 10,000 light-years from Earth in the direction of the constellation Sagittarius, emitting regularly timed radio pulses. Theory had predicted that due to their high magnetic fields--100 to 1,000 times stronger than typical radio pulsars--magnetars would be unlikely to send out radio waves.
The recent sighting, which relied on the Parkes radio telescope in Australia, is causing them to rethink fundamental theories about these extreme stars.
"Previous to our detection there were some theories that explained why you could not get radio emission from magnetars; obviously those are now incorrect," said Fernando Camilo of the Columbia Astrophysics Laboratory at Columbia University in New York.
The findings were detailed in the Aug. 24 issue of the scientific journal Nature.
Referred to as XTE J1810-197, the magnetar was first spotted by NASA's Rossi X-ray Timing Explorer in 2003 when the object abruptly 'came to life' with a strong burst of X-rays. Then, in 2004, astronomers using the National Science Foundation's Very Large Array radio telescope found the object was emitting radio waves.
To explain the anomaly, the scientists presumed the radio waves were emitted from a cloud of particles flung from the neutron star at the time of the X-ray burst. This theory was soon proved wrong when Camilo and his colleagues discovered that XTE J1810-197 was emitting strong radio pulsations every 5.5 seconds, which corresponds to the estimated rotation rate of this magnetar.
The research team suspects that the magnetar's mega magnetic field is twisting, causing the electric currents flowing along its magnetic field lines to change locations. These currents, they think, are fueling the detected radio pulses.
Beacons of light
Like their galactic cousins radio pulsars, magnetars are a type of spinning neutron star thought to result from the explosive death, or supernova, of a massive star. These spinning neutron stars emit a constant stream of electromagnetic particles from their magnetic poles. As the star whips around on its axis, the particles traveling at near-light-speed sweep out into space. When they shine toward Earth, astronomers pick up the ejections as pulses with radio and X-ray telescopes.
A magnetar's powerful magnetic field means that as the field decays, the star emits high-energy radiation in the form of X-rays. "The magnetic field from a magnetar would make an aircraft carrier spin around and point north quicker than a compass needle on Earth," said David Helfand of Columbia University.
More dazzling finds
With further probing, the researchers are finding even more bizarre traits of this celestial lighthouse.
"Perhaps even more surprising was the characteristic of this emission, which differs in many significant respects from 'normal' pulsar emission," said Camilo. For instance, the brightness of the radio pulses varies from day to day, a phenomenon not found in the 1,700 or so known radio pulsars.
Another surprise: Most pulsars get weaker at higher radio frequencies.
"For me, one of the most spectacular characteristics of the emission is that its spectrum is apparently flat," Camilo said. That means its brightness is the same at all frequencies observed, all the way from 350 megahertz to 140 gigahertz. "A typical pulsar would be about 15,000 times fainter at the higher frequency compared to the lower, and so it wouldn't be detected. It would be too faint," Camilo said.
In fact, 140 Ghz is the highest frequency ever detected from a neutron star, making XTE J1810-197 the brightest neutron star known.
Camilo doesn't expect the light show to last forever though. Like the X-rays, which have fizzled out since the 2003 outburst, the radio pulses will most likely fade as the star's rotation slows.
"It could be next month, which is why we've been collecting data with all the telescopes we can get our hands on, like crazy, in case it disappears quickly, and also because it's so cool. Or it could be in 100 years," Camilo said.
Do astronomers expect to find more oddball magnetars? "We shall see," said one of the co-authors, John Reynolds, officer-in-charge at the CSIRO Parkes Observatory. "This discovery will certainly heighten interest among pulsar astronomers throughout the world in observing magnetars. It will be a little surprising if more are not detected in the coming months."
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