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Strange Pulsing Star Puzzles Astronomers

Oddflashing pulses coming from a super-magnetic star called a magnetarhave astronomers glued to telescopes across the globe.

Magnetarsare particularly energetic versions of neutron stars, which are the burned outremnants of regular stars.

In March,astronomers detected a magnetar approximately 10,000 light-years from Earth inthe direction of the constellation Sagittarius, emitting regularly timed radiopulses. Theory had predicted that due to their high magneticfields--100 to 1,000 times stronger than typical radio pulsars--magnetarswould be unlikely to send out radio waves.

The recentsighting, which relied on the Parkes radio telescope in Australia, is causingthem to rethink fundamental theories about these extreme stars.

"Previousto our detection there were some theories that explained why you could not getradio emission from magnetars; obviously those are now incorrect," saidFernando Camilo of the Columbia Astrophysics Laboratory at Columbia Universityin New York.

Thefindings were detailed in the Aug. 24 issue of the scientific journal Nature.

Referred toas XTE J1810-197, the magnetar was first spotted by NASA's Rossi X-ray TimingExplorer in 2003 when the object abruptly 'came to life' with a strong burst ofX-rays. Then, in 2004, astronomers using the National Science Foundation's VeryLarge Array radio telescope found the object was emitting radio waves.

To explainthe anomaly, the scientists presumed the radio waves were emitted from a cloudof particles flung from the neutron star at the time of the X-ray burst. Thistheory was soon proved wrong when Camilo and his colleagues discovered that XTEJ1810-197 was emitting strong radio pulsationsevery 5.5 seconds, which corresponds to the estimated rotation rate of this magnetar.

Theresearch team suspects that the magnetar's mega magnetic field is twisting,causing the electric currents flowing along its magnetic field lines to changelocations. These currents, they think, are fueling the detected radio pulses.

Beaconsof light

Like theirgalactic cousins radio pulsars, magnetars are a type of spinning neutron starthought to result from the explosive death, or supernova, of a massive star.These spinning neutron stars emit a constant stream of electromagneticparticles from their magnetic poles. As the star whips around on its axis, theparticles traveling at near-light-speed sweep out into space. When they shinetoward Earth, astronomers pick up the ejections as pulses with radio and X-raytelescopes.

A magnetar'spowerful magnetic field means that as the field decays, the star emitshigh-energy radiation in the form of X-rays. "The magnetic field from a magnetarwould make an aircraft carrier spin around and point north quicker than acompass needle on Earth," said David Helfand of Columbia University.

Moredazzling finds

Withfurther probing, the researchers are finding even more bizarretraits of this celestial lighthouse.

"Perhaps evenmore surprising was the characteristic of this emission, which differs in manysignificant respects from 'normal' pulsar emission," said Camilo. For instance,the brightness of the radio pulses varies from day to day, a phenomenon notfound in the 1,700 or so known radio pulsars.

Anothersurprise: Most pulsars get weaker at higher radio frequencies.

"For me,one of the most spectacular characteristics of the emission is that itsspectrum is apparently flat," Camilo said. That means its brightness is thesame at all frequencies observed, all the way from 350 megahertz to 140gigahertz. "A typical pulsar would be about 15,000 times fainter at the higherfrequency compared to the lower, and so it wouldn't be detected. It would betoo faint," Camilo said.

In fact,140 Ghz is the highest frequency ever detected from a neutron star, making XTEJ1810-197 the brightest neutron star known.

VanishingAct

Camilodoesn't expect the light showto last forever though. Like the X-rays, which have fizzled out since the 2003outburst, the radio pulses will most likely fade as the star's rotation slows.

"It couldbe next month, which is why we've been collecting data with all the telescopeswe can get our hands on, like crazy, in case it disappears quickly, and alsobecause it's so cool. Or it could be in 100 years," Camilo said.

Doastronomers expect to find more oddball magnetars? "We shall see," said one ofthe co-authors, John Reynolds, officer-in-charge at the CSIRO ParkesObservatory. "This discovery will certainly heighten interest among pulsarastronomers throughout the world in observing magnetars. It will be a littlesurprising if more are not detected in the coming months."

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