PULSARS that only emit X-rays, once considered "anomalous", now officially outnumber those that emit radio waves. This is leading astronomers to rethink their ideas about what happens after a typical dying star explodes as a supernova.
A supernova explosion occurs when a star runs out of nuclear fuel and shrinks catastrophically under its own gravity. The result is a super-dense neutron star about the size of Mount Everest. Current theories of how these stars behave predict that they should all act as radio pulsars, sweeping a narrow beam of radio waves around the sky like a lighthouse a hundred times a second. So why didn't radio searches find more pulsars in supernova remnants?
"The trouble is that hardly more than 1 per cent of the 300-odd known young supernova remnants contain associated radio pulsars," says Eric Gotthelf of NASA's Goddard Space Flight Center near Washington DC. But now the reason they went missing is clear, Gotthelf says. Astronomers were looking in the wrong part of the electromagnetic spectrum.
In the past few years, astronomers using the Japanese-American ASCA satellite have found three "point-like" objects in the centres of supernova remnants which are emitting pulses of X-rays. Now Gotthelf says that he has just picked out three more of these "anomalous X-ray pulsars" (AXPs) in X-ray sources observed by the satellite. Add these three to the list and anomalous pulsars in supernovae remnants will outnumber the four known radio pulsars associated with supernovae remnants for the first time, he reports in a paper to appear in the journal Memorie della Societ' Astronomica Italiana.
These findings will mean that radio pulsars are the exception rather than the norm. "This is a complete reversal of our thinking," says Gotthelf. David Hough of Trinity University in San Antonio, Texas, confirms that these new findings mean that "the book on how pulsars are born in supernovae may have to be rewritten".
The X-rays coming from AXPs are produced by matter channelled by the star's magnetic field lines and heated to enormous temperatures. AXPs spin a thousand times slower than radio pulsars and are slowing down rapidly. This is puzzling, because when a star shrinks to the relatively tiny size of a neutron star it should automatically spin very fast. According to Gotthelf, the most likely explanation is that AXPs are indeed born spinning fast, but slow down quickly because they have a super-strong magnetic field, hundreds of times stronger than in radio pulsars.
"Such a strong magnetic field would drag material around as the star spins, sapping the star of rotational energy," says Gotthelf. A super-strong magnetic field would also prevent the formation of the electrons needed to produce radio waves. One possible explanation for the different magnetic field strengths in radio pulsars and X-ray pulsars is that stars start out with a natural variability in magnetic fields before they collapse.
At least two more sensitive X-ray satellites will be launched in the next few years, and Gotthelf believes they will find many more radio-quiet pulsars.
Author:Marcus Chown
New Scientist magazine, issue 3rd October 1998
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