Claire Bowles, firstname.lastname@example.org, 44 171 331 2751
Barbara Thurlow, email@example.com, (202) 452-1178
EMBARGOED FOR RELEASE: 30 SEPTEMBER 1998 at 2:00 p.m. EDT
Astronomers May Have Got It Wrong About Neutron Stars
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
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 Societa 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
New Scientist magazine, issue 3rd October 1998
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