Einstein's
predicted warping of space-time has been discovered around neutron stars, the
most dense observable matter in the universe.
The warping
shows up as smeared lines of iron gas whipping around the stars, University of Michigan and NASA astronomers say. The finding also indicates a size limit
for the celestial objects.
The same
distortions have been spotted around black holes and even around
Earth, so while the finding may not be a surprise, it is significant for
answering basic questions of physics, said study team member Sudip
Bhattacharyya of NASA's Goddard Space Flight Center in Greenbelt, Md. and the
University of Maryland, College Park.
"This
is fundamental physics," Bhattacharyya said. "There could be exotic
kinds of particles or states of matter, such as quark matter, in the centers of
neutron stars, but it's impossible to create them in the lab. The only way to
find out is to understand neutron stars."
Neutron
stars can pack more than a sun's worth of material into a city-sized
sphere. A few cups of neutron-star stuff would outweigh Mount Everest.
Astronomers use these collapsed
stars as natural laboratories to study how tightly matter can be crammed
under the most extreme pressures nature can offer.
To even
begin to address the mystery of what lies within these dying stars, scientists
must accurately and precisely measure their diameters and masses.
In two
concurrent studies, astronomers used the European Space Agency's XMM-Newton
X-ray Observatory and the Japanese/NASA Suzaku X-ray to survey three
neutron-star binaries: Serpens X-1, GX 349+2 and 4U 1820-30. They also studied
the spectral lines from hot iron atoms that whirl around in a disk just beyond
the neutron stars' surfaces at speeds reaching 40 percent light speed.
Normally,
the measured spectral line for the superheated iron atoms would show up as a
symmetrical peak. However, their results showed a skewed peak that was
indicative of distortion due to relativistic effects. The extremely fast motion
of the gas (and the related powerful
gravity), they say, causes the line to smear, shifting it to longer
wavelengths.
The
measurements allowed them to determine maximum star size. "We're seeing
the gas whipping around just outside the neutron star's surface," said
XMM-Newton team member Edward Cackett of the University of Michigan. "And since the inner part of the disk obviously can't orbit any closer than the
neutron star's surface, these measurements give us a maximum size of the
neutron star's diameter."
He said the
neutron stars can be no larger than about 20.5 miles (33 kilometers) across.
The
XMM-Newton paper was published in the Aug. 1 issue of Astrophysical Journal
Letters. The other paper has been submitted for publication in the same
journal.
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