If they succeed, the star's velocity would make it about the fastest
moving object of its kind. If they don't, a theory is going to need
rewriting, said research physicist Kevin Hurley of the Space Sciences
Laboratory at the University of California, Berkeley.
A progress report was presented Jan. 9 at the meeting of the American
Astronomical Society by Hurley and a team of scientists from NASA's
Marshall Space Flight Center, the University of Texas at Austin, the
United States Naval Observatory and Pennsylvania State University.
The star, which goes by the name SGR1900+14, is located in the constellation
of Aquila about 20,000 light-years away. It is an example of an intensely
magnetic star, known as a magnetar, which is thought to arise from
a fairly recent supernova explosion. Only four are known to exist for
certain within our Milky Way Galaxy.
Although many stars have come to an end in supernova explosions over
the eons, in most cases the result is a dense object known as a neutron
star, which has about the same diameter as the Washington D.C. beltway,
but contains as much matter as the sun. Neutron stars are observed
by radio astronomers as pulsars. A supernova remnant, the cosmic debris
of a supernova explosion, on the other hand, can be observed by optical
and radio astronomers as a slowly expanding, diffuse glow for up to
10 or 20 thousand years. After that, it fades into complete obscurity.
For reasons which are not entirely understood, Hurley said, in very
rare cases a supernova explosion may produce an intensely magnetized
neutron star or magnetar. A magnetar at the distance of the moon, for
example, would exert a stronger pull on metallic objects than a refrigerator
magnet, erasing credit cards and subway tickets, and pulling keys from
Although SGR1900+14 is at a "safe" distance of 20,000 light years,
it still has an effect on the Earth, Hurley said. It emits short pulses
of gamma-rays, an energetic type of x-ray, which are easily measured
by orbiting satellites. Several years ago, it unleashed a torrent of
radiation so intense that it produced a major disturbance in the Earth's
upper atmosphere. Scientists are understandably interested in finding
out when and where stars like this are born, and why.
"Normally, when you discover a magnetar, you look for the nearest
supernova remnant and assume that is its birthplace," Hurley said.
"In this case, though, we have a problem — the nearest remnant is very
far away. If it really was born there, it must hold some sort of speed
record, because it got out in a big hurry. And if it wasn't, then we
have another problem, because we don't know where it came from."
The distance between the magnetar and the nearest supernova remnant
is at least 15 light years. The nearest supernova remnant is probably
10,000 years old at most, or it would have faded to the point of being
invisible. So the velocity of the magnetar can be estimated on paper,
and it turns out to be at least 10 million miles an hour.
Still, astronomers can't actually perceive its motion, because it
is so distant. The idea is to take "snapshots" of it at intervals of
several years and compare them to one another. But even this is not
an easy matter. The snapshots must be exceedingly accurate if the motion
is to become at all apparent. Until a few years ago, there were just
two ways to achieve this. One is with optical pictures, which astronomers
routinely use to measure stellar velocities. The other is with radio
Unfortunately, the magnetar does not emit enough optical light to
pose for a portrait, Hurley said. That leaves radio pictures, but the
magnetar is not very photogenic in the radio either. It emitted a large
blast of radio waves several years ago, during its last giant outburst,
but it has been radio quiet ever since. So astronomers know where it
was a few years ago, and would like to know where it is today.
Enter the Chandra x-ray Observatory, one of NASA's "great observatories"
in space, launched two and a half years ago. Chandra takes the most
accurate x-ray pictures ever achieved, and fortunately, SGR1900+14
is a bright x-ray source. Chandra took an x-ray picture of the magnetar
last June. Now the trick will be to compare the x-ray picture with
the earlier radio picture to see if the magnetar has moved.
To do this, the positions of all the other x-ray sources in the Chandra
picture will have to be measured very accurately, so that a fixed background
can be established against which the motion of the magnetar will be
apparent. The required accuracy is about one tenth of one arcsecond,
which is the size of a dime at a distance of 12 miles. The first results
are just starting to emerge, but several more months of work will be
needed to draw any firm conclusion. If the large velocity is confirmed,
a new speed record will be established. If it is not, astronomers will
have to examine the relation between supernova explosions and magnetar
births with a more critical eye.
The final results are expected to become available by the time of
the next AAS meeting at Albuquerque in June.