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January 27, 2003

Astronomers Trace Microquasar's Path Back in Time
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Astronomers have traced the orbit through our Milky Way Galaxy of a 
voracious neutron star and a companion star it is cannibalizing, and 
conclude that the pair joined more than 30 million years ago and 
probably were catapulted out of a cluster of stars far from the 
Galaxy's center.

The pair of stars, called Scorpius X-1, form a "microquasar," in which 
material sucked from the "normal" star forms a rapidly-rotating disk 
around the superdense neutron star.  The disk becomes so hot it emits 
X-rays, and also spits out "jets" of subatomic particles at nearly the 
speed of light.

Using precise positional data from the National Science Foundation's 
Very Long Baseline Array (VLBA) and from optical telescopes, Felix 
Mirabel, an astrophysicist at the Institute for Astronomy and Space 
Physics of Argentina and French Atomic Energy Commission, and Irapuan 
Rodrigues, also of the French Atomic Energy Commission, calculated
that Scorpius X-1 is not orbiting the Milky Way's center in step with 
most other stars, but instead follows an eccentric path far above and 
below the Galaxy's plane.

Scorpius X-1, discovered with a rocket-borne X-ray telescope in 1962, 
is about 9,000 light-years from Earth.  It is the brightest continuous 
source of X-rays beyond the Solar System. The 1962 discovery and 
associated work earned a share of the 2002 Nobel Prize in physics for
Riccardo Giacconi.

Mirabel and Rodrigues used a number of published observations to 
calculate the path of Scorpius X-1 over the past few million years.

"This is the most accurate determination we have made of the path of 
an X-ray binary," said Mirabel.

By tracing the object's path backward in time, the scientists were 
able to conclude that the neutron star and its companion have been 
traveling together for more than 30 million years. They also 
speculated on the birthplace of Scorpius X-1.

"The neutron star, which is the remnant left over from the supernova 
explosion of an even more massive star, either came from the Milky 
Way's disk, or from a globular cluster at a considerable distance from 
the disk," said Rodrigues. Globular clusters are clumps of millions of 
stars in the outskirts of the Galaxy.

If it came from the Galaxy's disk, the scientists say, it would have 
had to receive a powerful one-sided "kick" from the supernova 
explosion to get into its present eccentric orbit. While this is 
possible, they conclude that a more likely scenario is that the 
neutron star came from a globular cluster.

"Probably, this neutron star picked up its companion and was thrown 
out of its globular cluster by a close encounter with other stars at 
the cluster's core," Mirabel said. The scientists published their 
results in the January 30 issue of the journal Astronomy and
Astrophysics.

The same pair of researchers traced a similar path of a black hole and 
its companion star in 2001. Also that year, other astronomers produced 
a "movie" showing motions in the jet of material ejected from the disk
around Scorpius X-1's neutron star.

The National Radio Astronomy Observatory is a facility of the National 
Science Foundation, operated under cooperative agreement by Associated 
Universities, Inc.

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