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.

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