Astronomers
using NASA's Swift X-ray telescope have detected a neutron star within 250 to
1,000 light-years of Earth, making it the closest neutron star ever known.
The object,
located in the constellation Ursa Minor, is nicknamed Calvera, after the villain
in the movie "The Magnificent Seven." If confirmed, it would be only
the eighth known "isolated neutron star," or one that does not have
supernova remnants, binary companions or radio pulsations.
"The
seven previously known isolated neutron
stars are known collectively as 'The Magnificent Seven' within the
community and so the name Calvera is a bit of an inside joke on our part,"
said Derek Fox, an astronomer at Penn State University who co-discovered the
mysterious object.
Neutron
stars are supernova leftovers that are too small to form black holes. Instead,
the leftover gas and dust crams into a glowing, incredibly dense body only a
few miles wide--a teaspoon, for example, would weigh millions of tons. A
study of the newly discovered neutron star will be detailed in an upcoming
edition of the Astrophysical Journal.
Robert
Rutledge of McGill University in Montreal, Quebec, originally called attention
to the curious X-ray source. He compared a catalog of 18,000 X-ray sources from
the German-American ROSAT satellite, which operated from 1990 to 1999, with
catalogs of objects that appear in visible light, infrared light and radio
waves. He realized that the ROSAT source known as 1RXS J141256.0+792204 did not
appear to have a counterpart at any other wavelength.
The group
aimed Swift at the object in August 2006, showing that it was still there and
emitting about the same amount of X-ray energy as it had during the 1990s. The
Swift observations, however, enabled the group to pinpoint the object's
position more accurately and show that it was not associated with any known
object.
"The
Swift observation of this source is what got the show going," said Andrew
Shevchuk, an astronomy student at Penn State and co-author of the study.
"As soon as I saw the data, I knew Calvera was a great neutron-star
candidate."
To show
that the object is not associated with any other energy source, the team
targeted the 8.1-meter Gemini North Telescope in Hawaii on Calvera. The team
also used the Chandra
X-ray Observatory, which showed the object as point-like—an observation
consistent with neutron-star appearance.
Rutledge said
there are no widely accepted alternate theories for objects bright in X-rays
and faint in visible light like Calvera. Exactly which type of neutron star it
is, however, remains a mystery.
"Either
Calvera is an unusual example of a known type of neutron star, or it is some
new type of neutron star, the first of its kind," Rutledge said.
Calvera's
location high above the plane of our Milky
Way Galaxy is part of its mystery, but researchers think the neutron star
is the remnant of a star before exploding as a supernova. In order to reach its
current position, it had to wander some distance out of the disk.
"The
best guess is that it is still close to its birthplace, and therefore close to
Earth," Rutledge said. If true, the object is 250 to 1,000 light-years
away--making it one of the closest known neutron stars.
"Because
it is so bright, and probably close to Earth, it is a promising target for many
types of observations," Fox said.
The team
added that Calvera could represent the tip of the iceberg for isolated neutron
stars.
"There
could easily be dozens," Fox said. "The key point is that until our
Swift survey, no one was able to refine the X-ray positions of large numbers of
ROSAT sources to the point where it became clear which ROSAT sources were
'missing' their optical counterparts."
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