NASA Astronomers Find Bizarre Planet-Mass Object Orbiting Neutron Star

September 12, 2007

PIO Contacts:
Robert Naeye / Rob Gutro
Goddard Space Flight Center, Greenbelt, Md.
301-286-4453/4044

PRESS RELEASE: 07-53

NASA ASTRONOMERS FIND BIZARRE PLANET-MASS OBJECT ORBITING NEUTRON STAR

Using NASA's Swift and Rossi X-ray Timing Explorer (RXTE)
satellites, astronomers have discovered one of the most bizarre
planet-mass objects ever found.

The object's minimum mass is only about 7 times the mass of
Jupiter. But instead of orbiting a normal star, this low-mass body
orbits a rapidly spinning pulsar. It orbits the pulsar every 54.7
minutes at an average distance of only about 230,000 miles (slightly
less than the Earth-Moon distance).

"This object is merely the skeleton of a star," says co-discoverer
Craig Markwardt of NASA's Goddard Space Flight Center in Greenbelt,
Md. "The pulsar has eaten away the star's outer envelope, and all the
remains is its helium-rich core."

Hans Krimm of NASA Goddard discovered the system on June 7, when
Swift's Burst Alert Telescope picked up an outburst of X rays and
gamma rays in the direction of the galactic center. The source was
named SWIFT J1756.9-2508 for its sky coordinates in the constellation
Sagittarius.

RXTE began observing SWIFT J1756.9 on June 13 with its Proportional
Counter Array (PCA). After analyzing the PCA data, Markwardt realized
that the object was pulsing in X rays 182.07 times per second, which
told him that it was a rapidly spinning pulsar. These so-called
millisecond pulsars are neutron stars that spin hundreds of times per
second, faster than a kitchen blender. Normally, the spin rate of
neutron stars slows down as they age, but much like we can pull a
string to "spin up" a top, gas spiraling onto a neutron star from
its companion can maintain or even increase its fast spin.

In the case of SWIFT J1756.9-2508, Markwardt detected subtle
modulations in the X-ray timing data that revealed a low-mass
companion tugging the pulsar toward and away from Earth. His
calculations show that the companion has a minimum mass about 7 times
that of Jupiter. Because we don't know the orbital inclination of the
system, the companion's actual mass is unknown, but it is extremely
unlikely to exceed 30 Jupiters.

MIT astronomers led by Deepto Chakrabarty also observed the system
with RXTE, before it faded to invisibility on June 21. Chakrabarty's
group reached identical conclusions, and the two teams have
coauthored a paper that has been accepted for publication in the
Astrophysical Journal Letters.

The system is only the eighth millisecond pulsar that is observed to
be accreting mass from a companion. Only one other such system has a
pulsar companion with such a low mass. The companion in this system,
XTE J1807-294, also has a minimum mass of about 7 Jupiters. "Given
that we don't know the exact mass of either companion, ours could be
the smallest," says Krimm.

The system probably formed several billion years ago, when it
consisted of a very massive star and a smaller star with perhaps 1 to
3 solar masses. The more massive star evolved quickly and exploded as
a supernova, leaving behind the neutron star. The smaller star
eventually started to puff up en route to becoming a red giant, and
the two objects became embedded in the extended stellar envelope.
This drained orbital energy, causing the two stars to draw ever
nearer, while simultaneously ejecting the envelope.

Today, the two objects are so close to each other than the neutron
star's powerful gravity produces a tidal bulge on its companion,
siphoning off gas that flows into a disk that surrounds the neutron
star. The flow eventually becomes unstable and dumps large quantities
of gas onto the neutron star, causing an outburst like the one
observed in June.

Evolution models by Christopher Deloye of Northwestern University
suggest that the low-mass companion is helium dominated. "Despite its
extremely low mass, the companion isn't considered a planet because
of its formation," says Deloye. "It's essentially a white dwarf that
has been whittled down to a planetary mass."

After billions of years, little remains of the companion star, and it
remains unclear whether it will survive. "It's been taking a beating,
but that's part of nature," adds Krimm.

With an estimated distance of roughly 25,000 light-years, the system
is normally too faint to be detected at any wavelength, and is only
visible during an outburst. SWIFT J1756.9 has never been seen to
erupt until this June, so as Markwardt points out, "We don't know how
long it will slumber before it wakes up again."

For related images to this story, please visit on the Web:

http://www.nasa.gov/centers/goddard/news/topstory/2007/millisecond_pulsar.html

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