Scientists at the Massachusetts Institute of
Technology have found a pulsar in a binary star system that has all
but completely whittled away its companion star, leaving this
companion only about 10 times more massive than Jupiter. The system
has one of the lowest-mass companions of any stellar binary. The
finding provides clear evidence that neutron stars can slowly
“accrete” (i.e., steal) material from their companions and
dramatically increase their spin rate, ultimately evolving into the
isolated, radiowave-emitting pulsars spinning a thousand times per
second — the type commonly seen scattered throughout the Milky Way
galaxy.

The maligned companion, once a bright orange gem probably more than
half the mass of our Sun (equivalent to 500 times the mass of
Jupiter), has slowly grown dimmer and dimmer and will eventually
vanish without even a whimper.

Dr. Ron Remillard of the MIT Center for Space Research discovered
the pulsar along with Drs. Jean Swank and Tod Strohmayer of NASA
Goddard Space Flight Center. The X-ray source, named XTE J0929-314,
was found in mid May, 2002, during a routine survey of the sky
with NASA’s Rossi X-ray Timing Explorer. Dr. Duncan Galloway, a
postdoctoral associate at MIT, performed the follow-up observation
that revealed the pulsar system’s unique properties. Other members
of the MIT observation and analysis team include Dr. Edward Morgan
and Professor Deepto Chakrabarty.

“This pulsar has been accumulating gas donated from its companion
for quite some time now,” said Galloway. “It’s exciting that we
are finally discovering pulsars at all stages of their evolution,
that is, some that are quite young and others that are
transitioning to a final stage of isolation.”

A pulsar is a neutron star that emits steady pulses of radiation
with each rotation. A neutron star is the skeletal remains of a
massive star that exhausted its nuclear fuel and subsequently
ejected its outer shell in a supernova explosion. The remaining
core, still possessing about a sun’s worth of mass, collapses to
a sphere no larger than Cambridge, about 12 miles in diameter.

Neutron stars in “low mass” binary star systems such as the one
observed here (where the companion has less mass than the Sun)
have been suspected as the sites where slowly spinning neutron
stars are spun-up to millisecond spin periods. A neutron star has
a powerful gravitational field, and it can accrete gas from its
companion. Matter spirals toward the neutron star in the form of
an accretion disk, a journey visible in X-ray radiation. In doing
so, it transfers its orbital energy to the neutron star, making it
spin faster and faster, in this case, 185 times per second.

In the XTE J0929-314 system — only the third known “accreting”
millisecond pulsar of its kind and the second identified with the
Rossi Explorer in the past two months — the pulsar orbits its
companion every 43 minutes. In fact, the entire binary system would
fit within the orbit of the Moon around the Earth, which takes a
month, making this one of the smallest binary orbits known.

While the first two accreting, millisecond pulsars discovered lie
near the direction of the galactic center, the latest discovery
lies in a completely different direction. “One advantage of
XTE J0929-314,” notes Morgan, “is that observations are less
affected by crowded star fields and interstellar gas and dust.”

“This binary system is a rare find”, says Chakrabarty, who works
extensively on neutron stars in the Galaxy. “It will help us to
understand the link between slow-spinning pulsars in binary systems,
which are quite common, and fast-spinning isolated pulsars, which
are commonly seen by radio astronomers.”

With XTE J0929-314 and its 10-Jupiter-mass companion, MIT scientists
have stumbled upon a pulsar that may be further along its path to
becoming isolated. The companion will eventually vanish as a result
of both the force of gravity pulling matter onto the neutron star
(accretion), and the pressure from the resulting X-ray radiation
emitted from the neutron star blowing matter away from the companion
(ablation).

Also, this is one of the faintest transients yet discovered with the
Rossi Explorer’s All-Sky Monitor. “It was found by superposing on
the sky the thousands of snapshots that our three panning cameras
provide in a given week of observations,” said Remillard. “The
results demonstrate the value of this analysis exercise and the fact
that important science is not confined to the sources with the
brightest or most dramatic outbursts.”

The Rossi Explorer’s All-Sky Monitor is an instrument designed and
constructed at MIT. Follow-up observations were made with the Rossi
Explorer’s Proportional Counter Array instrument, which was built
by a team at NASA Goddard.

IMAGE CAPTION: [http://web.mit.edu/newsoffice/nr/2002/pulsar.jpg (7KB)]
An artist’s conception of an X-ray pulsar in a binary star system:
Matter from a companion star is channeled onto a rapidly spinning
neutron star at its magnetic poles. Art Credit: NASA

NASA Quicktime Video, 9.5MB
[http://universe.gsfc.nasa.gov/press/images/neutron/cannibal.mov]
This video shows the fate of certain binary star systems with a
neutron star. One star, several times more massive than the sun,
experiences a core collapse, which produces a neutron star. This
neutron star slowly accretes matter from its companion star, spinning
faster as it absorbs that star’s rotational energy. Scientists say
that such neutron stars cannibalize their companions, becoming
isolate millisecond pulsars. Courtesy of NASA