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

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

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.