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
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.