Astronomers weigh "recycled" millisecond pulsar (Forwarded)

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Naval Research Laboratory
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1/12/2006

NRL Press Release: 2-06r

Astronomers weigh "recycled" millisecond pulsar

A team of U.S. and Australian astronomers is announcing today that they
have, for the first time, precisely measured the mass of a millisecond
pulsar -- a tiny, dead star spinning hundreds of times every second. This
result is of special interest because it gives new insight into the
production of millisecond pulsars and may shed light on the laws that
govern nuclear matter. The work is being presented today at the American
Astronomical Society meeting in Washington, DC, by Dr. Bryan A. Jacoby of
the Naval Research Laboratory in Washington, DC; Aidan W. Hotan of the
University of Tasmania in Hobart, Australia; Professor Matthew Bailes of
Swinburne University of Technology in Melbourne, Australia; Dr. Stephen M.
Ord of the University of Sydney in Sydney, Australia; and Professor
Shrinivas R. Kulkarni of the California Institute of Technology in
Pasadena, CA.

Pulsars are truly extreme, with significantly more mass than the sun but
so compact that they would fit inside the Washington, DC, beltway. These
spinning neutron stars are produced when a massive star is destroyed by a
supernova explosion at the end of its normal stellar life. As a pulsar
spins, it emits beams of radio waves that sweep through space like a
lighthouse beacon; astronomers use radio telescopes to observe the
apparent blinking of the pulsar and precisely measure the time when its
pulses arrive at the earth. A pulsar's spin slows at it ages, but If the
pulsar is in a binary system with another star, it can accumulate gas shed
by its companion star. This process of accretion, called "recycling," can
accelerate the pulsar's spin to hundreds of rotations per second -- faster
than a kitchen blender!

Using the 64-meter (210-foot) Parkes radio telescope in Parkes, Australia,
the research team made very precise measurements of the pulses from a
recently discovered millisecond pulsar called PSR J1909-3744, about 3700
light years away in the constellation Corona Australis. This pulsar spins
every 2.9 milliseconds, or 340 times per second; the pulsar and its white
dwarf companion star orbit their common center of gravity every 1.5 days.
These observations were carried out at a frequency of 1.4 GHz, which
provides a good compromise between the brightness of the pulsar and the
deleterious effects of the interstellar medium.

By taking exact measurements of the arrival time of pulses from PSR
J1909-3744 at regular intervals for nearly two years, and keeping count of
every pulse of radio waves during this time (about 19 billion pulses), the
astronomers precisely mapped out the pulsar's position on the sky and the
shape of its orbit. They also noticed something unusual: when the pulsar
is on the far side of its orbit, behind its white dwarf companion, its
pulses arrive at the earth about 14 millionths of a second later than
expected based on Newtonian mechanics. This effect, called Shapiro delay,
is a consequence of Einstein's general theory of relativity; essentially,
the light from the pulsar slows down when traveling through the companion
star's gravitational field. When the pulses have to go past the companion
on the way to the earth, they arrive late compared to when the companion
is behind the pulsar. This is in addition to the much larger Roemer delay
(3.8 seconds) arising simply because light from the pulsar has to travel
further when it is on the far side of its orbit. Disentangling these two
effects is only possible if the orbit is measured in incredible detail.
The orbit of PSR J1909-3744 is the most circular known in the universe:
the elliptical orbit is over one million kilometers across (about 1.5
times the size of the Moon's orbit around the Earth), but the major axis
is larger than the minor axis by only 10 microns, a fraction of the
thickness of a human hair.

By precisely measuring the size of the Shapiro delay and how it varies
throughout the orbit, the astronomers ascertained the mass of the white
dwarf companion and the angle that the pulsar's orbit makes with the sky.
Combined with Kepler's laws of motion, this information allowed them to
calculate the pulsar's mass: 1.44 times that of the sun. The uncertainty
in this measurement, 0.02 solar masses, is about 5 times smaller than the
uncertainty of the previous best measurement of a millisecond pulsar's
mass. This improvement was possible because this particular pulsar is well
suited to high-precision pulse arrival time measurements, because its
orbit is oriented so that we view it almost exactly from its edge
(maximizing the Shapiro delay), and because of the specialized
instrumentation developed at the California Institute of Technology and
the Swinburne University of Technology for these observations.

Before this result, only the masses of slower-spinning (non-millisecond)
pulsars had been measured precisely. Mildly-recycled pulsars, spinning a
few tens of times per second and thought to have accreted a relatively
small amount of matter from a companion, are between 1.31 and 1.44 times
the mass of the sun; reassuringly, none are more massive than PSR
J1909-3744 which should have accreted more material from its companion. In
only one case has the mass of a completely unrecycled pulsar, spinning
once every several seconds, been measured at 1.25 solar masses. If this
mass is typical, we can infer that a millisecond pulsar can be produced
with the accretion of less than 0.2 solar masses from its companion. If
this is the case, then the pulsar recycling process must be messy: more
than half a solar mass must have been lost from the companion as a wind of
ejected gas as it evolved from a normal main sequence star to a white
dwarf.

Because neutron stars behave, in many respects, like giant atomic nuclei,
measuring the physical properties of these exotic objects enhances our
understanding of fundamental physics. This result is exciting because "we
now have a more complete picture of how these exotic objects are formed,
and how they relate to other types of neutron stars," says Dr. Jacoby, who
is a National Research Council Postdoctoral Associate at NRL.

This research was supported by the NSF and NASA. Basic research in radio
astronomy at NRL is supported by the Office of Naval Research. The Parkes
telescope is part of the Australia Telescope which is funded by the
Commonwealth of Australia for operation as a National Facility managed by
CSIRO.

IMAGE CAPTION:
[http://www.nrl.navy.mil/pao/PressRel...Fig1-2-06r.jpg (30KB)]
Shapiro delay in the pulsar PSRJ 1909-3744's signal due to the
gravitational field of its companion. In the top panel, diagrams show the
configuration of the pulsar and its white dwarf companion in their orbits
relative to the line of sight to the observer on the Earth at two key
points in the orbit: when the pulsar is closest to the Earth (left, the
point of minimum Shapiro delay) and when the pulsar is on the far side of
its companion as viewed from Earth (right, the point of maximum Shapiro
delay). The bottom panel shows the variation in Shapiro delay through one
complete orbit of the pulsar-white dwarf system, with yellow arrows
highlighting the connection with the corresponding orbital diagrams in the
top panel. The size and shape of this delay curve allowed astronomers to
precisely calculate the mass of the pulsar. This result, obtained with the
64-meter (210-foot) Parkes radio telescope in Australia, were presented to
the American Astronomical Society meeting in Washington, DC on January 12,
2006. Note: diagrams in the top panel are not to scale.