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.