A binary pulsar is a pulsar with a binary
companion, often a white dwarf or neutron
star. (In at least one case, the double pulsar
PSR J0737-3039, the companion neutron star
is another pulsar as well.) Binary pulsars
are one of the few objects which allow physicists
to test general relativity because of the
strong gravitational fields in their vicinities.
Although the binary companion to the pulsar
is usually difficult or impossible to observe
directly, its presence can be deduced from
the timing of the pulses from the pulsar itself,
which can be measured with extraordinary accuracy
by radio telescopes.
== History ==
The first binary pulsar, PSR B1913+16 or the
"Hulse-Taylor binary pulsar" was discovered
in 1974 at Arecibo by Joseph Hooton Taylor,
Jr. and Russell Hulse, for which they won
the 1993 Nobel Prize in Physics. While Hulse
was observing the newly discovered pulsar
PSR B1913+16, he noticed that the rate at
which it pulsed varied regularly. It was concluded
that the pulsar was orbiting another star
very closely at a high velocity, and that
the pulse period was varying due to the Doppler
effect: As the pulsar was moving towards Earth,
the pulses would be more frequent; and conversely,
as it moved away from Earth fewer would be
detected in a given time period. One can think
of the pulses like the ticks of a clock; changes
in the ticking are indications of changes
in the pulsars speed toward and away from
Earth. Hulse and Taylor also determined that
the stars were approximately equally massive
by observing these pulse fluctuations, which
led them to believe the other object was also
a neutron star. Pulses from this system are
now tracked to within 15 μs.The study of
the PSR B1913+16 binary pulsar also led to
the first accurate determination of neutron
star masses, using relativistic timing effects.
When the two bodies are in close proximity,
the gravitational field is stronger, the passage
of time is slowed – and the time between
pulses (or ticks) is lengthened. Then as the
pulsar clock travels more slowly through the
weakest part of the field it regains time.
A special relativistic effect, time dilation,
acts around the orbit in a similar fashion.
This relativistic time delay is the difference
between what one would expect to see if the
pulsar were moving at a constant distance
and speed around its companion in a circular
orbit, and what is actually observed.
Prior to 2015 and the operation of Advanced
LIGO, binary pulsars were the only tools scientists
had to detect evidence of gravitational waves;
Einstein's theory of general relativity predicts
that two neutron stars would emit gravitational
waves as they orbit a common center of mass,
which would carry away orbital energy and
cause the two stars to draw closer together
and shorten their orbital period. A 10-parameter
model incorporating information about the
pulsar timing, the Keplerian orbits and three
post-Keplerian corrections (the rate of periastron
advance, a factor for gravitational redshift
and time dilation, and a rate of change of
the orbital period from gravitational radiation
emission) is sufficient to completely model
the binary pulsar timing.The measurements
made of the orbital decay of the PSR B1913+16
system were a near perfect match to Einstein's
equations. Relativity predicts that over time
a binary system's orbital energy will be converted
to gravitational radiation. Data collected
by Taylor and Joel M. Weisberg and their colleagues
of the orbital period of PSR B1913+16 supported
this relativistic prediction; they reported
in 1982 and subsequently that there was a
difference in the observed minimum separation
of the two pulsars compared to that expected
if the orbital separation had remained constant.
In the decade following its discovery the
system's orbital period had decreased by about
76 millionths of a second per year - this
means that the pulsar was approaching its
maximum separation more than a second earlier
than it would have if the orbit had remained
the same. Subsequent observations continue
to show this decrease.
== Effects ==
Sometimes the relatively normal companion
star of a binary pulsar will swell up to the
point that it dumps its outer layers onto
the pulsar. This interaction can heat the
gas being exchanged between the bodies and
produce X-ray light which can appear to pulsate,
in a process called the X-ray binary stage.
The flow of matter from one stellar body to
another often leads to the creation of an
accretion disk about the recipient star.
Pulsars also create a "wind" of relativistically
outflowing particles, which in the case of
binary pulsars can blow away the magnetosphere
of their companions and have a dramatic effect
on the pulse emission.
== See also ==
Astronomy
PSR B1913+16
PSR J0737-3039
Square Kilometre Array
== References ==
== External links ==
Prof. Martha Haynes Astro 201 Binary Pulsar
PSR 1913+16 Website
Nobel Prize for the binary pulsar discovery
Neutron Star Masses
D. Lorimer (2008). "Binary and millisecond
pulsars". Living Reviews in Relativity. 11:
8. arXiv:0811.0762. Bibcode:2008LRR....11....8L.
doi:10.12942/lrr-2008-8.
C. Will (2001). "The confrontation between
general relativity and experiment". Living
Reviews in Relativity. 4: 4. arXiv:gr-qc/0103036.
Bibcode:2001LRR.....4....4W. doi:10.12942/lrr-2001-4.
I. H. Stairs. Binary pulsars and tests of
general relativity. Proceedings of the International
Astronomical Union. 5. pp. 218–227. doi:10.1017/S1743921309990433.
