Discovery of the Youngest Ever Binary Pulsar (Forwarded)

Jodrell Bank Observatory
School of Physics & Astronomy
The University of Manchester
Macclesfield, Cheshire SK11 9DL
United Kingdom

Contact information:

Dr Duncan Lorimer, Jodrell Bank Observatory
00 44 (0)1477 572675

Dr Andrew Faulkner, Jodrell Bank Observatory
00 44 (0)1477 572677

Professor Andrew Lyne, Jodrell Bank Observatory
00 44 (0)1477 572640

12th January 2006

Discovery of the Youngest Ever Binary Pulsar

An international team of astronomers using a state-of-the-art receiver
system on the 305-m Arecibo radio telescope in Puerto Rico has discovered
a binary pulsar system which promises to be an important new test bed for
Einstein's theory of general relativity. The new binary system, known as
PSR J1906+0746, consists of a 144-ms pulsar in a 4-hr orbit around an as
yet unseen companion that is very likely another neutron star or a white
dwarf. What makes it different from other binary pulsars is the relative
youthfulness of the pulsar, about 112 thousand years, which increases
estimates of the rate at which such systems will spiral in and destroy
themselves. The discovery was presented at the American Astronomical
Society meeting in Washington, DC on January 12, 2006 and will be
published in the Astrophysical Journal in March.

The latest discovery was made during the early stages of a new large-scale
pulsar survey using the Arecibo L-band Feed Array (ALFA) system. ALFA uses
a set of 7 high-sensitivity receivers installed at the focus of the
world's largest radio telescope at Arecibo, Puerto Rico to simultaneously
observe 7 adjacent regions of the sky. This greatly increases the amount
of sky that the 305-m telescope can cover in a given time interval. "It is
equivalent to having seven Arecibo telescopes working together in
parallel", says Prof. Jim Cordes, of Cornell University, leader of the
international consortium of pulsar astronomers using ALFA. "The fact that
ALFA is attached to the most sensitive radio telescope on the Earth means
that we can carry out the deepest searches ever made of our Galaxy,"
Cordes adds.

The pulsar survey with ALFA (dubbed PALFA) will take at least 5 years to
complete and is expected to discover up to 1000 new pulsars. Observations
began in August 2004 and the new binary system was identified in a
preliminary, on-line analysis of data taken two months into the survey.
Dr. Duncan Lorimer of the University of Manchester, who helped install the
PALFA processing system, remarks: "The great sensitivity of Arecibo allows
us to use short dwell times (5 min) for each direction searched, which
minimizes changing Doppler shifts of compact binary systems and provides
excellent snapshots of any that might be present." The binary nature of
the new system was identified by inspecting archival data from an earlier
survey with the 64 meter Parkes radio telescope in New South Wales,
Australia. Subsequent observations with Arecibo and with the Lovell 76
meter telescope in the UK and the 100 meter Green Bank Telescope in West
Virginia were carried out to establish the precise nature of the orbit.

From the orbital parameters measured so far, the team has shown that the
pulsar's orbit precesses at a rate of 7.6 degrees per year. This
relativistic effect was first identified as part of the precession of the
orbit of Mercury around the Sun and its explanation was one of the initial
triumphs of Einstein's theory. However, as remarked by team member
Assistant Professor Ingrid Stairs at the University of British Columbia,
Canada, the size of this effect in the new binary system is 60,000 times
larger than for Mercury. "Using Einstein's theory, the precession allows
us to deduce the total mass of the binary system to be 2.6 solar masses,"
Stairs adds. Using other constraints from their observations, the team
concludes that the companion to PSR 1907+0647 is either another neutron
star or a white dwarf star. However, despite intensive searches for a
neutron star companion, the nature of this companion remains unclear.

What is most intriguing about the team's measurements of new system is
that the pulsar appears to be only about 112 thousand years old. "This is
1000 times younger than the double pulsar system," comments Dr. Paulo
Freire, a staff astronomer at the Arecibo Observatory, "and if the
companion is another neutron star, it might well be a baby (or extremely
young) version of the double pulsar". Regardless of the nature of the
companion star, the young age of the system implies a potentially large
rate of formation of similar binaries in our Galaxy. "Given that typical
lifetimes for pulsars are tens of millions of years, it would be extremely
fortuitous for us to see one in a binary system so young unless such
systems are very common," says Prof David Nice of Bryn Mawr College, who
adds that "our current estimate of the formation rate means that
gravitational wave detectors such as LIGO can expect to see twice as many
events as previously thought."

The new discovery highlights the fact that there are many uncertainties
concerning the binary pulsar population. "From observations over the next
few years, we expect to be able to measure the orbital decay and other
relativistic effects that will provide an important complement to the
existing binary pulsar tests of Einstein's theories" says Stairs. As
observed in the other relativistic binary systems, there appears to be
evidence for a wobble of the pulsar's spin axis due to another general
relativistic effect. Lorimer comments that "the pulse shape of the pulsar
appears to have changed between the current observations and that seen in
the archival Parkes data from seven years ago. Further observations will
allow us to confirm this effect."

Meanwhile, the search goes on at Arecibo for more pulsars. The new binary
system was found in the early stages of the survey, so more are expected.
In addition, it is expected that hyperfast pulsars -- those spinning up to
about 1000 times per second -- will be found, which will provide further
oppportunities to probe gravitational waves and magnetized matter at
densities completely inaccessible to terrestrial laboratories.

The Arecibo Observatory is part of the National Astronomy and Ionosphere
Center (NAIC) and is operated by Cornell University under a cooperative
agreement with the National Science Foundation (NSF).

Supplementary information

Pulsars are rapidly spinning, highly magnetized neutron stars that emit
beamed emission along their magnetic axes. Studied by astronomers across
the electromagnetic spectrum, but predominantly at radio wavelengths, the
clock-like stability of their radiation allows pulsars to be used as
probes in a wide variety of astrophysical settings that include binary
star systems.

The first binary pulsar system, B1913+16, a pair of neutron stars, was
discovered at Arecibo by Russell Hulse and Joe Taylor in 1974. In their
compact and highly relativisitic 7.75-hr orbit, the two neutron stars are
accelerated to speeds of up to 0.1% of the speed of light. Careful
measurements over 30 years of one of the neutron stars (which is visible
as a pulsar) show that the orbit shrinks at the rate of about 1 mm per
day. This effect is due to orbital energy being radiated away in the form
of gravitational waves -- small ripples in spacetime first predicted as a
consequence of Einstein's theory of general relativity in 1916. The
orbital decay measurements agree to within 0.1% with predictions from
Einstein's theory. The orbital decay means that binary will coalesce in
about 400 million years, undoubtedly producing a spectacular event for
future astronomers to detect as a gamma-ray burst and with gravitational
wave telescopes! Hulse and Taylor received the 1993 Nobel Prize in Physics
for their discovery.

Binary pulsars are fantastic laboratories for testing theories of gravity
and it is clear that as-yet undiscovered binaries will provide even better
opportunities to test general relativity. Astronomers have therefore been
searching the Milky Way Galaxy with increasingly better technology to
detect these observationally challenging systems. Not only are they rare
(out of over 1700 pulsars known, only 8 are in relativistic binary
systems), they are also very hard to detect due to the changing Doppler
shift of their signals as they move very rapidly on their orbital path.
One of the recent highlights from these searches was the discovery in 2003
of the first double-pulsar system, J0737-3039, in which both neutron stars
are visible as pulsars. Observations of the shrinkage of the orbit of this
pair have already reached the same level of agreement with Einstein's
theory as Hulse and Taylor's binary after only 3 years.