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).
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.