WORLD'S BIGGEST TELESCOPE PURSUES RELATIVISTIC BINARY PULSAR

The discovery of the youngest known binary pulsar was an early highlight of an ongoing large-scale survey being conducted by Canadian and international astronomers at the world's largest radio telescope in Arecibo, Puerto Rico. Laura Kasian, a PhD student at the University of British Columbia (UBC), will present the latest results from this survey on behalf of the collaboration at the annual Canadian Astronomical Society (CASCA) meeting in Calgary in early June.

The pulsar, referred to as PSR J1906+0746, travels around its companion at 200km/s and completes a full orbit in only 4 hours (by comparison, the Earth travels around the Sun at 30km/s once every 9,000 hours). Since its discovery, the team has been following the binary with several radio telescopes, including the 300-metre Arecibo telescope, the Australian 64-metre Parkes telescope, the 100-metre Green Bank telescope in the US and the Lovell 76-metre telescope in UK. In fact, an observation predating the discovery was found buried in archival data from the Parkes telescope.

Initially the astronomers could not be sure whether the pulsar's companion was a white dwarf or a neutron star. Yet with the growing set of observations, the pulsar's companion appears to be about as massive as the pulsar itself. "As we collect more data, it's looking more and more likely that the companion is a neutron star" says Kasian.

This binary system will provide a unique test bed for Einstein's theory of general relativity, mainly because of its short orbital period and youth. At about 112,000 years old, it is the youngest binary pulsar system ever discovered, which means it holds important information on the creation and lifetimes of binary pulsars. These systems typically live for tens of millions of years, which means that unless we are simply lucky to be seeing the pulsar at this stage, it may indicate that more of these systems exist than was previously thought.

The binary pulsar was discovered at the beginning of a large-scale pulsar survey using the seven-beam receiver system called ALFA (Arecibo L-band Feed Array) installed on the Arecibo radio telescope. ALFA is a powerful instrument which allows an observer to point the Arecibo dish in seven different directions at once. It allows extremely
sensitive data to be taken very efficiently, and is therefore excellent in performing deep surveys of the sky. The pulsar survey using ALFA (referred to as PALFA) has been ongoing since August 2004, and is expected to take at least three more years to complete. Several hundreds of pulsar discoveries are expected with this survey. "The PALFA survey will provide us with a goldmine for studying the numbers and types of pulsars in the Galaxy, as well as the structure of the Milky Way itself," says collaboration leader James Cordes of Cornell University.

To date, the PALFA survey has been successful in detecting around 100 pulsars, roughly 30 of which are new discoveries. The majority of these were detected using a quick on-line analysis of the data. Now that the team has begun to process this data more intensely using supercomputers stationed at various institutions - including both UBC and McGill University - they expect further discoveries of new and interesting pulsars to be made.

The pulsar search is ongoing at Arecibo, and there are high hopes for more exciting results in the future. With the ALFA receiver, Arecibo has the ability to detect some of the fastest pulsars, which spin nearly 1000 times per second. It also has the ability to find binary pulsars, which are generally difficult to observe due to doppler-shifting of the pulsar signal as the neutron star travels in its orbit. Such exotic pulsars provide valuable opportunities to probe binary evolution models, aspects of general relativity, and matter at extremely high densities.

Besides Kasian, the Canadian team members include postdoctoral fellow Joeri van Leeuwen and assistant professor Ingrid Stairs of UBC, as well as Ph.D student Jason Hessels, postdoctoral fellow David Champion and associate professor Victoria Kaspi of McGill University. The international collaboration counts roughly 35 members from Canada, the United States, the United Kingdom, the Netherlands, China, India, and Australia.

BACKGROUND

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. They can spin extremely quickly, as fast as 700 times per second, and have masses comparable to that of the sun. Pulsars are born from the death of massive stars, in which the stellar core collapses to a radius of ten kilometers, and have densities similar to that of an atomic nucleus.

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.

For more information:

Dr. Laura Kasian
UBC
Phone: (604) 822-3726
Email: kasian@astro.ubc.ca