The Double Pulsar - how was it discovered and why is it important?

The headline of a press release in early January 2004 read "First-Known Double Pulsar Opens Up New Astrophysics". The discovery, published in the journal Science, sent ripples of excitement through the astronomical community, but why? What is so exciting about a double pulsar?

What is a Pulsar?
A pulsar is a kind of neutron star, the remnant left when a massive star runs out of fuel and explodes as a supernova. They are incredibly dense objects - imagine something the mass of the Sun squashed into a ball the diameter of Greater Manchester and you're about there. Another analogy (which is just as mind-boggling) is that one eggcup full of neutron star material would have roughly the same mass as 35000 billion newspapers!

Pulsars can be thought of as cosmic lighthouses as they emit cones of radiation from their poles and rotate very quickly such that their beams of radiation sweep out arcs in space. If we are lucky, one of these beams crosses our line of sight during the pulsar's rotation and we see a flash each time the beam sweeps past. We can determine how quickly they rotate from measuring how rapidly the signal varies. Some pulsars have periods of the order of milliseconds - imagine our ball with the mass of the Sun and the diameter of Manchester rotating hundreds of times a second! And a double pulsar? Well, it's as simple as it sounds - two pulsars in orbit around each other, just like a double star.

Pulsar Searches
The double pulsar, known as PSR J0737-3039, was discovered during the Parkes Multibeam Survey (PMS) by an international collaboration including astronomers from the UK, Australia, Italy and the USA. The multibeam instrument consists of 13 receivers mounted at the focus of the 64 m diameter Parkes radio telescope in New South Wales, Australia. Since it began operation in 1997 the instrument has discovered over 700 new pulsars, doubling the total number of known pulsars, and making the PMS the most successful pulsar survey to date.

The Double Pulsar
Although over 1400 pulsars have been detected since they were accidentally discovered by Anthony Hewish and Jocelyn Bell at Cambridge in 1967, this particular system has perhaps caused the most excitement. Previous observations have detected several "pulsar binaries" in which a pulsar exists in an orbit around a companion "ordinary" neutron star - Russell Hulse and Joseph Taylor won the Nobel prize for their discovery of the first of these binaries, PSR 1913+16, in 1974 - but never before have two pulsars been detected in orbit around each other.

Pulsars, like all astronomical objects, are given catalogue numbers. The newly discovered double pulsar is officially known as PSR J0737-3039. The PSR signifies that the object is a pulsar while the J0737-3039 specifies the object's position (in RA and Dec) with respect to the J2000 standard. As there are actually two pulsars in this system they are known as PSR J0737-3039A and PSR J0737-3039B. The system lies in our Galaxy at a distance of around 2000 light years. The two pulsars are separated by roughly 800,000 km and orbit each other in just under two and a half hours.

Artistic impression of 
the double pulsar system
Artistic impression of the double pulsar system. Credit: Michael Kramer, JBO.

How was it detected?
Initially, this system was thought to be an ordinary pulsar binary. The first observations detected a single pulsar with a period of 23 milliseconds in orbit around a neutron star companion. It was only after follow-up observations were made, both with Parkes and the Lovell telescope, that a second, weaker set of pulses with a period of 2.8 seconds were detected coming from the companion object.

As luck would have it, the system is orbiting in such a way that the pulsars periodically eclipse one another so the weaker signal comes and goes as it is eclipsed by its companion. The reason why the second set of pulses was not discovered straight away is because of the weakness of the second signal. The fact that the orbits of the pulsars are nearly edge-on enables the researchers to study the atmospheres of these objects by watching how the signal alters as one pulsar passes behind the other.

Gravitational waves
One exciting aspect of modern physics that can be probed using this system is the possibility of gravitational waves, essentially ripples in the fabric of space-time, long predicted but not yet detected. As the two pulsars orbit each other they slowly spiral together, losing energy in the form of gravitational waves as they do so. Eventually, the two orbiting bodies will coalesce, producing large gravitational ripples as they do so. This energy loss was measured in PSR 1913+16 by Hulse and Taylor, but gravitational waves have yet to be directly measured.

J0737-3039 is a far more extreme system than 1913+16, as Dr Dick Manchester, head of the pulsar group at the Australia Telescope National Facility, put it: "It's just a fantastic natural laboratory, this system is really extreme." Theoretical models based on this new data show that the predicted coalescence events are more common than previously thought. The more common these events are, the more likely we are to actually detect evidence in the form of gravitational waves.

The basic layout of 
the LIGO detector
The basic layout of a LIGO detector. Credit: LIGO.

This is good news for the teams building gravitational wave detectors such as LIGO (Laser Interferometer Gravitational wave Observatory) run by the California Institute of Technology and Massachusetts Institute of Technology, an extrememly sensitive observatory consisting of two widely separated detectors. Each detector consists of two "arms" at right angles to each other, down which a laser beam is sent. The light is reflected back to the centre and combined in such a way that the light beams interfere producing a specific pattern, often referred to as "fringes". As a gravitational wave passes, the length of one arm will increase slightly compared to the other and this tiny change in the geometry of the detector will alter the pattern of fringes. It's a very delicate operation: the entire detector must be isolated from seismic effects as the expected change in the geometry is 10-16 cm!

So what does the future hold? Pulsar researchers are continuing to monitor J0737-3039 to refine their measurements and models, searches for more double pulsars are underway and who knows, the first detection of gravitational waves could be just around the corner.


JBO press release >
ATNF press release >
Pulsar group at JBO >
Pulsar group at ATNF >

Last updated: Tuesday, 03-Mar-2009 02:50:09 GMT