One of the culminations of European pulsar research was the discovery of the first ever double pulsar system. An artist's impression of this remarkable laboratory for relativistic and plasma physics is shown below.
In April of 2003, the first of the two pulsars, now known as J0737-3039A, was discovered by Italian members of PulSE team while processing data from a survey with the Parkes telescope. This pulsar, referred to henceforth as A, has a short 23-millisecond spin period. The large Doppler shifts measured showed that the pulsar was in a 2.45-hour, mildly eccentric orbit. General relativistic precession of the orbit was soon detected, at the phenomenal rate of 17 degrees per year, more than four times greater than that of any other pulsar binary system and 100,000 times greater than the relativistic precession of the orbit of Mercury. Measurement of the arrival time of A's pulses showed that its companion was almost certainly another neutron star, but it was not detected as a pulsar in the initial search data.
In October of 2003, while looking at a set of observations of pulsar A, Jodrell Bank PulSE team members found a second, much slower pulsation, with a period of 2.8 seconds. The Doppler shifts showed that this 2.8-s pulsar B was indeed A's previously unseen companion; the group had discovered the first ever double pulsar system! It turns out that the emission from B is only strong for roughly 20 minutes out of every 2.45-hour orbit, and it had not been active in the five-minute long discovery observation. It appears that the more energetic pulsar A alters the magnetic environment of pulsar B, allowing normal radio emission for only short amounts of time.
With a separation of only 870 million km, an orbit seen nearly edge-on and the unique ability to measure the Doppler shifts from both pulsars simultaneously, the double pulsar system provides the most accurate test ever of Einstein's theory of general relativity. This is now being persued by PulSE team members and is described further here.
In addition to providing stunning confirmation of Einstein's theories, this unique pulsar pair represents a long-predicted outcome from binary evolution theory. The currently observed system is thought to be the result of a binary system which has survived two supernova explosions. After the first explosion, material from the second star was transferred onto the newly formed pulsar spinning it up to a rapid spin rate and shielding its magnetic field. The result is thought to be the 22-ms pulsar A. Pulsar B was subsequently formed in the second supernova explosion which left a high magnetic field pulsar which spun down rapidly to the 2.7-second period now observed. This scenario is summarized in the movie shown below.
What is the ultimate fate of the double pulsar binary? Because it is so compact, profuse gravitational radiation is emitted from this system, causing the orbit to shrink. In fact, the orbit is probably 25% smaller now than it was when the binary system was born and the current rate of decay is 0.75 mm per orbit! As a result, in 85 million years, the two pulsars will inspiral and coalesce, resulting in a massive explosion of gravitational wave emission. A movie depicting this violent event is shown below.
These gravitational waves will be detectable to the farthest reaches of the Universe. Many similar systems to the double pulsar system are expected to be detectable by gravitational wave detectors such as the British-German GEO 600 in Hannover, Germany.