Double (Pulsar) Trouble

The discovery of a binary of two pulsars orbiting each other was a revolutionary finding, that opened up a fantastic opportunity to measure accurately several neutron star observables and test existing theories of gravity.

Among the astrophysical observables concerning neutron stars, their masses are one of the most accurately measured. This is because for neutron stars that are in binary, i.e., neutron stars orbiting another companion star, there are several known binary effects in gravity that depend on their masses. In contrast, masses of isolated neutron stars are more difficult to deduce.

Fig. 1: Animation of the two pulsars in the double pulsar system J0737-3039 orbiting each other [Source: Youtube]

There exist binaries where neutron stars orbit another neutron star, a white dwarf (another kind of compact object) or even a normal star [see Fig. 2]. But in 2003 came a surprising discovery at Australia's Parkes Observatory by a team led by Marta Burgay. They found a binary system in which two pulsars were orbiting each other. A pulsar is a neutron star that emits a radio beam close to its poles, that sweeps across the earth like a lighthouse [see animation in Fig. 1].

Fig. 2: Compilation of observed masses of neutron stars in binary [Lattimer & Prakash, e-Print: arXiv:1012.3208]

In 1974, Hulse and Taylor measured the decay in the orbit of a double neutron star system PSR B1913+16, now popularly known as Hulse-Taylor pulsar. In this system, one was a pulsar and the other an ordinary neutron stars. According to general relativity, if two neutron stars orbit each other, they lose energy in the form of gravitational waves, and orbit closer and closer to each other. Hulse and Taylor found the measured decrease in the orbits to be exactly according to the prediction by general relativity [see Fig. 3]. They were awarded the Nobel prize in 1993 for this remarkable discovery.

Fig. 3: Orbital decay of Hulse-Taylor pulsar [Living Reviews in Relativity "Binary and Millisecond Pulsars" by Duncan R. Lorimer]

A system of stars orbiting each other can be described by the well known Kepler's laws of planetary motion. But when objects such as neutron stars orbit another star, because of their strong gravitational effects, they require some additional parameters to accurately describe their system. These are known as Post-Keplerian parameters. Now with a system of two pulsars in the binary, astrophysicists had access to all the post-Keplerian parameters. This allowed them to deduce the pulsar masses, and in addition to test any modifications of Einstein's theory of gravity.

The other peculiarity of this system, was that the system was almost edge-on, i.e., the orbital disc was perpendicular to our line-of-sight. This resulted in an eclipse, every time the weaker pulsar B traversed in front of pulsar A. The origin of formation of such a peculiar astrophysical system is still under debate.