The double pulsar system: a unique laboratory for gravity

The PSR J0737-3039 is a double neutron star system in which both stars are detectable as active radio pulsars. The pair, consisting of an old, mildly recycled 23-ms pulsar and a young 2.8-s pulsar, orbit the common centre of mass in a slightly eccentric, compact orbit with a short orbital period of 147 min. The combination of system parameters makes this binary pulsar the most relativistic binary pulsar known and allows unique tests of general relativity and alternative theories of gravity. Hence, we summarize the importance of the system for such tests, and pay attention, for instance, to the observed measurement of relativistic spin precession which confirms the 'effacement' property of a spinning body. We also present a method to use measurements of the absolute position angle of the linearly polarized radio emission of the pulsars to measure the rate of the relativistic spin precession. We demonstrate how spin-orbit coupling will eventually allow us to determine the moment-of-inertia of pulsar A, and provide a general outlook into the prospects of future observations of the double pulsar.