The new binary millisecond pulsar PSR 0021-72A - A laboratory for gravitational physics

The recently discovered binary millisecond pulsar PSR 0021-72A in the globular cluster 47 Tuc has some remarkable properties. The orbit is viewed almost exactly face-on. The measured rate of precession of periastron and the predicted rate of geodetic precession are greater by nearly 2 orders of magnitude than in the classical binary pulsar PSR 1913+16, and tidally induced apsidal motion may play a role. The observability of pulses in directions nearly perpendicular to the orbital plane suggests that the system has large angles between the pulsar spin axis and the normal to the orbit. This, together with other considerations, implies that the system was probably formed by interaction of a third body with a low-mass X-ray binary in the cluster core. Accurate position measurements may well find it to be outside the cluster core. Its future lifetime is not likely to be more than 4 Myr.