Discovery of a young radio pulsar in a relativistic binary orbit

We report on the discovery of PSR J1141 - 6545, a radio pulsar in an eccentric, relativistic 5 hr binary orbit. The pulsar shows no evidence of being recycled, having a pulse period P = 394 ms, a characteristic age tau (c) = 1.4 x 10(6) yr, and an inferred surface magnetic dipole field strength B = 1.3 x 10(12) G. From the mass function and measured rate of periastron advance, we determine the total mass in the system to be 2.300 +/- 0.012 M(.), assuming that the periastron advance is purely relativistic. Under the same assumption we constrain the pulsar's mass to be M(p) less than or equal to 1.348 M(.), and the companion's mass to be M(c) > 0.968 M, (both with 99% confidence). Given the total system mass and the distribution of measured neutron star masses, the companion is probably a massive white dwarf that formed prior to the birth of the pulsar. Optical observations can test this hypothesis.