VERY-LOW BRAKING INDEX FOR THE VELA PULSAR

THE rotation rate of a pulsar is thought to decrease with time accordi ng to a simple power law, with a 'braking index' equal to 3 if rotatio nal energy is lost through radiation from a dipolar magnetic field(1-3 ). The age of the pulsar can accordingly be determined simply by measu ring the current rotation rate, and its current rate of change. Here w e report an analysis of the rotation rate of the Vela pulsar as observ ed over 25 years. We find that the braking index is 1.4+/-0.2, suggest ing that the braking cannot be attributed entirely to radiation from a constant magnetic dipole but is probably due to a changing, magnetic moment or effective moment of inertia. Taken at face value, the result implies that the Vela pulsar may be much older than previously though t, and that inferred velocities of the supernova ejecta(4) and an X-ra y jet from the pulsar(5) are correspondingly reduced. If other young p ulsars associated with supernova remnants have similarly low braking i ndices, then they too may be much older than believed, thereby reducin g their estimated velocities.