A second measurement of a pulsar braking index

The braking index, n, of a pulsar, which describes the dependence of the braking torque on rotation frequency, is a fundamental parameter of pulsar electrodynamics. Simple theoretical arguments, based on the assumption of a constant dipolar magnetic field, predict n = 3, whereas the only previously measured value (for the Crab pulsar) is 2.515 +/- 0.005. Here we present radio timing observations of the X-ray and radio pulsar, PSR1509-58, made over an approximately 2-year interval, which yield a second significant measurement of n = 2.83 +/- 0.03, more than that for the Crab pulsar, but less than the canonical value of 3. Despite the large period first derivative of PSR1509-58, there is no evidence for glitches or random irregularities in the period.