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.