MEASUREMENTS OF RADIO PULSAR BRAKING INDICES: A STATISTICAL APPROACH

A statistical technique for measuring the braking index of radio pulsars is presented. Our method employs the recently reported strong correlation (with correlation coefficient r = 0.95) between the observed second time derivative of the pulse rotation frequency (ν̈obs), obtained from fully phase-coherent timing analyses, and a timing noise statistic (σR23), used to quantify the amount of pulsar rotational fluctuations absorbed by the cubic term, to estimate the component of the measured braking index originating solely from pulsar timing irregularity (ν̈tno). The presumed deterministic braking index from electromagnetic torque braking processes (ν̈dip) is subsequently obtained as the magnitude of the difference between ν̈obs and ν̈tno. Application of this method to a sample of 27 radio pulsars, whose timing data span ∼ 9 − 13 years, show that (i) for 12 pulsars, the observed timing activity is too weak to allow for unambiguous braking index measurements, (ii) for 5 pulsars, the braking index appears to be significantly measured (n. 3), and (iii) for 10 pulsars, the braking indices have anomalous values. These results are discussed in the context of the prevailing standard model for radio pulsar spin-down. 1