Long-term Phase-coherent X-ray Timing of PSR B0540-69

Abstract

We present a new phase-coherent timing analysis for the young, energetic pulsar PSR B0540-69 using 7.6 yr of data from the Rossi X-Ray Timing Explorer. We measure the braking index, n=2.140+/-0.009, and discuss our measurement in the context of other discordant values reported in the literature. We present an improved source position from the phase-coherent timing of the pulsar, to our knowledge, the first of its kind from X-ray pulsar timing. In addition, we detect evidence for a glitch which has been previously reported but later disputed. The glitch occured at MJD 51335+/-12 with a fractional change in frequency of (1.4+/-0.2)E{-9} and a fractional change in frequency derivative of (1.33+/-0.02)E{-4}. We calculate that the glitch activity parameter for PSR B0540-69 is two orders of magnitude smaller than that of the Crab pulsar which has otherwise very similar properties. This suggests that neutron stars of similar apparent ages, rotation properties and inferred dipolar B fields can have significantly different internal properties.