Long‐Term Phase‐coherent X‐Ray Timing of PSR B0540−69

We present a new phase-coherent timing analysis for the young, energeticpulsar 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 inthe context of other discordant values reported in the literature. We presentan improved source position from the phase-coherent timing of the pulsar, toour knowledge, the first of its kind from X-ray pulsar timing. In addition, wedetect evidence for a glitch which has been previously reported but laterdisputed. The glitch occured at MJD 51335+/-12 with a fractional change infrequency of (1.4+/-0.2)E{-9} and a fractional change in frequency derivativeof (1.33+/-0.02)E{-4}. We calculate that the glitch activity parameter for PSRB0540-69 is two orders of magnitude smaller than that of the Crab pulsar whichhas otherwise very similar properties. This suggests that neutron stars ofsimilar apparent ages, rotation properties and inferred dipolar B fields canhave significantly different internal properties.Comment: 18 Preprint pages, 6 Figures. Accepted to the Astrophysical Journa