A phase-connected braking index measurement for the Large Magellanic Cloud pulsar PSR B0540-69

We report the results of an extensive monitoring campaign of PSR B0540-69, the 50 ms pulsar in the Large Magellanic Cloud, using data acquired with the Rossi X-Ray Timing Explorer. A set of 45 pointed X-ray observations spanning 1.2 yr reveals a steady spin-down interrupted by a single glitch event, the first recorded for this young, rotation-powered pulsar. The quality of the data allowed us to perform a pulse-timing analysis that successfully maintains cycle count between observations, after allowing for the observed glitch. The glitch event occurred on MJD 51,325 +/- 45 (90% CL error) and is characterized by a change in frequency of Delta nu/nu = 1.90 +/- 0.04 x 10(-9) and in frequency derivative of. By phase-linking the data set we provide a definitive measurement of the spin frequency of the pulsar and its first and second derivatives. These translate into a braking index of n = 1.81 +/- 0.07, significantly lower than reported previously for this source using measurements that could not be phase-linked unambiguously.