A Phase-connected Braking Index Measurement for the Large Magellanic Cloud Pulsar PSR B0540−69

Abstract

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 Deltanu/nu=(1.90+/-0.04)×10-9 and in frequency derivative of Deltanu/nu=(8.5+/-0.5)×10-5. 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.