ROSATTiming of the LMC Pulsar 0540−69

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Abstract
We present a timing study of the young rotation-powered pulsar 0540-69 in the Large Magellanic Cloud, based on 130 ks of archival ROSAT data spanning a ~3 yr period. We use "f-" techniques to measure the pulsar frequency as a function of frequency derivative at 17 independent epochs. From these measurements we derive a timing solution with a braking index n=2.5+ 0.6−0.7, and we compare this solution to previous timing studies of 0540-69. Using this frequency-based solution, we create 27 pulse profiles and perform a time-of-arrival (TOA) analysis to investigate further the pulsar's timing behavior. While we can fit smooth spin-down models to subsets of the TOAs spanning up to 2 yr successfully, we are unable to obtain acceptable phase-coherent fits to the entire 3 yr set of TOAs. This behavior provides the first clear evidence for timing noise in 0540-69. We discuss the implications of these results for understanding previous studies of the timing behavior of 0540-69.
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