ROSAT Timing of the LMC Pulsar 0540-69

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-ḟ'' 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.

This research has made use of data obtained from the High Energy Astrophysics Science Archive Center (HEASARC) provided by NASA's Goddard Space Flight Center.