We present a timing study of the young rotation-powered pulsar 0540-69 in the Large Magellanic Cloud, based on 130 kiloseconds of archival ROSAT data spanning a ∼3-year period. We use ``f−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=2.5−0.7+0.6, 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 successfully fit smooth spin-down models to subsets of the TOAs spanning up to 2 years, we are unable to obtain acceptable phase-coherent fits to the entire 3-year 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.
@article{arxiv.astro-ph/9707325,
title = {ROSAT Timing of the LMC Pulsar 0540-69},
author = {S. S. Eikenberry and G. G. Fazio and S. M. Ransom},
journal= {arXiv preprint arXiv:astro-ph/9707325},
year = {2009}
}