Long‐TermRossi X‐Ray Timing ExplorerMonitoring of Anomalous X‐Ray Pulsars

We report on the long-term monitoring of three anomalous X-ray pulsars usingthe Rossi X-ray Timing Explorer (RXTE). We present a phase-coherent timingephemeris for 4U 0142+61, and show that it has rotated with high stability over4.4 yr, with RMS phase deviations of 7% of the pulse period from a simple fitincluding only $\nu$ and $\dot{\nu}$. We report on the continued timingstability of 1E 2259.1+586, for which phase coherence has now been maintainedover 4.5 yr, as well as on the detection of a significant $\ddot{\nu}$ in 1.4yr of monitoring of RXS J170849.0-400910, consistent with recovery following aglitch. We note a correlation in which timing stability in AXPs decreases withincreasing $\dot{\nu}$. The timing stability of soft gamma repeaters inquiescence is consistent with this trend, given their large spin-down rates.This trend is similar to one seen in radio pulsars, suggesting a connectionbetween the three populations. We find no large variability in pulse morphologyas a function of time. We present high signal-to-noise ratio average pulseprofiles for each AXP, and consider them as a function of energy. We find avariety of different behaviors, and consider possible trends in the data. Wealso find no large variations in pulsed flux, and set 1 $\sigma$ upper limitsof ~20-30% of the mean. Assuming a constant pulsed fraction, either more thanone of the AXPs are coincidentally in a much more quiescent state during ourRXTE monitoring than in the past, or past claims of large flux variations areincorrect.Comment: 32 pages, 11 figures, accepted in the Astrophysical Journal. Figures 1,2,7,8 have been modified. Slight modifications to tex