Differences between the Two Anomalous X-Ray Pulsars: Variations in the   Spin Down Rate of 1E 1048.1-5937 and An Extended Interval of Quiet Spin Down   in 1E 2259+586
  We analysed the RXTE archival data of 1E 1048.1-5937 covering a time span of more than one year. The spin down rate of this source decreases by 30 percent during the observation. We could not resolve the X-ray flux variations because of contamination by Eta Carinae. We find that the level of pulse frequency fluctuations of 1E 1048.1-5937 is consistent with typical noise levels of accretion powered pulsars. Recent RXTE observations of 1E 2259+586 have shown a constant spin down with a very low upper limit on timing noise. We used the RXTE archival X-ray observations of 1E 2259+586 to show that the intrinsic X-ray luminosity times series is also stable, with an rms fractional variation of less than 15 percent. The source could have been in a quiet phase of accretion with a constant X-ray luminosity and spin down rate. 
A.Baykal (Middle East Technical University), T. Strohmayer   (NASA/GSFC), J.Swank (NASA/GSFC), A. Alpar (Sabanci Univ.), M.J...