Long-term Rossi X-ray Timing Explorer monitoring of the anomalous X-ray pulsar 1E 1048.1-5937

We report on long-term monitoring of the anomalous X-ray pulsar (AXP) 1E 10 48.1-5937 using the Rossi X-Ray Timing Explorer (RXTE). The timing behavior of this pulsar is different from that of other AXPs being monitored with R XTE. In particular, we show that the pulsar shows significant deviations fr om simple spin-down such that phase-coherent timing has not been possible o ver time spans longer than a few months. We find that the deviations from s imple spin-down are not consistent with single "glitch" type events nor are they consistent with radiative precession. We show that in spite of the ro tational irregularities, the pulsar exhibits neither pulse profile changes nor large pulsed flux variations. We discuss the implications of our result s for AXP models. We suggest that 1E 1048.1-5937 may be a transition object between the soft gamma-ray repeater and AXP populations and may be the AXP most likely to one day undergo an outburst.