LONG-TERM X-RAY CHANGES IN THE EMISSION FROM THE ANOMALOUS X-RAY PULSAR 4U 0142+61

We present results obtained from X-ray observations of the anomalous X-ray pulsar (AXP) 4U 0142+61 taken between 2000 and 2008 using XMM-Newton, Chandra, and Swift. These observations coincide with periods of long-term changes and burst epochs previously reported using the Rossi X-ray Timing Explorer (RXTE). In observations taken before 2006, we find that the pulse profile became more sinusoidal and the pulsed fraction increased with time. These results confirm those derived using RXTE and expand the observed evolution to energies below 2 keV. The total flux in the 0.5-10 keV band determined with XMM-Newton is observed to be nearly constant in observations taken before 2006, while an increase of similar to 10% is seen afterwards and coincides with the burst activity detected from the source in 2006-2007. After these bursts, the evolution toward more sinusoidal pulse profiles ceased while the flux and pulsed fraction returned to pre-bursts levels. No evidence for large-scale, long-term changes in the emission as a result of the bursts is seen. We also report on observations taken with the Gemini telescope after two bursts which show source magnitudes consistent with previous measurements. Our results demonstrate the wide range of X-ray variability characteristics seen in AXPs and we discuss them in light of current emission models for these sources.