Catching a Fading Transient Anomalous X-ray Pulsar in Transition

The recently discovered 5.5s pulsar XTE J1810197 is the first confirmedTransient Anomalous Xray Pulsar (TAXP), likely a magnetar with a huge magneticfield of 3E14 G. XTE J1810197 is currently in outburst but fading with a 300day efold time. Archival data spanning 24 yrs indicate that this is ashortlived active state, which implies the existence of a vast population ofunexplored young NSs. From its lack of pulsations and softer spectrum inquiescence, XTE J1810197 may also hold the key to unify the magnetar model forSGRs, AXPs, and central compact objects in SNRs. We propose to continue oursemiannual XMM monitoring campaign of XTE J1810197 to follow its flux, spin,and spectral evolution, to capture for the first time a complete cycle of a TAXP from quiescence, to outburst, and back. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]