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We report on 9 months of observations of the radio-emitting anomalous X-raypulsar XTE J1810-197 starting in 2006 May using the Nancay, Parkes, GBT, andVLA telescopes mainly at a frequency of 1.4 GHz. The torque experienced by theneutron star during this period, as inferred from a measurement of itsrotational frequency derivative, decreased by 60%, although not in a steadymanner. We have also observed very large ongoing fluctuations in flux densityand pulse shape. Superimposed on these, a general diminution of flux densityand a broadening of the pulse profile components occurred nearlycontemporaneously with a decrease in torque of about 10% that took place inlate 2006 July over an interval of 2 weeks. After a slight increase in averageflux density, since 2006 October the flux density has continued to decline andthe pulse profiles, while still varying, appear more uniform. In addition, asimultaneous observation of the pulsar with the Chandra X-ray Observatory andthe GBT allows us to show how the X-ray and radio profiles are aligned. Wediscuss briefly the implications of these results for the magnetosphericcurrents in this remarkable object.Comment: Accepted for publication in ApJ. Major changes with respect to  submitted (v1) version: includes an additional 4 months of timing, flux and  pulse shape data, and revised discussio

The Magnetar XTE J1810−197: Variations in Torque, Radio Flux Density, and Pulse Profile Morphology