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

We report on observations of the radio-emitting anomalous X-ray pulsar XTE J1810-197 during 2006 May-October using the Nancay, Parkes, GBT, and VLA telescopes at a frequency of 1.4 GHz. The torque experienced by the neutron star during this period, as inferred from a measurement of its rotational frequency derivative, decreased by 30%, although not in a steady manner. We have also observed very large ongoing fluctuations in flux density and pulse shape. Superimposed on these, a general diminution of flux density and a broadening of the pulse profile components occurred nearly contemporaneously with a decrease in torque of about 10% that took place in late July over an interval of two weeks. In addition, a simultaneous observation of the pulsar with the Chandra X-ray Observatory and the GBT allows us to show how the X-ray and radio profiles are aligned. We discuss briefly the implications of these results for the magnetospheric currents in this remarkable object.