We report on timing , flux density , and polarimetric observations of the transient magnetar and 5.54 s radio pulsar XTE J1810 - 197 using the Green Bank , Nançay , and Parkes radio telescopes beginning in early 2006 , until its sudden disappearance as a radio source in late 2008 .
Repeated observations through 2016 have not detected radio pulsations again .
The torque on the neutron star , as inferred from its rotation frequency derivative \dot { \nu } , decreased in an unsteady manner by a factor of 3 in the first year of radio monitoring , until approximately mid-2007 .
In contrast , during its final year as a detectable radio source , the torque decreased steadily by only 9 % .
The period-averaged flux density , after decreasing by a factor of 20 during the first 10 months of radio monitoring , remained relatively steady in the next 22 months , at an average of 0.7 \pm 0.3 mJy at 1.4 GHz , while still showing day-to-day fluctuations by factors of a few .
There is evidence that during this last phase of radio activity the magnetar had a steep radio spectrum , in contrast to earlier flat-spectrum behavior .
There was no secular decrease that presaged its radio demise .
During this time the pulse profile continued to display large variations , and polarimetry , including of a new profile component , indicates that the magnetic geometry remained consistent with that of earlier times .
We supplement these results with X-ray timing of the pulsar from its outburst in 2003 up to 2014 .
For the first 4 years , XTE J1810 - 197 experienced non-monotonic excursions in frequency derivative by at least a factor of 8 .
But since 2007 , its \dot { \nu } has remained relatively stable near its minimum observed value .
The only apparent event in the X-ray record that is possibly contemporaneous with the radio shut-down is a decrease of \approx 20 \% in the hot-spot flux in 2008–2009 , to a stable , minimum value .
However , the permanence of the high-amplitude , thermal X-ray pulse , even after the ( unexplained ) radio demise , implies continuing magnetar activity .