Discovery of Radio Emission from the Transient Anomalous X-Ray Pulsar XTE J1810-197

We report the first detection of radio emission from any anomalous X-raypulsar (AXP). Data from the Very Large Array (VLA) MAGPIS survey with angularresolution 6" reveals a point-source of flux density 4.5 +/- 0.5 mJy at 1.4 GHzat the precise location of the 5.54 s pulsar XTE J1810-197. This is greaterthan upper limits from all other AXPs and from quiescent states of softgamma-ray repeaters (SGRs). The detection was made in 2004 January, 1 yearafter the discovery of XTE J1810-197 during its only known outburst. AdditionalVLA observations both before and after the outburst yield only upper limitsthat are comparable to or larger than the single detection, neither supportingnor ruling out a decaying radio afterglow related to the X-ray turn-on. Anotherhypothesis is that, unlike the other AXPs and SGRs, XTE J1810-197 may power aradio synchrotron nebula by the interaction of its particle wind with amoderately dense environment that was not evacuated by previous activity fromthis least luminous, in X-rays, of the known magnetars.Comment: 13 pages, 1 figure, to appear in ApJ Letter