Discovery of a Transient Magnetar: XTE J1810-197

We report the discovery of a new X-ray pulsar, XTE J1810-197. The source wasserendipitously discovered on 2003 July 15 by the Rossi X-ray Timing Explorer(RXTE) while observing the soft gamma repeater SGR 1806-20. The pulsar has a5.54 s spin-period and a soft spectrum (photon index ~ 4). We detect the sourcein earlier RXTE observations back to 2003 January. These show that a transientoutburst began between 2002 November 17 and 2003 January 23 and that the pulsarhas been spinning down since then, with a high rate Pdot ~ 10^-11 s/s showingsignificant timing noise, but no evidence for Doppler shifts due to a binarycompanion. The rapid spin-down rate and slow spin-period imply a super-criticalmagnetic field B=3x10^14 G and a young characteristic age < 7600 yr. Theseproperties are strikingly similar to those of anomalous X-ray pulsars and softgamma repeaters, making the source a likely new magnetar. A follow-up Chandraobservation provided a 2".5 radius error circle within which the 1.5 mRussian-Turkish Optical Telescope RTT150 found a limiting magnitude ofR_c=21.5, in accord with other recently reported limits. The source is presentin archival ASCA and ROSAT data as well, at a level 100 times fainter than the\~ 3 mCrab seen in 2003. This suggests that other X-ray sources that arecurrently in a state similar to the inactive phase of XTE J1810-197 may also beunidentified magnetars awaiting detection via a similar activity.Comment: Submitted to ApJL; 4 pages; 4 figure