The Discovery of a Transient Magnetar

The newly discovered X‐ray transient XTE J1810‐197 exhibits almost all characteristics of magnetars. It possesses a relatively long spin period of 5.54 s and a rapid spin down rate of ≈ 10−11 s s−1, while showing no evidence for Doppler shifts due to a binary companion. This yields a magnetar‐strength dipole field B = 3 × 1014 G and a young characteristic age τ ⩽ 7600 yr. The spectrum of the source is notably soft (photon index ≈ 4) and optical observations with the 1.5 m Russian‐Turkish Optical Telescope RTT150 revealed a limiting magnitude of Rc = 21.5, both consistent with those of Soft gamma repeaters and anomalous X‐ray pulsars. However, the source shows a significant flux decline for over nine months and is present in archival ASCA and ROSAT observations at nearly two orders of magnitude fainter luminosity. Putting all evidence together shows that we have found the first confirmed transient magnetar. This suggests the presence of other unidentified transient magnetars in a state similar to XTE J1810...