Transient pulsed radio emission from a magnetar

Anomalous X-ray pulsars (AXPs) are neutron stars whose properties are best explained by the magnetar model. Their very large X-ray luminosities exceed those available from braking of the stellar rotation by magnetic torque, and are generated instead by decay of ultra-strong magnetic fields. The ubiquitous radio pulsars, which are powered by rotational energy loss, display no magnetar-like X-ray activity and have enduring radio emission, while magnetars have lacked any radio emission. XTE J1810-197 was identified as a transient AXP in early 2003 when its X-ray luminosity increased by a factor of ~100 compared to the quiescent level, and X-ray pulsations were observed with period 5.54s. The detection from XTE J1810-197 of a point-like radio source of unknown origin one year after the X-ray outburst further distinguishes it from all other magnetars. Here we show that XTE J1810-197 emits remarkable radio pulsations. These are sharply modulated at the rotation period with peak flux density >1Jy and are highly linearly polarised, conclusively establishing the first detection of magnetospheric radio emission from a magnetar. In contrast to ordinary pulsars, there is no evidence of radio activity prior to the 2003 X-ray outburst, and the intrinsic flux varies on short timescales with an approximately flat spectrum such that, at >20GHz, XTE J1810-197 is currently the brightest neutron star known. This discovery provides a unique opportunity to probe in detail the dynamic magnetosphere of a neutron star with extreme properties