The 2018 x-ray and radio outburst of magnetar XTE J1810-197

We present the earliest X-ray observations of the 2018 outburst of XTE J1810-197, the first outburst since its 2003 discovery as the prototypical transient and radio-emitting anomalous X-ray pulsar (AXP). The Monitor of All-sky X-ray Image (MAXI) detected XTE J1810-197 immediately after a November 20-26 visibility gap, contemporaneous with its reactivation as a radio pulsar, first observed on December 8. On December 13 the Nuclear Spectroscopic Telescope Array (NuSTAR) detected X-ray emission up to at least 30 keV, with a spectrum well-characterized by a blackbody plus power-law model with temperature kT =0.74 ±0.02 keV and photon index Γ =4.4 ±0.2 or by a two-blackbody model with kT =0.59 ±0.04 keV and kT =1.0 ±0.1 keV, both including an additional power-law component to account for emission above 10 keV, with Γ h =-0.2 ±1.5 and Γ h =1.5 ±0.5, respectively. The latter index is consistent with hard X-ray flux reported for the nontransient magnetars. In the 2-10 keV bandpass, the absorbed flux is 2 ×10 -10 erg s -1 cm -2 , a factor of 2 greater than the maximum flux extrapolated for the 2003 outburst. The peak of the sinusoidal X-ray pulse lags the radio pulse by ≈0.13 cycles, consistent with their phase relationship during the 2003 outburst. This suggests a stable geometry in which radio emission originates on magnetic field lines containing currents that heat a spot on the neutron star surface. However, a measured energy-dependent phase shift of the pulsed X-rays suggests that all X-ray emitting regions are not precisely coaligned.