Deep X-Ray and Radio Observations of the First Outburst of the Young Magnetar Swift J1818.0-1607

Swift J1818.0-1607 is a radio-loud magnetar with a spin period of 1.36 s and a dipolar magnetic field strength of B ~ 3 × 10¹⁴ G, which is very young compared to the Galactic pulsar population. We report here on the long-term X-ray monitoring campaign of this young magnetar using XMM-Newton, NuSTAR, and Swift from the activation of its first outburst in 2020 March until 2021 October, as well as INTEGRAL upper limits on its hard X-ray emission. The 1-10 keV magnetar spectrum is well modeled by an absorbed blackbody with a temperature of kT _BB ~ 1.1 keV and apparent reduction in the radius of the emitting region from ~0.6 to ~0.2 km. We also confirm the bright diffuse X-ray emission around the source extending between ~50″ and ~110″. A timing analysis revealed large torque variability, with an average spin-down rate $\dot{\nu }\,\sim $ -2.3 × 10^-11 Hz² that appears to decrease in magnitude over time. We also observed Swift J1818.0-1607 with the Karl G. Jansky Very Large Array on 2021 March 22. We detected the radio counterpart to Swift J1818 measuring a flux density of S _v = 4.38 ± 0.05 mJy at 3 GHz and a half-ringlike structure of bright diffuse radio emission located at ~90″ to the west of the magnetar. We tentatively suggest that the diffuse X-ray emission is due to a dust-scattering halo and that the radio structure may be associated with the supernova remnant of this young pulsar, based on its morphology.