Chandra Observations of the Newly Discovered Magnetar Swift J1818.0–1607

Swift J1818.0–1607 is a new radio-loud magnetar discovered by the Swift Burst Alert Telescope on 2020 March 12. It has a magnetic field B ∼ 2.5 × 1014 G, spin-down luminosity $\dot{E}$ ∼ 7.2 × 1035 erg s−1, and characteristic age τc ∼ 470 yr. Here we report on the Chandra observations of Swift J1818.0–1607, which allowed for a high-resolution imaging and spectroscopic study of the magnetar and its environment. The 1–10 keV spectrum of the magnetar is best described by a single blackbody model with a temperature of 1.2 ± 0.1 keV and an unabsorbed flux of 1.9−0.3+0.4 × 10−11 erg cm−2 s−1. This implies an X-ray luminosity of 9.6−1.5+2.0×1034 d6.52 erg s−1 and efficiency of ${L}_{{\rm{X}}}/\dot{E}$ ∼ 0.13 d6.52 at a distance of 6.5 kpc. The Chandra image also shows faint diffuse emission out to $\geqslant 10^{\prime\prime} $ from the magnetar, with its spectrum adequately described by a power law with a photon index of 2.0 ± 0.5 and a luminosity of ∼8.1 × 1033 ${d}_{6.5}^{2}$ erg s−1. The extended emission is likely dominated by a dust-scattering halo and future observations of the source in quiescence will reveal any underlying compact wind nebula. We conclude that Swift J1818.0–1607 is a transient source showing properties between high-B pulsars and magnetars, and could be powered at least partly by its high spin-down, similar to rotation-powered pulsars.