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

Abstract

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 × 10 14  G, spin-down luminosity  ∼ 7.2 × 10 35 erg s −1 , and characteristic age τ c  ∼ 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 ×10 34   d 6.5 2 erg s −1 and efficiency of  ∼ 0.13  d 6.5 2 at a distance of 6.5 kpc. The Chandra image also shows faint diffuse emission out to 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 × 10 33   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.