The Timing Evolution of the Magnetar Swift J1818.0–1607 during a Period of Reduced Activity

Abstract We report results from an observational campaign of the radio-loud magnetar Swift J1818.0−1607 using the Green Bank Telescope at 2.0 GHz that began in 2021 November, during a period of reduced activity approximately 20 months after its 2020 March outburst. Over the ∼60 day duration reported here, the integrated pulse profile remained consistently stable, exhibiting a single, narrow peak with a small precursor component and no evidence of a postcursor one. This pulse profile is in sharp contrast to the double-peaked morphology observed during an observing campaign ∼120 days preceding ours. Along with this change in the integrated pulse profile shape, we also measured a slower spin-down rate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> ) compared to the end of that preceding campaign. Together, these differences suggest that a mode-switching event likely occurred between the end of that campaign and the start of ours. Finally, we derived a phase-connected timing solution from our data, from which we inferred a characteristic age of τ c ∼ 2500 yr—about 2.5 times older than the most recent published estimate—and a surface dipole magnetic field strength of B field ∼ 1 × 10 14 G, nearly 3 times weaker. These updated estimates reflect the short-term variations in the magnetar’s spin-down rate, from which both its age and magnetic field strength are inferred, rather than intrinsic changes in the magnetar itself.