NICER Observation of the Temporal and Spectral Evolution of Swift J1818.0-1607: A Missing Link between Magnetars and Rotation-powered Pulsars

We report on the hard X-ray burst and the first ∼100 days of NICER monitoring of the soft X-ray temporal and spectral evolution of the newly discovered magnetar Swift J1818.0-1607. The burst properties are typical of magnetars with a duration of T₉₀ = 10 ± 4 ms and a temperature of kT = 8.4 ± 0.7 keV. The 2-8 keV pulse shows a broad, single-peak profile with a pulse fraction increasing with time from 30% to 43%. The NICER observations reveal strong timing noise with $\dot{\nu }$ varying erratically by a factor of 10, with an average long-term spin-down rate of $\dot{\nu }=(-2.48\pm 0.03)\times {10}^{-11}$ s^-2, implying an equatorial surface magnetic field of 2.5 × 10¹⁴ G and a young characteristic age of ∼470 yr. We detect a large spin-up glitch at MJD 58928.56 followed by a candidate spin-down glitch at MJD 58934.81, with no accompanying flux enhancements. The persistent soft X-ray spectrum of Swift J1818.0-1607 can be modeled as an absorbed blackbody with a temperature of ∼1 keV. Its flux decayed by ∼60% while the modeled emitting area decreased by ∼30% over the NICER observing campaign. This decrease, coupled with the increase in the pulse fraction, points to a shrinking hot spot on the neutron star surface. Assuming a distance of 6.5 kpc, we measure a peak X-ray luminosity of 1.9 × 10³⁵ erg s^-1, lower than its spin-down luminosity of 7.2 × 10³⁵ erg s^-1. Its quiescent thermal luminosity is ≲1.7 × 10³⁴ erg s^-1, lower than those of canonical young magnetars. We conclude that Swift J1818.0-1607 is an important link between regular magnetars and high-magnetic-field, rotation-powered pulsars.