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

摘要

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 T _(90)?=?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 varying erratically by a factor of 10, with an average long-term spin-down rate of s~(?2), implying an equatorial surface magnetic field of 2.5?×?10~(14) 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?×?10~(35) erg s~(?1), lower than its spin-down luminosity of 7.2?×?10~(35) erg s~(?1). Its quiescent thermal luminosity is ?1.7?×?10~(34) 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.