NICER Observation of the Temporal and Spectral Evolution of Swift J1818.0−1607: A Missing Link between Magnetars and Rotation-powered Pulsars
Chin‐Ping Hu, Beste Begiçarslan, Tolga Güver, Teruaki Enoto, George Younes, T. Sakamoto, Paul S. Ray, Tod E. Strohmayer, Sébastien Guillot, Zaven Arzoumanian, D. M. Palmer, Keith C. Gendreau, Christian Malacaria, Zorawar Wadiasingh, Gaurava K. Jaisawal, Walid A. Majid
Abstract
Abstract 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.