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^1,2,3*, Beste Becicarslan⁴, Tolga Guver⁴, Teruaki Enoto², George Younes^5,6, Takanori Sakamoto⁷, Paul S. Ray⁸, Tod E. Strohmayer⁹, Sebastien Guillot¹⁰, Zaven Arzoumanian¹¹, David M. Palmer¹², Keith C. Gendreau¹¹, C. Malacaria^13,14, Zorawar Wadiasingh^11,15, Gaurava K. Jaisawal¹⁶

¹Department of Physics, National Changhua University of Education, Changhua, Taiwan
²Extreme Natural Phenomena RIKEN Hakubi Research Team, RIKEN, Saitama, Japan
³Department of Astronomy, Kyoto University, Kyoto, Japan
⁴Department of Astronomy and Space Sciences, Istanbul University, Istanbul, Turkey
⁵Department of Physics, The George Washington University, Washington, USA
⁶Astronomy, Physics and Statistics Institute of Science, The George Washington University, Washington, USA
⁷Department of Physics and Mathematics, Aoyama Gakuin University, Kanagawa, Japan
⁸U.S. Naval Research Laboratory, N/A, Washington DC, USA
⁹Astrophysics Science Division and Joint Space-Science Institute, NASA’s Goddard Space Flight Center, Greenbelt, USA
¹⁰IRAP, CNRS, Toulouse Cedex, France
¹¹Astrophysics Science Division, NASA’s Goddard Space Flight Center, Greenbelt, USA
¹²Los Alamos National Laboratory, N/A, Los Alamos, USA
¹³NASA Marshall Space Flight Center, NSSTC, Huntsville, USA
¹⁴Science and Technology Institute, Universities Space Research Association, Huntsville, USA
¹⁵Universities Space Research Association (USRA), N/A, Columbia, USA
¹⁶National Space Institute, Technical University of Denmark, Lyngby, Denmark
¹⁷Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA

* Presenter:Chin-Ping Hu, email:cphu0821@cc.ncue.edu.tw

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 ν˙ varying erratically by a factor of 10, with an average long-term spin-down rate of (−2.48±0.03)×10^−11 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 × 1035 erg s-1, lower than its spin-down luminosity of 7.2 × 1035 erg s-1. Its quiescent thermal luminosity is lower than 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.

Keywords: pulsars, magnetars, neutron stars, X-rays