NICER Observation of the Temporal and Spectral Evolution of Swift J1818.0-1607: A Missing Link between Magnetars and Rotation-powered Pulsars
Chin-Ping Hu1,2,3*, Beste Becicarslan4, Tolga Guver4, Teruaki Enoto2, George Younes5,6, Takanori Sakamoto7, Paul S. Ray8, Tod E. Strohmayer9, Sebastien Guillot10, Zaven Arzoumanian11, David M. Palmer12, Keith C. Gendreau11, C. Malacaria13,14, Zorawar Wadiasingh11,15, 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,
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