NICER detection of 1.36 sec periodicity from a new magnetar, Swift J1818.0-1607

ATel #13551; Teruaki Enoto (RIKEN), Takanori Sakamoto (AGU), George Younes (GWU), Chin-Ping Hu (Kyoto Univ./RIKEN/JSPS Fellow), Wynn C. G. Ho (Haverford), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Tolga Guver (Istanbul Univ.), Sebastien Guillot (IRAP/CNRS), Diego Altamirano (University of Southampton), Paul S. Ray (NRL), Mason Ng (MIT), Deepto Chakrabarty (MIT), and Gaurava K. Jaisawal (DTU), Slavko Bogdanov (Columbia), report on behalf of the NICER Team
on 13 Mar 2020; 14:40 UT
Credential Certification: Teruaki Enoto (teruaki.enoto@gmail.com)

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Following the Swift/BAT detection of a short burst from a new soft gamma-ray repeater (SGR) candidate, Swift J1818.0-1607, at 21:16:47 UT on March 12, 2020 (GCN circular 27373), NICER started a series of follow-up observations of this source at 01:38 UT on March 13, four hours after the Swift notification.

During the initial pointings, we detected a background-subtracted average source count rate of 1.5 cps in the 2-10 keV band. We barycentered the event data using (RAJ 18:18:00.12, DecJ -16:07:52.8) and searched for variability. We find a coherent periodicity at 0.733417(4) Hz with significance exceeding 5-sigma. The X-ray pulse profile exhibits a single broad peak. Taking into account the short burst detected with Swift, we suggest that Swift J1818.0-1607 is a new magnetar with a spin period of 1.36 sec, which is the shortest among the known magnetars, but longer than high-B rotation-powered pulsars with magnetar-like activity.

The NICER 2-10 keV spectrum can be approximated by an absorbed blackbody model, with best fit spectral parameters N_H=(9.8 +/-1.2)x10^22 cm^-2, kT=0.92 +/-0.07 keV, and an absorbed X-ray flux of 1.43(-0.30, +0.01) x10^-11 ergs/s/cm^2 in the 2-10 keV range. Errors represent 1 sigma uncertainties. These parameters are in the range of known magnetars.

All these results indicate that Swift J1818.0-1607 is a new magnetar although its period derivative has yet to be measured. The NICER team is planning to monitor this target to measure its period derivative and expected flux decay (as part of NICER GO2 program #3056). There is no clear evidence for its flux decay over the last 12 hours. The NICER visibility is not good until Tuesday, March 17th, but will improve after that.

A nearby magnetar candidate AX J1818.8-1559 (Mereghetti et al. 2012, A&A, 546, 30) is 14.9 arcmin away from Swift J1818.0-1607, and out of the NICER field of view (6 arcmin diameter). Thus, Swift J1818.0-1607 is thought to be a new magnetar.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.