[vsnet-grb-info 28023] Initial results of a NICER X-ray monitoring of the magnetar, Swift J1555.2-5402

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30126
SUBJECT: Initial results of a NICER X-ray monitoring of the magnetar, Swift J1555.2-5402
DATE:    21/06/04 20:37:36 GMT
FROM:    Teruaki Enoto at Kyoto U/Astro Dept  <teruaki.enoto at gmail.com>

Teruaki Enoto (RIKEN), Mason Ng (MIT), Chin-Ping Hu (NCUE, RIKEN), Karishma Bansal (JPL), Keith Gendreau, Zaven Arzoumanian, Tod Strohmayer, Zorawar Wadiasingh (NASA/GSFC), Tolga Guver (Istanbul Univ), Aaron B. Pearlman (McGill University, McGill Space Institute, Caltech), Walid A. Majid (JPL, Caltech), Chryssa Kouveliotou (George Washington University),  Francesco Coti Zelati, Alice Borghese, Nanda Rea (ICE-CSIC), Gian Luca Israel (INAF-OAR), Paolo Esposito (IUSS Pavia), Gaurava K. Jaisawal (DTU Space) report on behalf of the NICER Team:

Following Swift’s discovery of a new magnetar candidate, Swift J1555.2-5402, on June 3, 2021 (GCN circular 30120, 30124), NICER discovered a 3.86-sec periodicity from this source (GCN 30121). NICER has been monitoring Swift J1555.2-5402 since 13:43 UT on June 3 with a total 4.2 ks exposure as of 11:48 UT June 4. Here we report refined timing analyses after barycentric correction, as well as on the X-ray spectral and bursting properties of this source.   

The background-subtracted average source count rate is 3.69 cps in the 1-10 keV band. After barycentric correction of the event data at the reported coordinates (RA: 15:55:08.66s, Dec: -54:03:41.1) in GCN 30122, we found a coherent periodicity at 0.2590004(17) Hz with a significance of approximately 30 sigma. The X-ray pulse profile has a single peak. Its period derivative has yet to be measured. 

The NICER 1-10 keV spectrum can be approximated by an absorbed blackbody model, with best fit spectral parameters N_H=(7.7 +/- 0.3)e+22 cm-2, kT=1.24 +/- 0.03 keV, with an apparent emitting radius at 10 kpc of R=1.8 +/- 0.1 km. The absorbed and unabsorbed X-ray fluxes of the source in the same energy range are (4.4 +/- 0.1)e-11 and (6.94 +/- 0.12)e-11 ergs/s/cm2, respectively. Errors represent 1 sigma uncertainties. These parameters are in the range of known magnetars in outburst. 

We detected four short burst (< 1 s) candidates with significance > 5 sigma during the observation. The source is still active, and multiwavelength coordinated observations are encouraged. 

All these results confirm that Swift J1555.2-5402 is a new magnetar in outburst. The NICER team is planning to monitor this target to measure its spin-down rate and flux decay.

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.