Initial results of a NICER X-ray monitoring of the magnetar, Swift J1555.2-5402

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.