TITLE:   GCN CIRCULAR
NUMBER:  30121
SUBJECT: NICER discovery of 3.86 s pulsations from a new magnetar: SGR J1555.2-5402
DATE:    21/06/03 16:59:30 GMT
FROM:    Nanda Rea at CSIC-IEEC  <nanda.rea@gmail.com>

F. Coti Zelati, A. Borghese, N. Rea (ICE-CSIC), G. L. Israel
(INAF-OAR), P. Esposito (IUSS Pavia), T. Enoto (RIKEN), K. Gendreau
(NASA/GSFC) S. Campana (INAF-OAB) on behalf of a larger collaboration


On 2021 June 03 at 09:45:46 UT, the Swift Burst Alert Telescope (BAT)
triggered on a short X-ray burst from a direction close to the
Galactic plane. The short duration of the burst led to its tentative
association to a newly discovered magnetar, SGR J1555.2-5402 (GCN
#30120). NICER started observing the field at 12:32:00 UT, and
Swift/XRT observed at 10:53:43 UT, only a few hours after the
Swift/BAT trigger. We report on the preliminary analysis of the data
available so far, which total to an exposure time of about 1.6 ks for
NICER, and 1.6ks for Swift/XRT.

Swift/XRT finds a new X-ray source at a position (3.67 arcsec error) of:
RA (2000) = 15 55 08.556,
Dec (2000) -54 03 38.74

SGR J1555.2-5402 is detected in NICER at a count rate of about 4.5
counts/s (not background-subtracted) in the energy range 1-8 keV. We
also detected coherent pulsations at high confidence level at a period
of P=3.8611(1) s (uncertainty in parenthesis at 1 sigma; we used event
times not corrected for the solar system barycentre), which confirms
that the short burst was indeed emitted by a new magnetar. Results
from a more detailed analysis will be reported later on.