GCN Circular 30120

D.M. Palmer (LANL), P.A. Evans (U. Leicester), 
N.P.M. Kuin (MSSL/UCL), and K.L. Page (U. Leicester) report on behalf of the Swift Team:

At 09:45:46 UT on 2021-06-03, the Swift Burst Alert Telescope (BAT) triggered and
located an unknown burst source (trigger=1053220).  Swift slewed immediately to the
source.  The BAT on-board calculated location is 
RA, Dec 238.819, -54.046 which is 
  RA(J2000) = 15h 55m 17s
  Dec(J2000) = -54d 02' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT lightcurve shows a single-bin (64 ms) 
spike with a nominal peak of 2000 counts per second. However, the BAT 
trigger timescale was very short (8 ms) and moderately intense 
(29 sigma in count rate), indicating a very short spike emission.

The XRT began observing the field at 09:47:23.3 UT, 96.8 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.

UVOT took a finding chart exposure of 61 seconds with the White filter starting
100 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers none of the
XRT error circle.  The list of sources is typically complete to about 18.0 mag. 
No correction has been made for the large expected extinction.

The fast lightcurve shape, combined with the source's location near the 
Galactic plane (lon, lat = 327.90, -0.34) leads us to the conclusion that 
this is a new SGR, which we name Swift SGR J1555.2-5402 based on the BAT position.

More information on this source will be provided once the ground pass
data has been analyzed.