GCN Circular 30796

N. J. Klingler (GSFC/UMBC/CRESSTII), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 10:11:55 on 2021-09-10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a burst from the Soft Gamma Repeater Swift J1555.2-5402 (trigger=1072706). 
Swift slewed immediately to the burst.  The BAT on-board calculated location is 
RA, Dec 238.773, -54.055 which is 
   RA(J2000) = 15h 55m 05s
   Dec(J2000) = -54d 03' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single high
bin at the 64 ms timescale, consistent with the 4 ms trigger duration. 
The peak count rate on the 64 ms timescale was ~1300 counts/sec 
(15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 10:13:14.4 UT, 78.8 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source 
with an enhanced position: RA, Dec 238.78912, -54.06236
which is equivalent to:
   RA(J2000)  = 15h 55m 09.39s
   Dec(J2000) = -54d 03' 44.5"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 43 arcseconds from the BAT onboard position, within the BAT
error circle, and consistent with the known position of Swift J1555.2-5402. 
This position may be improved as more data are received; the latest position 
is available at https://www.swift.ac.uk/sper.  

A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 82 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 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain, extinction expected.