GCN Circular 30296

M. G. Bernardini (INAF-OAB), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (PSU), N. P. M. Kuin (UCL-MSSL),
S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC),
A. Melandri (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL),
T. Sbarrato (INAF-OAB), B. Sbarufatti (PSU), M. H. Siegel (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 17:04:36 on 2021-06-21 UT, the Swift Burst Alert Telescope (BAT) triggered
on a burst from the Soft Gamma Repeater Swift J1555.2-5402 
(trigger=1057131).  Swift was already observing the source at that time. 
The BAT on-board calculated location is 
RA, Dec 238.812, -54.063 which is 
   RA(J2000) = 15h 55m 15s
   Dec(J2000) = -54d 03' 45"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The immediately available BAT light curve
showed a single high bin on the 64 ms timescale, consistent with
the on-board trigger duration of 8 milliseconds for a single short spike. 
The peak count rate was ~1000 counts/sec (15-350 keV) on the 64 ms
timescale, at ~0 sec after the trigger. 

Using promptly downlinked data we find an X-ray source with an
enhanced position: RA, Dec 238.7871, -54.0615 which is equivalent to:
   RA(J2000)  = 15h 55m 08.90s
   Dec(J2000) = -54d 03' 41.4"
with an uncertainty of 2.0 arcseconds (radius, 90% containment), which is
consistent with the 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 exposures of 150 seconds with the white
filter and 250 seconds with the U filter.  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. Because of the density
of catalogued stars, further analysis  is required to report an upper
limit for any afterglow in the sub-image. 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 18.0 mag. 
No  correction has been made for the large, but uncertain, extinction
expected. 

This source lies within the current (Sector 39) field-of-view of TESS camera 2. 

The current list of bursts from this source detected by BAT
now stands at:

2021-06-03 09:45:46 GCN #30120 (discovery)
2021-06-05 23:52:04 ATEL #14690
2021-06-07 12:33:40 ATEL #14690
2021-06-13 06:35:22  not previously reported
2021-06-16 14:44:30  not previously reported
2021-06-21 17:04:36 This GCN