[vsnet-grb-info 28608] Swift detection of an outburst from SGR Swift J1555.2-5402

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30711
SUBJECT: Swift detection of an outburst from SGR Swift J1555.2-5402
DATE:    21/08/25 07:13:16 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at star.le.ac.uk>

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
P. A. Evans (U Leicester), J. A. Kennea (PSU), H. A. Krimm (NSF),
A. Melandri (INAF-OAB), K. L. Page (U Leicester), T. Sakamoto (AGU),
T. Sbarrato (INAF-OAB) and M. H. Siegel (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 06:53:27 UT, the Swift Burst Alert Telescope (BAT) triggered 
on a burst from the Soft Gamma-ray Repeater Swift J1555.2-5402 
(trigger=1070298).  Swift slewed immediately to the burst.  
The BAT on-board calculated location is 
RA, Dec 238.836, -54.070 which is 
   RA(J2000) = 15h 55m 21s
   Dec(J2000) = -54d 04' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single 
structure with a duration of about 0.1 sec.  The peak count rate
was ~2700 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 06:54:47.7 UT, 80.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 238.7859, -54.0617
which is equivalent to:
   RA(J2000)  = 15h 55m 08.61s
   Dec(J2000) = -54d 03' 42.2"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 109 arcseconds from the BAT onboard position, within the
BAT error circle. This position is consistent with the previously-reported
position of Swift J1555.2-5402 (GCN 30122). This position may be improved as
more data are received; the latest position is available at
https://www.swift.ac.uk/sper.  We cannot determine whether the source
is fading at the present time. 

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 85 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 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.