TITLE:   GCN CIRCULAR
NUMBER:  28055
SUBJECT: Trigger 980513: Swift detection of SGR Swift J1818.0-1607
DATE:    20/07/02 07:41:35 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 07:09:38 UT on 2020-07-02, the Swift Burst Alert Telescope (BAT) 
triggered and located a burst from Soft Gamma Repeater 
SGR Swift J1818.0-1607 (trigger=980513).  Swift slewed immediately 
to the source.  The BAT on-board calculated location is 
RA, Dec 274.471, -16.167 which is 
   RA(J2000) = 18h 17m 53s
   Dec(J2000) = -16d 09' 59"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 100 ms.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 07:11:03.2 UT, 84.4 seconds after
the BAT trigger. Using promptly downlinked data we find an 
X-ray source with an enhanced position: RA, Dec 274.5017, -16.1317
which is equivalent to:
   RA(J2000)  = 18h 18m 00.40s
   Dec(J2000) = -16d 07' 54.2"
with an uncertainty of 3.0 arcseconds (radius, 90% containment). This
location is 165 arcseconds from the BAT onboard position, within the
BAT error circle, and consistent with the catalogued position of SGR 
Swift J1818.0-1607. 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 87 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 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. 

This is the fourth time Swift has triggered on SGR Swift J1818.0-1607;
with 5 additional burst detections that did not reach the level needed for
a burst response.  These detections were:
2020-02-28T22:19:32
2020-03-12T21:16:47 GCN 27373
2020-04-13T19:34:18
2020-05-06T17:36:50 GCN 27696
2020-05-16T15:03:28 GCN 27746
2020-05-17T14:29:54
2020-05-25T20:23:45
2020-06-01T16:52:09