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GCN Circular 29516

Subject
Swift Trigger 1033144: Detection of SGR 1830-0645
Date
2021-02-17T08:28:25Z (3 years ago)
From
Kim Page at U.of Leicester <klp5@leicester.ac.uk>
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:11:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located the soft gamma repeater SGR 1830-0645 (trigger=1033144).  
Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 277.646, -6.735 which is 
   RA(J2000) = 18h 30m 35s
   Dec(J2000) = -06d 44' 05"
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 0.1 sec.  The peak count rate
was ~10000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 08:12:33.2 UT, 71.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 277.67287, -6.75438 which is equivalent
to:
   RA(J2000)  = 18h 30m 41.49s
   Dec(J2000) = -06d 45' 15.8"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
location is 118 arcseconds from the BAT onboard position, within the
BAT error circle, and consistent with the known location of SGR 1830-0645. 
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 gives a column density in excess of the Galactic value (1.11 x
10^22 cm^-2, Willingale et al. 2013), with an excess column of 2.7
(+4.47/-2.40) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 76 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. Because of the density of
catalogued stars, further analysis is required to report an upper limit for any
afterglow in the region. No correction has been made for the large, but
uncertain, extinction expected. 

BAT most recently triggered on SGR 1830-0645 on 2021 Feb 13 (GCN 29500).
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