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GCN Circular 29500

Subject
Swift Trigger 1032350: Detection of SGR 1830-0645
Date
2021-02-13T00:39:23Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL) and
B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 00:10:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a burst form the soft gamma repeater SGR 1830-0645 (trigger=1032350). 
Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 277.660, -6.748 which is 
   RA(J2000) = 18h 30m 38s
   Dec(J2000) = -06d 44' 52"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single 
short peak in one 0.128 s timebin.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 00:11:14.4 UT, 60.6 seconds after
the BAT trigger. Using promptly downlinked data we find an
X-ray source with an enhanced position: RA, Dec 277.6734, -6.7541 which
is equivalent to:
   RA(J2000)  = 18h 30m 41.61s
   Dec(J2000) = -06d 45' 14.9"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 52 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 8.1
(+9.28/-7.67) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 63 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 25% of
the BAT 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 BAT 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's most recent detection of this SGR was on 2020-11-22, following 
its discovery by Swift on 2020-11-10 (Page et al., GCN Circ. 28594).
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