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GCN Circular 19433

Subject
Trigger 686761: Swift detection of a bright burst from SGR 1935+2154
Date
2016-05-18T09:46:00Z (8 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
P. D'Avanzo (INAF-OAB), P.A. Evans (U Leicester), J. A. Kennea (PSU),
A. Melandri (INAF-OAB), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 09:09:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a burst from the Soft Gamma Repeater SGR 1935+2154 
(trigger=686761).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 293.722, +21.891 which is 
   RA(J2000) = 19h 34m 53s
   Dec(J2000) = +21d 53' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed single short spike
of 0.192 ms duration. The peak count rate was 350k counts/s
for 0.064 s, at ~0.1 sec after the trigger. 

The XRT began observing the field at 09:10:22.9 UT, 59.1 seconds after
the BAT trigger. Using promptly downlinked data we find a catalogued
X-ray source located at RA, Dec 293.73098, 21.89458 which is equivalent
to:
   RA(J2000)  = 19h 34m 55.44s
   Dec(J2000) = +21d 53' 40.5"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
position may be improved as more data are received; the latest position
is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.27
x 10^22 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 64 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 100% 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. 

We note that this burst is ~100x brighter in BAT than the burst from
this source 36 hours earlier.
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