GCN Circular 17485

D. N. Burrows (PSU), P. D'Avanzo (INAF-OAB), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
F. E. Marshall (NASA/GSFC), T. Sakamoto (AGU) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 12:16:26 UT and 12:31:11, the Swift Burst Alert Telescope (BAT) triggered
twice on an outburst from SGR 1935+2154 (triggers=632158, 632159).  
Swift slewed immediately to the location. The BAT on-board calculated location 
is RA, Dec 293.724, +21.888 which is 
   RA(J2000) = 19h 34m 54s
   Dec(J2000) = +21d 53' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve for the first trigger
shows a single-peaked structure with a duration of about 0.2 sec.  
The peak count rate was ~3200 counts/sec (15-350 keV), at ~0 sec 
after the trigger.  The second light curve also shows a single-peaked 
structure with a duration of about 0.2 sec.  The peak count rate was 
~11,000 counts/sec (15350 keV), at ~0 sec after the second trigger. 

The XRT began observing the field at 12:17:48.1 UT, 82.0 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source 
with an enhanced position: RA, Dec 293.7312, 21.8973 which
is equivalent to:
   RA(J2000)  = 19h 34m 55.50s
   Dec(J2000) = +21d 53' 50.3"
with an uncertainty of 2.5 arcseconds (radius, 90% containment). This
location is 41 arcseconds from the BAT onboard position, within the BAT
error circle. This position is consistent with the XRT position reported 
for this source in Cummings et al. (ATel #6294). This position may be 
improved as more data are received; the latest position is available 
at http://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 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 86 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. 

This source was discovered by Swift in July 5, 2014 (Cummings et al, ATel #6294). 
This is the first activity since that time.