TITLE:   GCN CIRCULAR
NUMBER:  29907
SUBJECT: Swift Trigger 1045571 is not a GRB
DATE:    21/04/27 14:08:54 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

P. A. Evans (U Leicester), C. Gronwall (PSU), A. Y. Lien (GSFC/UMBC)
and K. L. Page (U Leicester) report on behalf of the Neil Gehrels
Swift Observatory Team:

At 06:56:48 UT on 2021-04-27, the Swift Burst Alert Telescope (BAT) 
triggered and located on a marginal image peak due to noise fluctuation 
when Swift entered the SAA (trigger=1045571). Swift slewed to the event. 
The BAT on-board calculated location is 
RA, Dec 263.109, 54.058 which is 
   RA(J2000) = 17h 32m 26s
   Dec(J2000) = +54d 03' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve shows the background
increase due to the SAA, but does not show any significant burst structure. 
Using BAT ground data from T-61 to T+100 s, the image significance 
decreases to 4.2 sigma, which is consistent with noise fluctuation. 

No source was detected in ~700 s of Photon Counting mode data, starting 1.4 ks
after the trigger time, supporting the conclusion that this was not an 
astrophysical event. 

(Some information of this trigger was received at a delayed time due to 
a TDRSS downlink gap.)