GCN Circular 20832

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), L. M. Z. Hagen (PSU),
J. A. Kennea (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL),
J. L. Racusin (NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team:

At 20:24:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located the possible GRB 170307A (trigger=741528).  Swift did not slew
due to an observing constraint.  The BAT on-board calculated location is 
RA, Dec 13.535, +9.537, which is 
   RA(J2000)  =  00h 54m 08s
   Dec(J2000) = +09d 32' 11"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows that Swift was 
emerging from the SAA at this time, giving a quickly-decreasing
count rate that would swamp any variation expected from the source
at the nominal intensity implied by the image strength. 

Due to a Sun observing constraint, Swift cannot slew to the BAT
position until 00:33 UT on 2017 May 24. There will thus be no XRT or
UVOT data for this trigger before this time. 

Because of high and variable background rates from the SAA, and
in the absence of follow-up observations by the XRT, the 
immediately-available data does not determine whether this 
was a true astronomical event or an image fluctuation 
(with a nominal significance of 7.2 sigma). 
A determination of the nature of this source will require the
full downlinked data set. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)