GCN Circular 23194

A. D'Ai (INAF-IASFPA), J.D. Gropp (PSU), J. A. Kennea (PSU),
S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
M. J. Moss (George Washington University), D. M. Palmer (LANL),
B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 21:28:32 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180904A (trigger=859282).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 274.250, +46.623 which is 
   RA(J2000) = 18h 17m 00s
   Dec(J2000) = +46d 37' 24"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 21:30:35.3 UT, 123.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 274.2495,
46.6298 which is equivalent to:
   RA(J2000)  = 18h 16m 59.87s
   Dec(J2000) = +46d 37' 47.4"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 24 arcseconds from the BAT onboard position, within the BAT
error circle. 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 in excess of the Galactic value (3.37 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.3
(+3.26/-2.80) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 4.47e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 131 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 expected extinction corresponding to E(B-V) of
0.04. 

Burst Advocate for this burst is A. D'Ai (antonino.dai AT ifc.inaf.it). 
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.)