GCN Circular 29925

J.D. Gropp (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 17:39:33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located MAXI J1803-298 (trigger=1046414). Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 270.742, -29.818 which is 
   RA(J2000) = 18h 02m 58s
   Dec(J2000) = -29d 49' 05"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, 
the available BAT light curve shows no significant structure. 

The XRT began observing the field at 17:46:59.5 UT, 446.4 seconds after
the BAT trigger. XRT found a bright X-ray source located
at RA, Dec 270.7590, -29.8302 which is equivalent to:
   RA(J2000)  = 18h 03m 2.16s
   Dec(J2000) = -29d 49' 48.7"
with an uncertainty of 5.9 arcseconds (radius, 90% containment). This
location is 68 arcseconds from the BAT onboard position, within the BAT
error circle and consistent with being MAXI J1803-298. No event data are 
yet available to determine the column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 7.51e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 454 seconds after the BAT trigger. There is a candidate counterpart in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:03:02.79 = 270.76162
  DEC(J2000) = -29:49:49.7  = -29.83046
with a 90%-confidence error radius of about 0.61 arc sec. This position is 8.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
15.82 with a 1-sigma error of about  0.14. No correction has been made for the
large, but uncertain, extinction expected.