TITLE: GCN CIRCULAR NUMBER: 29925 SUBJECT: Swift Detection of MAXI J1803-298 including an optical counterpart DATE: 21/05/02 18:05:46 GMT FROM: Amy Lien at GSFC 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.