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Swift Detection of MAXI J1803-298 including an optical counterpart

ATel #14591; 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:
on 2 May 2021; 18:11 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: Optical, X-ray, Transient

Referred to by ATel #: 14594, 14597, 14598, 14601, 14602, 14606, 14607, 14609, 14627, 14629, 14630, 14660, 14706, 14994

This is cross posted from GCN #29925.

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.