TITLE: GCN CIRCULAR NUMBER: 16706 SUBJECT: GRB 140818A: Swift-XRT and UVOT potential afterglow detection DATE: 14/08/18 15:05:41 GMT FROM: Craig Swenson at PSU/Swift Swenson, C.A. (PSU) and Sbarufatti, B. (INAF-OAB/PSU) report on behalf of the Swift team Swift-XRT has observed the error circle of the MAXI GRB 140818A in a series of observations tiled on the sky. The total exposure time is 2.3 ks spread over 4 fields; the maximum exposure within the sky observed was 1.0 ks. The observations started 10.5 ks after the MAXI trigger (Honda et al., GCN Circ. 16702). Within these data we detect a bright, uncatalogued X-ray source at RA, Dec= 199.56275, 6.92254 which is equivalent to: RA (J2000.0) = 13h 18m 15.06s Dec (J2000.0) = +06d 55' 21.1" with an uncertainty of 2.1 arcsec (radius, 90% confidence). This is an enhanced position, determined by using UVOT field stars to determine the astrometry.The exposure at this location was 746 s. This previously uncatalogued source is significantly brighter than the RASS limit, and we thus consider it the likely GRB afterglow. The results of the automatic processing for this source are available at http://www.swift.ac.uk/xrt_products/TILED_GRB00029/index_1.php The Swift/UVOT began settled observations of the field of GRB 140818A 10467 s after the MAXI trigger. A source consistent with the XRT position is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 13:18:15.08 = 199.56285 (deg.) Dec (J2000) = +06:55:22.6 = 6.92295 (deg.) with an estimated uncertainty of 0.50 arc sec. (radius, 90% confidence). Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag u 10467 11094 617 19.21 ± 0.12 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.04 in the direction of the burst (Schlegel et al. 1998). This circular is an official product of the Swift team.