GRB 230818A: Swift/UVOT Detection

The Swift/UVOT began settled observations of the field of GRB 230818A 144 s after the BAT trigger (Tohuvavohu et al., GCN Circ. 34479). A source consistent with the optical position (Lipunov et al., GCN Circ. 34476) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 19:03:33.13 = 285.88806 (deg.) Dec (J2000) = +40:53:49.0 = 40.89695 (deg.) with an estimated uncertainty of 0.48 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits 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 white 144 294 147 19.09 +/- 0.07 v 379 572 39 18.40 +/- 0.30 b 477 1513 117 19.68 +/- 0.20 u 452 1487 117 >19.5 w1 428 1463 97 >20.2 w2 1026 4987 255 >20.1 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.101 in the direction of the burst (Schlegel et al. 1998).