TITLE: GCN CIRCULAR NUMBER: 22010 SUBJECT: BAT trigger 778435 is not a GRB. DATE: 17/10/13 16:03:27 GMT FROM: Boris Sbarufatti at PSU B. Sbarufatti (PSU), A. Y. Lien (GSFC/UMBC) and S. R. Oates (Uni. of Warwick) report on behalf of the Swift team: We report on further analysis of BAT trigger 778435/possible GRB 171013A (Sbarufatti et al., GCN Circ. 22007). The BAT ground analysis using the data set from T-240 to T+962 s shows nothing significant in the mask-weighted light curve. Also, the BAT image significance in 15-350 keV is only 4.3 sigma. The XRT ground analysis on 6.5 ks of data observed between T+107 s to T+21.6 ks. No X-ray source was detected inside the BAT error circle, down to a 3 sigma upper limit of 5.4e-3 cts s^-1, corresponding to a 0.3-10 keV observed flux of 2.7e-13 erg cm^-2 s^-1 (assuming a typical GRB spectrum). The Swift/UVOT began settled observations of the field of GRB 171013A 110 s after the BAT trigger. No optical afterglow consistent with the BAT position is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 110 260 147 >20.2 u_FC 322 572 246 >19.1 white 110 621 167 >20.1 v 652 672 19 >17.0 b 577 597 19 >18.0 u 322 572 246 >19.2 m2 677 696 19 >17.2 w2 628 647 19 >17.5 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.97 in the direction of the burst (Schlegel et al. 1998). We therefore conclude that BAT trigger 778435 was caused by a noise fluctuation. This circular is an official product of the Swift Team