TITLE: GCN CIRCULAR NUMBER: 10297 SUBJECT: Swift Trigger 381434 is not a GRB DATE: 09/12/29 19:21:53 GMT FROM: Vanessa Mangano at INAF-IASFPA S. D. Barthelmy (GSFC), C. B. Markwardt (GSFC/UMD), V. Mangano, B. Sbarufatti (INAF-IASF PA), A. Breeveld (MSSL/UCL) report on behalf the Swift team: Using the full data set from telemetry downlinks, we report that Swift Trigger 381434 is due to a cosmic ray shower event in the BAT instrument. It is not due to a GRB nor any other astrophysical source. There were two showers ~8 msec apart which caused the 64msec trigger to fire. The XRT observed the field of the target starting 68 s after the trigger. In the initial 7 ks in PC mode no source is detected within the 3 arcmin BAT error circle down to a limiting flux of 8 x 10^-14 erg cm^-2 s^-1. Three X-ray sources are detected with more than 3 sigma significance in the whole XRT field of view, the brightest one likely associated to 1RXS J050001.1-550121, and the other two unidentified. All of these sources are further than 4 arcmin from the target position. The UVOT began settled observations of the field of the target 65 s after the BAT trigger. No new source is detected. Preliminary 3-sigma upper limits using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 65 215 147 >21.1 u_FC 277 526 246 >20.4 white 65 1699 411 >21.6 v 606 1748 136 >19.4 b 532 1834 123 >20.3 u 277 1822 363 >20.6 w1 656 1798 136 >19.9 m2 631 1773 136 >19.8 w2 582 1551 117 >20.1 The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998). The lack of a bright X-ray/optical counterpart of Trigger 381434 in prompt observations strengthens the spurious trigger interpretation.