TITLE: GCN CIRCULAR NUMBER: 28253 SUBJECT: Swift trigger 990358 is not an astrophysical source DATE: 20/08/17 02:14:59 GMT FROM: David Palmer at LANL M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB), M. H. Siegel (PSU) and T. N. Ukwatta (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:53:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located on an image peak that is likely due to a noisy detector element (trigger=990358). Swift slewed immediately to the event. The BAT on-board calculated location is RA, Dec 154.131, -33.921 which is RA(J2000) = 10h 16m 31s Dec(J2000) = -33d 55' 16" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single spike that appears only in the 25-50 keV band, which is likely due to the presence of the noisy detector element. The full downlinked data will allow the noisy element to be removed from analysis. The XRT began observing the field at 01:54:44.4 UT, 83.4 seconds after the BAT trigger. No source was detected in 769 s of promptly downlinked data. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 150 seconds with the White filter starting 87 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.10. This marginal significance burst (6.58 sigma) was not confirmed by ground imaging of the scaled detector map. This, combined with the presence of a hot detector pixel in the data and the non-detection of a source by XRT leads us to believe that this is not an astrophysical source.