TITLE: GCN CIRCULAR NUMBER: 23401 SUBJECT: FACT follow-up of IceCube20181023A DATE: 18/10/31 15:24:39 GMT FROM: Daniela Dorner at U of Wuerzburg A. Biland (ETH Zurich) reports on behalf of the FACT Collaboration: On Tue 23 Oct 18 16:38:11 UTC, the IceCube Collaboration reported the detection of a high-energy neutrino (run number 131653, event number 53411354). An updated position was communicated by IceCube on 18/10/23 19:38:53 GMT (GCN #23375). FACT observed the updated position for one hour on 2018/10/26. The automatic Quick Look Analysis (http://www.fact-project.org/monitoring) does not show any significant signal from the observed position. The weather conditions during the observation were not ideal and the zenith distance range during the observation was from 52 degree to 63 degree. An analysis of these data yields an upper limit of 84% of the flux of the Crab Nebula with a confidence level of 99%. Contact person: Daniela Dorner (dorner@astro.uni-wuerzburg.de) The First G-APD Cherenkov Telescope (FACT) is an imaging air Cherenkov telescope monitoring blazars at TeV energies. It is located in the Observatorio del Roque de los Muchachos on the Canary Island La Palma.