TITLE: GCN CIRCULAR NUMBER: 23375 SUBJECT: IceCube-181023A - IceCube observation of a high-energy neutrino candidate event DATE: 18/10/23 19:38:53 GMT FROM: Erik Blaufuss at U. Maryland/IceCube The IceCube Collaboration (http://icecube.wisc.edu/) reports: On 23 October 2018 UTC IceCube detected a track-like, very-high-energy event with a high probability of being of astrophysical origin. The event was identified by the  Extremely High Energy (EHE) track event selection.  The IceCube detector was in a normal operating state. EHE events typically have a neutrino interaction vertex that is outside the detector, produce a muon that traverses the detector volume, and have a high light level (a proxy for energy). After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon/53411354_131653.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to: Date: 18/10/23 Time: 16:37:32.65 UTC RA: 270.18 (-1.70,+2.00 deg  90% PSF containment) J2000 Dec: -8.57  (-1.30,+1.25 deg 90% PSF containment) J2000 The closest sources to this position among Fermi-LAT catalogs 3FGL, 2FGL, and 3FGL and Roma-BZCat is the unassociated source 3FGL J1804.5-0850 (1.3 degrees from best fit refined direction).  We encourage follow-up of this alert by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino. The original GCN Notice contained an incorrect statement that “this EHE event is the same as the previously reported HESE event.”  This is a unique EHE alert trigger and unrelated to previously reported HESE alerts. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu