TITLE: GCN CIRCULAR NUMBER: 30862 SUBJECT: IceCube-210922A - IceCube observation of a high-energy neutrino candidate track-like event DATE: 21/09/22 19:45:10 GMT FROM: Dr. Massimiliano Lincetto at Ruhr-Universitaet Bochum <lincetto@astro.rub.de> The IceCube Collaboration (http://icecube.wisc.edu/) reports: On 2021-09-22 at 18:17:20.948 UT IceCube detected a track-like event with a high probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_GOLD alert stream. The average astrophysical neutrino purity for Gold alerts is 50%. This alert has an estimated false alarm rate of 0.1472 events per year due to atmospheric backgrounds. The IceCube detector was in a normal operating state at the time of detection. After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon_g_b/135736_30987826.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to: Date: 2021-09-22 Time: 18:17:20.948 RA: 60.73 (+0.96/-0.66 deg 90% PSF containment) J2000 Dec: -4.18 (+0.42/-0.55 deg 90% PSF containment) J2000 We encourage follow-up by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino. There are no Fermi 4FGL or 3FHL catalog sources in the 90% uncertainty region. The nearest gamma-ray source in either catalog is 4FGL J0357.2-0319 / 3FHL J0357.3-0316 at RA: 59.32 deg, Dec: -3.32 deg (1.64 deg away from the best-fit event position). The source is classified as having a possible blazar origin. This neutrino candidate event has an estimated neutrino energy of ~750 TeV and an estimated signalness of >90%, making it one of the most interesting neutrino alerts issued in the last year. 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