TITLE: GCN CIRCULAR NUMBER: 28468 SUBJECT: IceCube-200921A: IceCube observation of a high-energy neutrino candidate event DATE: 20/09/21 20:36:22 GMT FROM: Cristina Lagunas Gualda at DESY The IceCube Collaboration (http://icecube.wisc.edu/) reports: On 20/09/21 at 19:07:12.89 UT IceCube detected a track-like event with a moderate probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_Bronze alert stream. The average astrophysical neutrino purity for Bronze alerts is 30%. This alert has an estimated false alarm rate of 1.586 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/134512_71996695.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to: Date: 20/09/21 Time: 19:07:12.89 UT RA: 195.29 (+ 2.35 - 1.73 deg 90% PSF containment) J2000 Dec: 26.24 (+ 1.51 - 1.77 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 is one Fermi 4FGL/3FHL source located within the 90% localization region, 4FGL J1303.0+2434, located at RA: 195.76 deg, Dec: 24.58 deg (J2000), at a distance of 1.71 deg from the best-fit event position. 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