TITLE: GCN CIRCULAR NUMBER: 26696 SUBJECT: IceCube-200109A: IceCube observation of a high-energy neutrino candidate event DATE: 20/01/10 01:14:56 GMT FROM: Robert Stein at DESY The IceCube Collaboration (http://icecube.wisc.edu/) reports: On 20/01/09 at 23:41:39.94 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 0.64 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/133609_37927131.amon ), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to: Date: 20/01/09 Time: 23:41:39.94 UT RA: 164.49 (+4.94 -4.19 deg 90% PSF containment) J2000 Dec: 11.87 (+1.16 -1.36 deg 90% PSF containment) J2000 Due to the unique topology of this event, with a partially-contained track in the detector, it was challenging for online algorithms to reconstruct. The initially-reported direction was offset from this updated best-fit position by approximately 3 degrees. 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 source within the 90% localization region, 4FGL J1103.0+1157 at RA: 165.77 deg, Dec: 11.97 deg (1.26 deg away from the best-fit event position). This source, which is associated with the quasar TXS 1100+122 at z=0.91, is also found in the 3FHL catalog. 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