TITLE: GCN CIRCULAR NUMBER: 20929 SUBJECT: IceCube-170321A - IceCube observation of a high-energy neutrino candidate event DATE: 17/03/22 01:02:06 GMT FROM: Erik Blaufuss at U. Maryland/IceCube Erik Blaufuss (University of Maryland) reports on behalf of the IceCube Collaboration (http://icecube.wisc.edu/). On 21 March, 2017 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/80305071_129307.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to: Date: 2017-03-21 Time: 07:32:20.69 UT RA: 98.30 (+/- 1.2 deg 90% PSF containment) J2000 Dec: -15.02(+/- 1.2 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. This event was found to be close to the edge of the instrumented detector volume, which has increased the overall direction uncertainty for this event. 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