TITLE: GCN CIRCULAR NUMBER: 30887 SUBJECT: IceCube-210926A: No neutrino counterpart candidates in ANTARES search DATE: 21/09/27 08:32:10 GMT FROM: Antoine Kouchner at ANTARES Collaboration <kouchner@apc.in2p3.fr> Alexis Coleiro (APC/Universite de Paris) and Damien Dornic (CPPM/CNRS) on behalf of the ANTARES Collaboration. Using data from the ANTARES detector, we have performed a follow-up analysis of the recently reported cascade event IceCube-210926A (<https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/135747_58376130.amon>). At the time of the alert, the reconstructed origin was -70.6 degrees below the horizon for ANTARES. No up-going muon neutrino candidate events were recorded within the 90% error box of the IceCube event during a +/- 1h time-window centered on the IceCube event time, and over which the potential source remained visible all time. This leads to a preliminary 90% confidence level upper limit on the muon-neutrino fluence from a point source of 21 GeV.cm^-2 over the energy range 2.3 TeV – 3.2 PeV (the range corresponding to 5-95% of the detectable flux) for an E^-2 power-law spectrum, and 30 GeV.cm^-2 (380 GeV - 240 TeV) for an E^-2.5 spectrum. A search over an extended time window of +/- 1 day has also yielded no detection (73% visibility). ANTARES <http://antares.in2p3.fr/> is the largest undersea neutrino detector (Mediterranean Sea) and it is primarily sensitive to astrophysical neutrinos in the TeV-PeV energy range. At 10 TeV, the median angular resolution for muon neutrinos is about 0.5 degrees. In the range 1-100 TeV ANTARES has a competitive sensitivity to this position in the sky.