TITLE: GCN CIRCULAR NUMBER: 30954 SUBJECT: AMON IC_HAWC NuEm-211020A: No neutrino counterpart detected with ANTARES DATE: 21/10/21 14:30:28 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 AMON IC-HAWC coincidence alert NuEm-211020A (https://gcn.gsfc.nasa.gov/gcn3/30950.gcn3). No up-going muon neutrino candidate events were detected within 3 degrees from the coincidence coordinate over a time period [10h52 - 14h13 UT] corresponding to 55.8% of the HAWC transit time mentioned in the AMON notice, during which the potential source remained visible in the up-going field of view of ANTARES. This leads to a preliminary 90% confidence level upper limit on the muon neutrino radiant fluence from a point source located at the coordinates of the coincidence as reported by AMON, of about 15 GeV.cm^-2 over the energy range 4 TeV - 4 PeV (the range corresponding to 5-95% of the detectable flux) for an E^-2 power-law spectrum, and about 26 GeV.cm^-2 (750 GeV - 380 TeV) for an E^-2.5 spectrum. These indicative limits have been computed for the end of the HAWC transit time. A search over an extended time window of +/-1 day has also yielded no detection (45% visibility). ANTARES (https://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.