GCN Circular 21631
M. Ageron (CPPM/CNRS), B. Baret (APC/CNRS), A. Coleiro (IFIC & APC), D. Dornic (CPPM/CNRS), A. Kouchner (APC/Universite Paris Diderot), T. Pradier (IPHC/Universite de Strasbourg) report on behalf of the ANTARES Collaboration: The refined location of the LIGO/Virgo G298048 event (90% confidence level contour Bayestar-HLV) falls outside the optimum field of view of ANTARES (as defined with upward going events). GCN #21522 reported the results of the upward going neutrino search. As a consequence, we have performed a dedicated analysis looking for down-going neutrino candidates in the on-line ANTARES data stream. Cuts on the quality of the track reconstruction and an energy proxy have been used to reduce the false alarm rate in such a way that one event matching in time (+/- 500 s) and direction (90% C.L contour) with G298048 would result in a 3-sigma excess. The ANTARES visibility, for down-going events, at the time of the alert together with the 90% contour of the probability map are shown in: https://www.cppm.in2p3.fr/~dornic/events/G298048/gw190817_visi_down.png <https://www.cppm.in2p3.fr/~dornic/events/G298048/gw190817_visi_down.png> (gwantares/GW@ANT31). No down-going muon neutrino candidate events were recorded by ANTARES within the 90% C.L. contour and during a +/- 500 s time-window centered on the G298048 event time. An extended search during +/- 1 hour yields no coincidence. An estimate of the upper limit on the associated neutrino fluence will be sent in a subsequent circular. ANTARES, being installed in the Mediterranean Deep Sea, is the largest neutrino detector in the Northern Hemisphere. It is primarily sensitive to astrophysical neutrinos in the TeV-PeV energy range. At 10 TeV, the median angular resolution for muon neutrinos is below 0.5 degrees.