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.