GCN Circular 21659

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:

Using on-line data from the ANTARES detector, we have performed a follow-up analysis of the recently reported LIGO/Virgo G298936 event
using the bayestar probability map at event time. The ANTARES visibility at the time of the alert together with the 50% and 90% contours
of the probability map are shown in: https://www.cppm.in2p3.fr/~dornic/events/G298936/visi_230817.png <https://www.cppm.in2p3.fr/~dornic/events/G298936/visi_230817.png> (gwantares/GW@ANT40). Considering 
the location probability provided by the LIGO/VIRGO collaboration, there is 50% chance that the GW emitter was in the ANTARES field of view.

No up-going muon neutrino candidate events were recorded in the ANTARES sky during a +/- 500s time-window centered on the G298936 event
time. The expected number of atmospheric background events in the region visible by ANTARES is ~1.1e-2 in the +/- 500s time window. An
extended search during +/- 1 hour gives no up-going neutrino coincidence.

The results of a second analysis covering the full-sky as well as 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.
In the range 1-100 TeV, ANTARES has the best sensitivity to a large fraction of the Southern sky.