//////////////////////////////////////////////////////////////////////// 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 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 (). 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 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. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 30895 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCubeCascade-210926A DATE: 21/09/29 07:56:48 GMT FROM: Simone Garrappa at DESY S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and R. de Menezes (Univ. of Sao Paulo, Univ. of Wuerzburg) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC210926A neutrino event () with all-sky survey data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope. The IceCube event was detected on 2021-09-26 at 05:26:10.94 UT (T0) with J2000 position RA = 249.4993 deg, Decl. = -35.7347 deg (centered at an error region with 10.18⁰ radius, 90% PSF containment). Several cataloged gamma-ray (>100 MeV) sources are located within the 90% IC210926A localization region (4FGL-DR2, The Fermi-LAT collaboration 2020, ApJS, 247, 33). We searched for intermediate (days to month) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (> 5 sigma) new excess emission (> 100 MeV) at the IC210926A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC210926A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 6.3e-9 (< 5.6e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For these observations the Fermi-LAT contact persons are S. Garrappa (simone.garrappa at desy.de) and S. Buson (sara.buson at uni-wuerzburg.de). The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.