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 <simone.garrappa@desy.de> 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 (<https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/135747_58376130.amon>) 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.