TITLE: GCN CIRCULAR NUMBER: 40276 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-250426A DATE: 25/04/29 11:02:04 GMT FROM: Leo Pfeiffer at University of Würzburg <pfeiffer.leo@gmail.com> L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg), S. Garrappa (Weizmann Institute of Science) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC250426A neutrino event (GCN 40254) 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 2025-04-26 17:44:58.16 UTC (T0) with J2000 position RA = 296.37 (+0.53, -0.50) deg, Decl. = 20.7 (+0.52, -0.52) deg 90% PSF containment. No cataloged gamma-ray sources are found within the 90% IC250426A localization error (The Fourth Fermi-LAT catalog, 4FGL-DR4, The Fermi-LAT collaboration 2023, arXiv:2307.12546). We searched for the existence of intermediate (days to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (>5 sigma) new excess emission (> 100 MeV) within the IC250426A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250426A best-fit position, the >100 MeV flux upper limit (95% confidence) is <6.95e-10 ph cm^-2 s^-1 for ~16-years (2008-08-04 / T0), <4.67e-08(<3.56e-07) 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 region will continue. For this analysis, the Fermi-LAT contact person is L. Pfeiffer (leonard.pfeiffer 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/40276. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...