TITLE: GCN CIRCULAR NUMBER: 42020 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-250926A DATE: 25/09/28 17:52:33 GMT FROM: Sara Buson at DESY, Univ. of Wurzburg <sara.buson@gmail.com> L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg) and 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 IC250926A neutrino event (GCN 42003) 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 25-09-26 at 12:28:55.17 UTC (T0) with J2000 position RA = 131.0 (+0.56, -0.63) deg, Decl. = 9.97 (+0.55, -0.71) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC250926A localization error (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, 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 (>5sigma) new excess emission (> 100 MeV) within the IC250926A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250926A best-fit position, the >100 MeV flux upper limit (95% confidence) is <1.8e-10 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), <4.4e-09 (<4.2e-08) 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 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/42020. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...