TITLE: GCN CIRCULAR NUMBER: 35625 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-240123A DATE: 24/01/25 07:57:28 GMT FROM: chiara.bartolini-1@unitn.it C. Bartolini (Univ of Trento & INFN Bari), S. Garrappa (Weizmann Institute of Science), S. Buson (Univ of Wuerzburg), L. Pfeiffer (Univ of Wuerzburg) and J. Sinapius (DESY) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the IC240123A high-energy neutrino event (GCN 35604) 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 2024-01-23 at 11:25:36.05 UT (T0) with J2000 position RA = 357.54 (+1.93, -1.71)deg, Decl. = +4.26 (+0.80, -0.76)deg (90% PSF containment). According to the fourth Fermi LAT source catalog (4FGL-DR4), there are no 4FGL-DR4 cataloged gamma-ray (>100 MeV; The Fermi-LAT collaboration 2022, ApJS, 260, 53) sources in the 90% IC240123A uncertainty localization region. The nearest 4FGL cataloged gamma-ray source is 4FGL J2349.4+0534 (TXS 2346+052) at 1.33deg away from the neutrino best-fit position. Based on a preliminary analysis of the LAT data over one month and day timescale prior T0, this object is not significantly detected at gamma rays. We searched for intermediate (days to years) timescale emission from a new gamma-ray source. Preliminary analysis indicates no significant (> 5 sigma) new excess emission (> 100 MeV) at the IC240123A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC240123A best-fit position, the >100 MeV flux upper limit (95% confidence) is <4.36e-10 ph cm^-2 s^-1 for ~15-years (2008-08-04 to 2024-01-23 UTC), and <2.42e-9 (<9.60e-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 region will continue. For this observation the Fermi-LAT contact person is C. Bartolini (chiara.bartolini at ba.infn.it). 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/35625. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...