TITLE: GCN CIRCULAR NUMBER: 43157 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-251218A DATE: 25/12/20 15:25:16 GMT FROM: chiara.bartolini-1@unitn.it C. Bartolini (Univ. of Trento and INFN Bari), 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 IC251218A neutrino event (GCN 43151) 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 25-12-18 at 20:21:00.63 UTC (T0) with J2000 position RA = 353.60 (+0.90, -0.74) deg, Decl. = 44.15 (+0.46, -0.45) deg 90% PSF containment (J2000). One cataloged gamma-ray (>100 MeV) source is located within the 90% IC251218A localization region (4FGL-DR4, The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546). This is the unassociated gamma-ray source 4FGL J2331.6+4430. Based on a preliminary analysis of the LAT data over the timescale of 1-day and 1-month prior to T0, this object is not significantly detected (> 5 sigma). 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 IC251218A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC251218A best-fit position, the >100 MeV flux upper limit (95% confidence) is <9.6e-11 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), <9.2e-09 (<8.8e-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 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/43157. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...