TITLE: GCN CIRCULAR NUMBER: 23378 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-181023A DATE: 18/10/24 06:56:36 GMT FROM: Sara Buson at GSFC/Fermi S. Buson (Univ. of Wuerzburg), S. Garrappa (DESY-Zeuthen), S. Cutini (Univ. of Perugia) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the very high-energy IC181023A neutrino event (GCN 23375) 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 2018-10-23 16:37:32.65 UTC (T0) with J2000 position RA = 270.18 (-1.70,+2.00) deg, Decl. = -8.57 (-1.30,+1.25) deg 90% PSF containment. One cataloged >100 MeV gamma-ray source is located within the 90% IC181014A localization error, at a distance of roughly 0.97deg. This is the unassociated object 3FGL J1804.5-0850 (Ackermann et al. 2015, ApJS, 810, 14). Based on a preliminary analysis of the LAT data over the past month, this object is not significantly detected in gamma-rays. We searched for the existence of intermediate (months to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (>5sigma) new excess emission (0.1 - 300 GeV) within the IC181023A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.2 fixed) for a point source at the IceCube best-fit position, the >100 MeV flux upper limit (95% confidence) is < 2.4e-8 (< 1.2e-8) ph cm^-2 s^-1 for a 1-month (6-month) integration time before T0. Within the error circle for the direction of the neutrino, 0.3deg offset from the best-fit IC181023A position, a ~4 sigma excess of gamma rays, Fermi J1801-0849, was detected in an analysis of the integrated LAT data (0.1 - 300 GeV) between 2008-08-04 and 2018-03-16. Assuming a power-law spectrum, the candidate gamma-ray source has best-fit localization of RA: 270.42 Dec: -8.82 (0.14 deg 99% containment) with best fit spectral parameters flux = (7 +/- 3)e-9 ph cm^-2 s^-1, index = 2.3 +/- 0.1. This region is located near the Galactic plane, where the emission of the gamma-ray diffuse background is bright and might present some structure. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this source the Fermi-LAT contact persons are S. Buson (sara.buson at gmail.com), S. Garrappa (simone.garrappa at desy.de) and S. Cutini (sara.cutini at pg.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.