TITLE: GCN CIRCULAR NUMBER: 26728 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-200109A and detection of a possible new gamma-ray source, Fermi J1055.8+1034 DATE: 20/01/13 09:54:53 GMT FROM: Simone Garrappa at DESY S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and V. Paliya (DESY-Zeuthen) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC200109A neutrino event (GCN 26696) 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 2020-01-09 at 23:41:39.94 UT (T0) with J2000 position RA = 164.49 (+4.94, -4.19) deg, Decl. = 11.87 (+1.16, -1.36) deg 90% PSF containment. Two cataloged >100 MeV gamma-ray sources (The Fermi-LAT Collaboration 2019, arXiv:1902.10045) are located within the 90% IC200109A localization error. These are the objects 4FGL J1103.0+1157 and 4FGL J1114.6+1225. Based on a preliminary analysis of the LAT data over the timescales of 1-day and 1-month prior to T0, these objects are not significantly detected at 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 (> 5 sigma) new excess emission (> 100 MeV), at the IC200109A 90% best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IceCube best-fit position, the >100 MeV flux upper limit (95% confidence) is < 4e-10 ph cm^-2 s^-1 for ~11-years (2008-08-04 / 2020-01-09 UTC), < 8e-9 (< 7e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. Within the error circle for the direction of the neutrino, ~1.2deg offset from the best-fit IC200109A position, a ~4 sigma excess of gamma rays, Fermi J1055.8+1034 was detected in an analysis of the integrated LAT data (> 100 MeV) between 2008-08-04 and  2020-01-09. Assuming a power-law spectrum, the best-fit localization is (J2000) RA:  163.97, Dec: 10.58 (0.19 deg 99% containment, 0.09 deg 68% containment), with best-fit spectral parameters flux = (1.6 +/- 0.8)e-9 ph cm^-2 s^-1 and index = 2.06 +/- 0.18. In a preliminary analysis of the LAT data over one day and one month prior T0, Fermi J1055.8+1034  is not significantly detected in the LAT data. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For these sources the Fermi-LAT contact persons are S. Garrappa (simone.garrappa at desy.de ) and S. Buson (sara.buson 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.