TITLE: GCN CIRCULAR NUMBER: 31194 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-211208A DATE: 21/12/09 20:26:24 GMT FROM: Simone Garrappa at DESY S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg), J. Sinapius (DESY-Zeuthen) and M. Kadler (Univ. of Wuerzburg) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC211208A neutrino event (GCN 31191) 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 2021-12-08 at 20:02:51.1 UT (T0) with J2000 position RA = 114.52 (+2.82, -2.50) deg, Decl. = 15.56 (+1.81, -1.39) deg (90% PSF containment). Two cataloged gamma-ray (>100 MeV) sources are located within the 90% IC211208A localization region (4FGL-DR2, The Fermi-LAT collaboration 2020, ApJS, 247, 33). These are 4FGL J0738.4+1539, associated with the radio source of unknown type NVSS J073824+153839 at 0.13 deg and 4FGL J0743.1+1713, associated with the BL Lac object TXS 0740+173 at 2.05 deg. Based on a preliminary analysis of LAT data at timescales of 1-month and 1-day prior to T0, these objects are not significantly detected (>5sigma). We searched for intermediate (days 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 IC211208A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC211208A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 3.0e-10 ph cm^-2 s^-1 for ~13-years (2008-08-04 to 2021-12-08 UTC), and < 4.5e-9 (< 7.0e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. In the vicinity of the IC211208A best-fit position, outside the 90% localization region at 2.1 deg from the best-fit IC211208A localization, the cataloged >100 MeV gamma-ray source 4FGL J0738.1+1742 is significantly detected (> 5 sigma) integrating 1-day LAT data before T0. This source is associated with the BL Lac object PKS 0735+17 at z = 0.45 (Gattano et al, 2018 A&A, 618, A80). Preliminary analysis indicates that on 2021-12-08 the gamma-ray source was observed with a daily averaged flux (E>100 MeV) of (3+/-1) x 10^-7 photons cm^-2 s^-1 (statistical uncertainty only), about 5 times greater than the average flux reported in the fourth Fermi LAT source catalog (4FGL-DR2). The source was significantly detected at 1-day timescale on 2021-12-01 and was observed on 2021-12-04 with peak daily flux value of (5+/-2) x 10^-7 photons cm^-2 s^-1 (statistical uncertainty only), about 10 times greater than the average 4FGL-DR2 flux. The source was reported to be in optical brightening on 2021-11-08 and is currently at its brightest state in R band https://vo.astro.spbu.ru/sites/default/files/optic/pks0735R.png (see also ATel #15021). Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For these observations the Fermi-LAT contact persons are S. Garrappa (simone.garrappa at desy.de) and  S. Buson (sara.buson at gmail.com). 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.