TITLE: GCN CIRCULAR NUMBER: 32565 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-220918A and detection of a new gamma-ray source, Fermi J0502.5+0037 DATE: 22/09/19 20:54:50 GMT FROM: Simone Garrappa at DESY S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and J. Sinapius (DESY-Zeuthen) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the IC220918A high-energy neutrino event (GCN 32562) 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 2022-09-18 at 12:46:05.32  UT (T0) with J2000 position RA = 75.15 (+3.79 , -3.38) deg, Decl. = 3.58 (+3.70 , -3.40) deg (90% PSF containment). Seven cataloged gamma-ray (>100 MeV) sources are located within the 90% IC220918A localization region (4FGL-DR3; The Fermi-LAT collaboration 2022, ApJS, 260, 53). 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 IC220918A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC220918A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 5.9e-10 ph cm^-2 s^-1 for ~14-years (2008-08-04 to 2022-09-18 UTC), and < 3.5e-9 (<5.8e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. Based on a preliminary analysis of the LAT data integrating over the time interval 1-month prior to T0, the catalogued sources 4FGL J0509.4+0542  (associated with the object TXS 0506+056) and 4FGL J0505.3+0459 (associated with the flat-spectrum radio object PKS 0502+049) are  significantly detected (> 5 sigma). The observed fluxes are comparable to the average ones measured in the 4FGL catalog. Integrating over the time interval  1-day prior to T0, 4FGL J0509.4+0542 is detected at a statistical significance of ~4.5 sigma, at a flux level comparable to the average one measured in the 4FGL catalog. Within the 90% confidence localization of the neutrino, 3 deg offset from the best-fit IC220918A position, a new excess of gamma rays, Fermi J0502.5+0037, was detected in an analysis of the ~14-years integrated LAT data (100 MeV - 1 TeV) prior to T0. This putative new source is detected at a statistical significance > 5 sigma (calculated following the prescription adopted in the the Fourth Fermi-LAT catalog, The Fermi-LAT collaboration 2020, ApJS, 247, 33). Assuming a power-law spectrum, the excess has best-fit localization of RA = 75.63 deg, Decl. = 0.62 deg (99% containment radius = 0.1 deg) with best-fit spectral parameters, flux = (2.8 +/- 0.9)e-9 ph cm^-2 s^-1, index = 2.2 +/-0.1. In a preliminary analysis of the LAT data over one month and one day prior to T0, Fermi J0502.5+0037 is not significantly detected in the LAT data. All values include the statistical uncertainty only. A possible counterpart of Fermi J0502.5+0037  is the radio source PKS B0500+006, at RA=75.641042 deg, Dec=0.704861 deg (Condon et al. 1998 AJ, 115, 5). This source is located about 0.08 deg from the Fermi J0502.5+0037 best-fit position, and within the gamma-ray 99% positional uncertainty. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this region will continue. For these observations 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.