TITLE: GCN CIRCULAR NUMBER: 45695 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-260805A DATE: 26/09/22 19:59:02 GMT FROM: Sara Buson at DESY, Univ. of Wurzburg 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 IC260805A neutrino event (GCN 45294) 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 2026-08-05 at 14:01:39.70 UTC (T0) with J2000 position RA = 60.47 (+0.91, -0.80) deg, Decl. = 26.07 (+0.54, -0.56) deg 90% PSF containment (J2000). One cataloged >100 MeV gamma-ray source is located within the 90% IC260805A localization error. This is the object FL16Y J0402.5+2615 (FL16Y, The Fermi-LAT 16 Year Source List, The Fermi-LAT collaboration 2026, arXiv:2602.22148). The FL16Y catalog does not report an associated counterpart at other wavelengths for FL16Y J0402.5+2615. We performed a new analysis of the full ~18yr Fermi-LAT mission data, integrating the period from 2008-08-04 to T0, in the 100 MeV–1 TeV energy range. The best-fit position of FL16Y J0402.5+2615 is (J2000) RA, Dec = 60.83, +25.99 (0.11 deg 68% containment, 0.18 deg 95% containment, 0.22 deg 99% containment). The source 5BZQ J0403+2600 (also known as OF 200), located at (RA, Dec) = (60.7732753, +26.0004174), lies 0.053 deg from the best-fit gamma-ray position, within its 68% localization uncertainty. This positional agreement further supports 5BZQ J0403+2600 as the likely counterpart of the gamma-ray excess, as also reported in the previous Fermi-LAT catalog (4LAC-DR3; The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope: Data Release 3, The Fermi-LAT collaboration 2022, DOI: 10.3847/1538-4365/ac9523). Based on a preliminary analysis of the LAT data over the timescales of 1-day and 1-month prior to T0, this object is not significantly detected (> 5 sigma). 5BZQ J0403+2600 is a flat-spectrum radio quasar at z=2.109 (Massaro et al. 2015, Ap&SS, 357, 75; Schmidt 1997, ApJ, 217, 358). The Fermi-LAT spectrum indicates that its gamma-ray emission peaks toward the MeV range, while the source is relatively faint at GeV energies, suggesting that a substantial fraction of its high-energy emission may emerge at lower MeV/X-ray energies. The LAT light curve, integrated over 3-year time bins, shows a mild enhancement in recent years. Given the source's spectral shape, this increase at GeV energies could potentially indicate enhanced activity at lower MeV/X-ray energies. Its X-ray activity over recent years remains largely unconstrained, as only one X-ray observation is available in the archives, obtained by the Neil Gehrels Swift Observatory in 2007. New X-ray observations would help characterize its current activity state and better constrain the high-energy peak of its SED. The source has also been regularly observed at radio wavelengths and is included in the OVRO monitoring program (Richards et al. 2011, ApJS, 194, 29). Recent radio observations could provide useful complementary information on its long-term activity and help assess whether the GeV enhancement is part of a broader multi-wavelength evolution. 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 IC260805A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC260805A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 3.0e-10 ph cm^-2 s^-1 for ~18-years (2008-08-04 / T0), <8.2e-09 (<3.7e-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 points of contact are L. Pfeiffer (leonard.pfeiffer at uni-wuerzburg.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.