TITLE: GCN CIRCULAR NUMBER: 42371 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-251018A DATE: 25/10/19 23:49:18 GMT FROM: Simone Garrappa at Weizmann Institute of Science <simone.garrappa@weizmann.ac.il> S. Garrappa (Weizmann Institute of Science), L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg) and C.Bartolini (Univ. of Trento & INFN Bari) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the IC251018A high-energy neutrino event (GCN 42351) 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 25-10-18 at 05:05:42.97 UT (T0) with J2000 position RA = 321.11 (+3.09/-2.94) deg, Decl. = +41.31 (+2.46/-3.51) deg (90% PSF containment). There are three catalogued gamma-ray sources (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546) located within the 90% IC251018A localization region. These are: 4FGL J2118.3+4055: unassociated,at 1.2 deg distance from the best-fit neutrino position; 4FGL J2120.5+4331: unassociated, at 2.3 deg distance from the best-fit neutrino position; 4FGL J2136.5+4259: associated with the blazar of uncertain type TXS 2134+428, at 2.8 deg from the best-fit neutrino position; Based on a preliminary analysis of the LAT data over the timescale of one day and one month prior T0, these objects are not significantly detected at gamma-rays. 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) at the IC251018A bestfit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC251018A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 8.4 e-10 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), < 7.4e-09 (<5.9e-08) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. In the analysis of the ~17-years integrated LAT data (100 MeV - 1 TeV), a 5 sigma new excess of gamma rays, Fermi J2125.8+3940 was detected 1.65 deg offset from the best-fit IC251018A position and within the 90% confidence localization of the direction of the neutrino. Assuming a power-law spectrum, the best-fit localization is (J2000) RA = 321.46 deg, Dec = +39.69 deg (6 arcmin 99% containment, 3 arcmin 68% containment). The gamma-ray best-fit spectral parameters are flux = (8 +/- 3)e-10 ph cm^-2 s^-1 and index = 1.84+/-0.12. In a preliminary analysis of the LAT data over 1-day and 1-month prior T0, Fermi J2125.8+3940 is not significantly detected in the LAT data. All values include the statistical uncertainty only. The statistical significance is calculated following the prescription adopted in the 4FGL (Abdollahi et al. 2020, ApJS, 247, 33). Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this region will continue. For these observations the Fermi-LAT contact person is S. Garrappa (simone.garrappa at weizmann.ac.il). 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/42371. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...