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GCN Circular 33882

Fermi-LAT gamma-ray observations of IceCube-230524A
2023-05-26T21:44:19Z (a year ago)
Sara Buson at GSFC/Fermi <>
J. Sinapius (DESY-Zeuthen), S. Garrappa (Ruhr-Universitaet Bochum) and S. Buson (Univ. of Wuerzburg) on behalf of the Fermi-LAT collaboration:
We report an analysis of observations of the vicinity of the IC230524A  high-energy neutrino event (GCN 33633) 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 2023-05-24 at 11:09:36.24 UT (T0) with J2000 position RA = 318.43 (+3.12, -2.59) deg, Decl. = +2.84 (+2.55, -2.43) deg (90% PSF containment).  Five cataloged gamma-ray (>100 MeV; The Fermi-LAT collaboration 2022, ApJS, 260, 53) sources are located within the 90% IC230524A  localization region. These sources are 4FGL J2110.3+0404, at a distance of 1.5 deg from the IC best-fit position, 4FGL J2109.6+0440, at a distance of 2.1 deg, 4FGL J2122.5+0345, at a distance of 2.4 deg, 4FGL J2104.7+0108, at a distance of 2.8 deg, and 4FGL J2125.6+0458 at a distance of 3.7 deg. Based on a preliminary analysis of the LAT data over a month and day timescale prior T0, these objects are not significantly detected at gamma rays. 
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 IC230524A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC230524A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 3.6e-10 ph cm^-2 s^-1 for ~14-years (2008-08-04 to 2023-05-24 UTC), and < 3.7e-9 (<6.0e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.
Within the 90% confidence localization of the neutrino, 2.3 deg offset from the best-fit IC230524A position, an excess of gamma rays, Fermi J2106.3+0125 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 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 = 316.58 deg, Decl. = 1.43 deg (95% containment radius = 0.13 deg) with best-fit spectral parameters, flux = (8.6 +/- 3.9)e-10 ph cm^-2 s^-1, index = 1.9 +/-0.1. In a preliminary analysis of the LAT data over one month prior to T0, Fermi J2106.3+0125 is not significantly detected. All values include the statistical uncertainty only.

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 <>), J. Sinapius (jonas.sinapius at <>) and S. Buson (sara.buson at <>).
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.
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