Fermi-LAT Gamma-ray Observations of IceCube-260916A

ATel #18078; L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg), S. Garrappa (Weizmann Institute of Science) on behalf of the Fermi-LAT collaboration
on 23 Sep 2026; 21:14 UT
Credential Certification: Sara Buson (sara.buson@gmail.com)

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We report an analysis of observations of the vicinity of the high-energy IC260916A neutrino event (GCN 45598) 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 26-09-16 at 17:33:51.90 UTC (T0) with J2000 position RA = 156.18 (+0.51, -0.44) deg, Decl. = 4.22 (+0.49, -0.50) deg 90% PSF containment (J2000).

No cataloged >100 MeV gamma-ray source is located within the 90% IC260916A localization error (FL16Y, The Fermi-LAT 16 Year Source List, The Fermi-LAT collaboration 2026, arXiv:2602.22148). Preliminary analysis indicates no significant (>5 sigma) new excess emission (> 100 MeV) at the IC260916A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source, the >100 MeV flux upper limit (95% confidence) is <4.5e-10 ph cm^-2 s^-1 for ~18-years (2008-08-04 / T0), <9.3e-08(<2.4e-07) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

In the analysis of the ~18-years integrated LAT data (100 MeV - 1 TeV), a new 5.0 sigma excess emission, Fermi J1026+0359, was detected at a separation of 0.47 deg from the neutrino best-fit position and within the IC260916A 90% confidence localization. The statistical significance is calculated following the prescription adopted in the Fourth Fermi-LAT catalog (The Fermi-LAT collaboration 2022, ApJS, 260, 53).

Assuming a power-law spectrum, the best-fit localization is (J2000) RA: 156.59 deg, Dec: 3.99 deg (0.17 deg 99% containment, 0.08 deg 68% containment). The gamma-ray best-fit spectral parameters are flux = (1.4 +/- 0.8)e-9 ph cm^-2 s^-1 and index = 2.17 +/- 0.21 (statistical uncertainty only). The most likely counterpart to Fermi J1026+0359 is the blazar 3HSP J102523.0+040229 (The 3HSP catalogue of extreme and high-synchrotron peaked blazars, Chang Y.-L. et al., 2019), separated 0.25 deg away from the best-fit position of the gamma-ray excess, and within its 99% containment. In a preliminary analysis of the LAT data over 1-day and 1-month prior T0, Fermi J1026+0359 is not significantly detected in the LAT data.

Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this analysis, the Fermi-LAT contact person is L. Pfeiffer (leonard.pfeiffer 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.