TITLE: GCN CIRCULAR NUMBER: 40124 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-250406A DATE: 25/04/08 19:44:10 GMT FROM: Leo Pfeiffer at University of Würzburg pfeiffer.leo@gmail.com
L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg), C. Bartolini (INFN Bari), 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 IC250406A neutrino event (GCN 40103) 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 2025-04-06 at 22:50:35.34 UTC (T0) with J2000 position RA = 81.47 (+0.54, -0.53) deg, Decl. = 16.96 (+0.42, -0.50) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC250406A localization error (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546).
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 IC250406A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250406A best-fit position, the >100 MeV flux upper limit (95% confidence) is <1.72e-10 ph cm^-2 s^-1 for ~16-years (2008-08-04 / T0), <2.13e-08 (<1.58e-07) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.
The closest catalogued gamma-ray source to the IC250406A best-fit position is 4FGL J0521.2+1637 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), associated with the compact steep spectrum quasar 3C 138, located approximately 1.15° away from the IceCube event, outside the 90% neutrino localization region. Since October 2024, the source has exhibited strong and prolonged gamma-ray flaring activity (see ATel #16845, ATel #17107). A preliminary analysis confirms that this enhanced activity is ongoing, with a daily averaged gamma-ray flux (E > 100 MeV) of (0.39± 0.29) × 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), as observed on April 6, 2025. This represents an increase in flux by a factor of approximately 93 compared to the average flux reported in the 4FGL-DR4.
Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this source, 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.
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