TITLE: GCN CIRCULAR NUMBER: 39637 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-250309A DATE: 25/03/09 17:38:48 GMT FROM: Sara Buson at DESY, Univ. of Wurzburg sara.buson@gmail.com
S. Buson (DESY, Univ. of Wuerzburg), S. Garrappa (Weizmann Institute of Science), L. Pfeiffer (Univ. of Wuerzburg), N. Di Lalla (Stanford Univ.), M. Arimoto (Kanazawa University), R. Gupta (NASA GSFC) and A. Holzmann Airasca (UniTrento and INFN Bari) on behalf of the Fermi-LAT collaboration:
We report an analysis of observations of the vicinity of the high-energy IC250309A neutrino event (GCN 39631) 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 2025-03-09 07:36:04.75 UTC (T0) with J2000 position RA = 211.07 (+0.31, -0.30) deg, Decl. = -10.73 (+0.26, -0.30) deg 90% PSF containment. No cataloged gamma-ray sources are found within the 90% IC250309A localization error (The Fourth Fermi-LAT catalog, 4FGL-DR4, 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 IC250309A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IceCube best-fit position, the >100 MeV flux upper limit (95% confidence) is <2.4e-10 ph cm^-2 s^-1 for ~16-years (2008-08-04 / T0), <6.8e-09 (<9.3 e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.
The IceCube event IC250309A occurred near the GRB 250309B (GCN 39635), with a detection time 145.91 seconds after the Fermi-GBM trigger and a positional separation of 0.50 deg. An analysis of the LAT data conducted over a ±1000-second window centered on T0, shows no significant excess emission, neither associated with IC250309A nor GRB 250309B. The >100 MeV flux upper limit (95% confidence) at IC250309A best-fit position for this time interval is < 5.3 e-6 ph cm^-2 s^-1.
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 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.
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