TITLE: GCN CIRCULAR NUMBER: 36349 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-240424A DATE: 24/04/29 19:18:19 GMT FROM: Sara Buson at DESY, Univ. of Wurzburg sara.buson@gmail.com

S. Garrappa (Weizmann Institute of Science), C. Bartolini (INFN Bari), S. Buson (Univ. of Wuerzburg), L. Pfeiffer (Univ. of Wuerzburg) and J. Sinapius (DESY) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the IC240424A high-energy neutrino event (GCN 36283) 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 2024-04-24 at 01:49:26 UT (T0) with J2000 position RA = 327.08 (+2.06, -1.70) deg, Decl. = +3.06 (+1.37, -1.33) deg (90% PSF containment). Two catalogued gamma-ray (>100 MeV; 4FGL, The Fermi-LAT collaboration 2022, ApJS, 260, 53) sources are located within the 90% IC240204A localization region. These are 4FGL J2149.6+0323 (associated with the FSRQ PKS B2147+031) and 4FGL J2146.8+0425 (associated with the FSRQ MG1 J214653+0427). Based on a preliminary analysis of the LAT data over the timescales of 1-month and 1-day prior to T0, these objects are not significantly detected (> 5 sigma).

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 IC240424A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC240424A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 1.2e-10 ph cm^-2 s^-1 for ~15-years (2008-08-04 to 2024-04-24 UTC), and < 2.2e-8 (<3.2e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

In the analysis of the ~15-years integrated LAT data (100 MeV - 1 TeV), a 4.5 sigma new excess of gamma rays, Fermi J2151.3+0220 was detected 1 deg offset from the best-fit IC240424A 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: 327.83 deg, Dec: 2.34 deg (10 arcmin 99% containment, 5 arcmin 68% containment). The gamma-ray best-fit spectral parameters are flux = (2.5 +/- 1.9)e-9 ph cm^-2 s^-1 and index = 1.7+/-0.2. In a preliminary analysis of the LAT data over 1-day and 1-month prior T0, Fermi J2151.3+0220 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.

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 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.

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