TITLE: GCN CIRCULAR NUMBER: 40242 SUBJECT: Fermi-LAT gamma-ray observations of IceCube-250421A DATE: 25/04/24 20:30:21 GMT FROM: Leo Pfeiffer at University of Würzburg pfeiffer.leo@gmail.com

L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg), S. Garrappa (Weizmann Institute of Science) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the IC250421A high-energy neutrino event (GCN 40195) 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 25-04-21 at 17:06:08.07 UT (T0) with J2000 position RA = 240.91 (+2.62, -4.44) deg, Decl. = +28.67 (+1.70, -1.70) deg (90% PSF containment). There are five catalogued gamma-ray sources (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546) located within the 90% IC250421A localization region. These are:

4FGL J1606.5+2717, associated with B2 1604+27;

4FGL J1556.1+2812, associated with NVSS J155611+281134;

4FGL J1612.2+2828, associated with TXS 1610+285;

4FGL J1545.5+2839, associated with WISEA J154520.76+283508.6;

4FGL J1555.3+2903 (unassociated).

Based on a preliminary analysis of the LAT data over the timescale of one day and one month prior T0, these objects are not significantly detected at gamma-rays.

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) at the IC250421A bestfit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250421A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 1.21 e-10 ph cm^-2 s^-1 for ~16-years (2008-08-04 / T0), < 4.61e-09 (<1.01e-07) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Within the error circle for the direction of the neutrino, 1.39 deg offset from the best-fitIC250421 position, a ~14 sigma excess of gamma rays, Fermi J1605.3+2959, was detected in an analysis of the integrated LAT data (0.1 - 300 GeV) between 2008-08-04 and T0. Assuming a power-law spectrum, the candidate gamma-ray source has best-fit localization of RA = 241.39 deg, Dec = +30.00 deg (0.16 deg 99% containment, 0.13 deg 95% containment, 0.08 deg 68% containment) with best fit spectral parameters flux = (1.20± 0.12) × 10^-8 ph cm^-2 s^-1, index = 3.18 ± 0.23. A possible counterpart for Fermi J1605.3+2959 is the FSRQ CGRaBS J1605+3001 (a.k.a. BZQ J1605+3001, Stephen E. & Healey et al 2007, ApJS, 171, 61) with a redshift of 2.41 (Sloan Digital Sky Survey 2017, ApJS, 233, 25), located at 0.03 deg from the Fermi J1605.3+2959 best-fit position, within the 68% localization error.

A preliminary analysis of the temporal variability at the position of Fermi J1605.3+2959 reveals that the source underwent an approximately six-months long period of enhanced activity between late 2023 and early 2024.

In addition, in a preliminary analysis of the LAT data over 1-month timescale, we test the presence of an additional point source at the position of the optical transient SN2025cbj, a ~60-days old SN of Type-IIn which was reported in spatial coincidence with IC250421A in GCN#40208. No significant gamma-ray emission was detected. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the SN2025cbj best-fit position, the >100 MeV flux upper limit (95% confidence) is < 1.13e-08 ph cm^-2 s^-1 for a 1-month integration time before T0.

Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this region will continue. For these observations 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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