IceCube-250926A

L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg) and 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 IC250926A neutrino event (GCN 42003
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 ) 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 25-09-26 at 12:28:55.17 UTC (T0) with J2000 position RA = 131.0 (+0.56, -0.63) deg, Decl. = 9.97 (+0.55, -0.71) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC250926A localization error (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546
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 ).

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 IC250926A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250926A best-fit position, the >100 MeV flux upper limit (95% confidence) is <1.8e-10 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), <4.4e-09 (<4.2e-08) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

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.

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-250926A (https://gcn.nasa.gov/circulars/42003
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 ) in a time range of 1000 seconds centered on the alert event time (2025-09-26 12:20:35.170 UTC to 2025-09-26 12:37:15.170 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-250926A. We report a p-value of 1.00 in this time window. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.3e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-250926A in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 2e+02 GeV and 1e+05 GeV. 

A subsequent search was performed including 2 days of data centered on the alert event time (2025-09-25 12:28:55.170 UTC to 2025-09-27 12:28:55.170 UTC). In this case, we report a p-value of 1.00, consistent with no significant excess of track events. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.5e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-250926A in a 2 day time window. 

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu.

[1] IceCube Collaboration, R. Abbasi  et al., ApJ 910 4 (2021)

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

On 25-09-26 at 12:28:55.17 UT IceCube detected a track-like event with a moderate probability of being of astrophysical origin.   The event was selected by the ICECUBE_Astrotrack_BRONZE alert stream.  The average astrophysical neutrino purity for Bronze alerts is 30%.  This alert has an estimated false alarm rate of 3.2659 events per year due to atmospheric backgrounds.  The IceCube detector was in a normal operating state at the time of detection.

After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon_g_b/141390_64842455.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 25-09-26
Time: 12:28:55.17 UT
RA: 131.0 (+0.56/-0.63 deg 90% PSF containment) J2000
Dec: 9.97 (+0.55/-0.71 deg 90% PSF containment) J2000

We encourage follow-up by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino.

No known gamma-ray sources listed in the Fermi 4FGL-DR4 or 3FHL catalogs are located within the 90% uncertainty region of the event.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica.  The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu