General information
IceCube-251218A is a track-like high-energy neutrino candidate detected by IceCube on 2025-12-18 20:21:00.63 UTC, distributed via the Astrotrack BRONZE stream; follow-up reports so far provide only upper limits with no confirmed EM counterpart.
Wavelength coverage
- Neutrino: IceCube reported a refined localization and searched for additional coincident track-like events, finding none in ±500 s and in a 2-day window.
- Gamma-ray (GeV): Fermi-LAT survey data show no significant (>5σ) new >100 MeV excess within the localization; flux upper limits are reported for multiple timescales.
- Optical: COLIBRÍ (r, z) and ZTF (r) reported non-detections with limiting magnitudes around r≈21.9 (COLIBRÍ) and typical ZTF depth ∼21.0 mag.
No X-ray, UV, IR, or radio observations are mentioned in the cited circulars.
Lightcurve and spectrum
- Fermi-LAT: assuming a power-law spectrum with photon index 2.0, the >100 MeV 95% flux UL is <9.6e-11 ph cm^-2 s^-1 (2008-08-04 to T0), <9.2e-09 ph cm^-2 s^-1 (1 month pre-T0), and <8.8e-08 ph cm^-2 s^-1 (1 day pre-T0).
- Optical: COLIBRÍ reports 10σ limits r>21.9 and z>20.8 (AB; not extinction-corrected); ZTF reports 300 s r-band imaging with typical depth ∼21.0 mag and no candidates.
Redshift
What’s special vs typical