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VERITAS Follow-Up Observations of the Ultra-High-Energy Neutrino Event KM3-230213A

Connor Mooney

TL;DR

VERITAS followed up the ultra-high-energy neutrino KM3-230213A with targeted VHE gamma-ray observations to search for a shared hadronic counterpart. The campaign covered the KM3NeT 90% localization with a wobble pointing strategy and amassed 17.6 hours of data, but found no significant TeV emission; the derived 99% CL upper limit above $550$ GeV is $9.49 \times 10^{-13}$ cm$^{-2}$ s$^{-1}$, or $1.81\%$ of Crab Nebula flux. The non-detection constrains persistent TeV emitters and dampens cosmogenic-only explanations, while allowing a transient or beamed extragalactic origin within the region. The result underscores the importance of rapid, real-time multi-messenger follow-up and forecasts the improved capabilities of CTAO and IceCube-Gen2 for future UHE neutrino sources.

Abstract

The recent announcement of the detection of the ultra-high-energy (UHE) neutrino event KM3-230213A by the KM3NeT telescope represents a critical opportunity to explore the origins of cosmic neutrinos and their potential gamma-ray counterparts. With an inferred neutrino energy exceeding 100 PeV, this event stands as the most energetic neutrino observed to date. The large offset from the galactic plane (11 degrees) and the presence of several blazars with temporally correlated multiwavelength counterparts within the 3 degrees localization region raise the possibility of an extragalactic origin. Additionally, the event's apparent tension with IceCube constraints suggests that it could be transient in nature rather than cosmogenic. VERITAS conducted a targeted follow-up campaign to search for very-high-energy (VHE, greater than 100 GeV) gamma-ray emission associated with KM3-230213A. Observations were performed in February and March 2025, using a four-point wobble strategy centered on the best-fit neutrino position, covering nearly the entire 90 percent confidence region. These observations probe potential hadronic gamma-ray emission from a common origin with the neutrino, placing constraints on particle-acceleration scenarios. We present the results of this search, including upper limits on very-high-energy gamma-ray flux and their implications for possible source models of KM3-230213A.

VERITAS Follow-Up Observations of the Ultra-High-Energy Neutrino Event KM3-230213A

TL;DR

VERITAS followed up the ultra-high-energy neutrino KM3-230213A with targeted VHE gamma-ray observations to search for a shared hadronic counterpart. The campaign covered the KM3NeT 90% localization with a wobble pointing strategy and amassed 17.6 hours of data, but found no significant TeV emission; the derived 99% CL upper limit above GeV is cm s, or of Crab Nebula flux. The non-detection constrains persistent TeV emitters and dampens cosmogenic-only explanations, while allowing a transient or beamed extragalactic origin within the region. The result underscores the importance of rapid, real-time multi-messenger follow-up and forecasts the improved capabilities of CTAO and IceCube-Gen2 for future UHE neutrino sources.

Abstract

The recent announcement of the detection of the ultra-high-energy (UHE) neutrino event KM3-230213A by the KM3NeT telescope represents a critical opportunity to explore the origins of cosmic neutrinos and their potential gamma-ray counterparts. With an inferred neutrino energy exceeding 100 PeV, this event stands as the most energetic neutrino observed to date. The large offset from the galactic plane (11 degrees) and the presence of several blazars with temporally correlated multiwavelength counterparts within the 3 degrees localization region raise the possibility of an extragalactic origin. Additionally, the event's apparent tension with IceCube constraints suggests that it could be transient in nature rather than cosmogenic. VERITAS conducted a targeted follow-up campaign to search for very-high-energy (VHE, greater than 100 GeV) gamma-ray emission associated with KM3-230213A. Observations were performed in February and March 2025, using a four-point wobble strategy centered on the best-fit neutrino position, covering nearly the entire 90 percent confidence region. These observations probe potential hadronic gamma-ray emission from a common origin with the neutrino, placing constraints on particle-acceleration scenarios. We present the results of this search, including upper limits on very-high-energy gamma-ray flux and their implications for possible source models of KM3-230213A.

Paper Structure

This paper contains 5 sections, 3 figures, 1 table.

Table of Contents

  1. Introduction and Motivation
  2. VERITAS Observations
  3. Results
  4. Discussion and Conclusion
  5. Acknowledgements

Figures (3)

  • Figure 1: Sky map of KM3-230213A showing the 90% containment region (green dotted circle), the VERITAS fields of view from observations conducted in wobble mode with 0.7$^\circ$ camera offsets (black dashed circles), and the positions of candidate blazars. Figure adapted from KM3NeT Coll. et al. (2025).
  • Figure 2: VERITAS significance map of the KM3-230213A field with the 90% containment region contained in a black dashed circle. Candidate sources are labeled and color-coded by their highest confirmed emission: optical (black triangles), X-ray (blue squares), and GeV gamma-ray (purple circles).
  • Figure 3: Swift-XRT 0.3–10 keV lightcurve of MRC 0614-083. The vertical dashed line marks the time of KM3-230213A. The shaded region shows the VERITAS observation window. Error bars represent 1$\sigma$ statistical uncertainties in count-rate.