Characterizing Candidate Blazar Counterparts of the Ultra-High-Energy Event KM3-230213A

TL;DR

This work investigates a possible blazar origin for the ultra-high-energy neutrino KM3-230213A by assembling the most complete multiwavelength dataset to date within the 99% localization region. It combines fresh VLBI radio imaging, long-term radio light curves, infrared/optical surveys, X-ray monitoring (ROSAT, eROSITA, Swift, Chandra), and Fermi-LAT gamma-ray analysis to identify 17 candidate blazars and characterize their variability. Three sources show notable temporal activity near the neutrino time (a radio flare in PMN J0606-0724 with a pre-trial p-value of 0.26%, a gamma-ray flare in 0605-085, and X-ray activity in MRC 0614-083), but none provide conclusive evidence for association; the paper discusses the implications for blazar-based neutrino production and the importance of improved localization. Overall, the study demonstrates the power and necessity of comprehensive, contemporaneous multiwavelength baselines to refine counterpart candidates for future high-energy neutrino events and to test hadronic emission scenarios in blazars.

Abstract

High-energy astrophysical neutrinos serve as crucial messengers for understanding hadronic acceleration processes and identifying the origins of cosmic rays, with blazars among the most promising neutrino sources. The KM3NeT experiment reported the detection of an ultra-high-energy neutrino with an energy estimate of ~ 220 PeV, the most energetic yet observed. The neutrino arrival direction has a 99% confidence region of 3° radius centered at RA 94.3°, Dec -7.8° (J2000). In this work, seventeen candidate blazars located within this region are identified. Comprehensive new observations and archival data analysis for these sources are presented. The study provides a complete multiwavelength coverage across radio, optical, X-ray, and gamma-ray bands, including proprietary data and dedicated follow-up observations. This study highlights flaring behavior in several candidate counterparts. One object exhibits a radio flare coinciding with the neutrino arrival time, with a pre-trial chance probability of 0.26%. Another candidate displays a rising trend in X-ray flux in a one-year window around the neutrino arrival time, while a third undergoes a gamma-ray flare during the same period. Based on the observational findings here presented, while none of these candidates can conclusively be linked to the neutrino, the implications of a possible blazar origin for the KM3NeT event are discussed.

Figures (25)

• Figure 1: 3$\,\sigma$ upper limits derived from the tiling of the neutrino error region with SVOM-MXT over the 0.5--10 keV energy band. The concentric red circles indicate the 90% and 99% error regions of KM3-230213A, respectively. The numbers refer to the candidate blazars listed in \ref{['tab:candidate-blazars']}.
• Figure 1: Likely blazar candidates within the KM3-230213A 99% localization area, based on their parsec-scale radio emission properties. See \ref{['tab:candidate-blazars']} and \ref{['s:vlbi_sources']} for details. Two blazars are labeled in the plot: PMN J0606-0724 exhibits the strongest temporal correlation of radio flares to the neutrino arrival (analyzed in \ref{['s:flaretest']}); 0605-085 is the brightest in terms of parsec-scale emission.
• Figure 1: The KM3-230213A localization in the sky (68%, 90% and 99% containment areas shaded in cyan) together with the RFC bright (above 150mJy) sources (\ref{['s:vlbi_sources']}), displayed along with the sources monitored by the OVRO program (\ref{['s:ovro_data']}). The direction of the event is close to the boundary of the OVRO monitoring area, $\pm \qty{10}{\degree}$ from the Galactic Plane (dashed black line). All bright radio blazars in this region are in the CGRaBS sample and have radio light curves available.
• Figure 1: Multi-wavelength light curves of MRC 0614-083 (#1). The panels display from top to bottom: Fermi-LAT gamma-ray light curves integrated over a 6-months time bin; X-ray data from Swift-XRT and eROSITA; optical data from CRTS, ZTF, ATLAS and Gaia; IR data from WISE/NEOWISE. The cyan stripe highlights the arrival time of KM3-230213A.
• Figure 2: SVOM-ECLAIRs $3\,\sigma$ upper limits on the energy flux in different energy ranges for the candidate blazars listed in \ref{['tab:candidate-blazars']}. The average upper limits for the seventeen targets are shown with red markers. The horizontal hatched bands cover the interval between the minimum and maximum upper limits for the candidate sample.