GeV-TeV Neutrino Counterparts

• GeV–TeV neutrino counterparts are astrophysical emissions in the 1 GeV to multi-TeV range produced through hadronic interactions that also generate high-energy gamma rays.
• Detection strategies using IceCube and its sub-arrays employ advanced event reconstruction and real-time alert systems to isolate transient neutrino signals.
• Joint multimessenger analyses integrate gamma-ray and neutrino data to constrain cosmic-ray acceleration mechanisms and refine models of hadronic processes in diverse sources.

A GeV–TeV neutrino counterpart refers to the manifestation of astrophysical neutrino emission in the 1 GeV–multi-TeV energy range, typically correlated with high-energy gamma-ray or cosmic-ray activity in astrophysical sources. The identification and characterization of these counterparts is central to the emerging field of multimessenger astrophysics, enabling constraints on cosmic-ray acceleration mechanisms, hadronic processes, and source populations spanning Galactic and extragalactic environments.

1. Physical Production Mechanisms

High- and very-high-energy (VHE) neutrinos are predominantly generated via the decay of charged pions and kaons resulting from hadronic interactions—either proton–proton ($p$–$p$) or proton–photon ($p$–$\gamma$)—in energetic astrophysical environments. The coincident decay of neutral pions yields high-energy gamma rays, establishing a direct correlation between neutrino and gamma-ray emission.

Notable channels include:

• $$p + p \rightarrow \pi^{\pm,0} + X \rightarrow (\mu\,\nu,\,\gamma\gamma,...)$$
• $$p + \gamma \rightarrow \Delta^+ \rightarrow n + \pi^+$$ (Greisen–Zatsepin–Kuzmin interactions in $p$–$\gamma$ systems)

The correspondence between leptonic (neutrino) and electromagnetic (photon) secondaries is foundational: equal energy partition in gamma rays and neutrinos is often assumed for first-order estimates, modulo oscillation and branching corrections (Abbasi et al., 2022, Neronov et al., 2018). This underpinning motivates the search for neutrino counterparts in the same energy domain as observed gamma-ray emission.

2. Detection Strategies and Instrumental Capabilities

The IceCube Neutrino Observatory has pioneered detection in this regime, with the DeepCore sub-array providing sensitivity down to $\sim$10 GeV. Track-based and cascade-based event selections—utilizing both containment and directional vetoes—allow for background suppression and enable point-source and transient searches.

Key technical parameters include:

• Effective area $A_{\mathrm{eff}}(E)$, rising from $\sim 10^{-3}$ m² at 5 GeV to $\sim 1$–$10^2$ m² in the TeV band depending on energy and event class (Larson et al., 2021, Abbasi et al., 2022, Collaboration et al., 2019).
• Angular resolution: $\sim$30–40° at $\mathcal{O}$(10 GeV), improving to sub-degree at $\gtrsim$TeV energies (Larson et al., 2021, Collaboration et al., 2019).
• Energy resolution: 30–50% in $\log_{10}E$ (Larson et al., 2021).
• Real-time analysis pipelines and alert generation enable coordinated multimessenger follow-up (Larson et al., 2021, Solares, 2019).

Observational stacking (across transients such as novae, GRBs, or blazar flares) improves sensitivity, particularly as instrumental exposure increases and new data streams become available (Abbasi et al., 2022, Larson et al., 2021).

3. Empirical Results: Limits and Populations

Multiple classes of astrophysical sources—novae, GRBs, blazars, and star-forming galaxies—have been targeted for GeV–TeV neutrino counterpart searches.

Novae

• Stacking and individual event analyses with IceCube-DeepCore set 90% CL upper limits on time-integrated $E^2F_{\nu}(1\,\text{TeV})$ in the range $\sim 4$–$200$ GeV cm⁻², exceeding even optimistic hadronic shock model predictions by more than an order of magnitude (Abbasi et al., 2022).

GRBs

• Searches for neutrino emission coincident with GRB 221009A reported 90% CL differential upper limits on $E^2\Phi_{\nu}$ (e.g., $<0.02$ GeV cm⁻² in the 100–300 GeV bin), with no excess found. These limits rule out baryon loading parameters $\xi_p \gtrsim 60$ for moderate bulk Lorentz factors ($\Gamma \lesssim 780$) in fireball models (Kruiswijk et al., 2023).
• Time-variable neutron-loaded jet simulations predict a spectral peak at $E_\nu \sim 10$–30 GeV with high-energy tails into the TeV regime for strong Lorentz factor modulations. The neutrino radiative efficiency $\eta_\nu$ reaches $\sim$0.1–10% for gamma-bright GRBs and up to $\sim$20% for X-ray-rich, photon-poor jets (Nakama et al., 11 Dec 2025).

Blazars and Extragalactic Sources

• A 10-year Fermi-LAT study finds that blazar duty cycles are $\sim$23%; time-averaged neutrino flux predictions must be weighted accordingly. Individual blazars are rarely observable in the TeV neutrino band except during bright flares (Sacahui et al., 2020).
• For the 2014–2015 IceCube neutrino flare near TXS 0506+056, coincident GeV flaring from PKS 0502+049 suggests possible joint contribution, with expected $$E_\nu^2 dN_{\nu_\mu}/dE_\nu \sim 2.2 \times 10^{-8}$$ GeV cm⁻² s⁻¹ at 100 TeV, enough to explain several events (Liang et al., 2018).

Diffuse and Galactic Contribution

• Spatial and spectral correlation studies using Fermi-LAT and IceCube data demonstrate close matching of the multi-TeV $\gamma$ and $\nu$ fluxes at high Galactic latitudes, supporting a hadronic Galactic origin for a significant fraction of the IceCube “diffuse” neutrino flux (Neronov et al., 2018).

4. Joint Multimessenger Analysis Frameworks

Dedicated joint-likelihood analyses (e.g. AMON) leverage sub-threshold TeV $\gamma$-ray hotspots (HAWC) and neutrino tracks (IceCube) (Solares, 2019). Salient features include:

• Combined spatial and temporal likelihood ranking, sub-degree localization ($\sim$0.2°), and controlled false-alarm rates (FAR).
• Cross-messenger efficiency and alert generation with $\sim$6 h latency enable rapid follow-ups and enhance the probability of identifying true associations, expected at the $\mathcal{O}$(1) yr⁻¹ level for bright transients.

Multimessenger synergies, such as prompt GCN circulars from $\mathcal{O}$(10–1000 GeV) neutrino clusters, facilitate cross-wavelength campaigns that can test hadronic vs. leptonic emission scenarios and probe transient engine physics (Larson et al., 2021, Solares, 2019).

5. Constraints on Astrophysical Models and Source Physics

Current null results in the GeV–TeV band place stringent upper limits on hadronic contributions to gamma-ray and neutrino emission in Galactic and extragalactic transients. Examples:

• In novae, the non-observation of neutrinos implies relativistic hadron energy fractions $\ll 1$ in radiative shocks (Larson et al., 2021, Abbasi et al., 2022).
• For GRB jets, IceCube limits rule out the highest nucleon-loading/quasi-thermal emission models and cap baryon loading in fireball scenarios below $\xi_p \sim 60$ for standard $\Gamma$ (Kruiswijk et al., 2023).
• For blazars, the aggregate neutrino flux per source is suppressed by the duty cycle, with individual-source detections during quiescence highly unlikely (Sacahui et al., 2020).

A plausible implication is that many candidate sources produce neutrino spectra with breaks or cutoffs between $\sim$10 GeV and 1 TeV, rather than unbroken power laws, limiting concurrent GeV and PeV emission (Raab et al., 11 Jul 2025).

6. Future Prospects and Directions

Planned and ongoing detector upgrades (IceCube-Upgrade, IceCube-Gen2, KM3NeT/ORCA) will improve effective area and angular resolution by factors of a few at sub-TeV energies, allowing stacking analyses across longer timescales and more sensitive targeted searches (Abbasi et al., 2022, Kruiswijk et al., 2023).

Improved low-energy event reconstruction, multi-messenger follow-up coordination, and expanded catalogs of transient and persistent sources will continue to strengthen constraints on GeV–TeV neutrino counterparts and maximize discovery potential in the multimessenger paradigm.