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High-energy neutrinos from shocked circumnuclear material around optically-bright and infrared-only tidal disruption events

This paper proposes that infrared-only tidal disruption events, occurring in dense, obscured nuclear environments, can efficiently accelerate cosmic rays and produce high-energy neutrinos through radiative compression, potentially contributing significantly to the diffuse neutrino flux and becoming detectable via joint stacking searches with future multi-km³ neutrino observatories.

Original authors: Mainak Mukhopadhyay, Shigeo S. Kimura, Tatsuya Matsumoto

Published 2026-10-02

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CC BY 4.0

Original authors: Mainak Mukhopadhyay, Shigeo S. Kimura, Tatsuya Matsumoto

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the quiet centers of galaxies, where supermassive black holes lie dormant, a violent drama can unfold when a wandering star strays too close. The black hole's immense gravity tears the star apart, stretching it into a stream of gas that swirls around the hole before being swallowed. This event, known as a tidal disruption event, creates a brilliant flash of light that astronomers can see across the universe. For years, scientists have watched these flashes in visible light and ultraviolet, but a newer class of these events has been discovered that shines brightly only in infrared, the heat radiation we cannot see with our eyes. These hidden events suggest the star was torn apart inside a thick, dusty cloud that blocks the visible light but glows warmly as it absorbs the energy. While these cosmic collisions are famous for their light, a new study asks a different question: do they also fire off invisible, high-energy particles that can travel across the universe and be detected here on Earth?

The researchers focused on the aftermath of these stellar deaths, specifically the powerful winds of gas that the black hole launches as it devours the debris. As this wind rushes outward, it slams into the surrounding gas and dust that naturally fills the space around the black hole. This collision creates a shock wave, a boundary where the fast-moving wind crashes into the stationary material. In this chaotic zone, protons—tiny particles that make up the core of atoms—can be accelerated to incredible speeds, becoming cosmic rays. When these speeding protons crash into other protons in the surrounding gas, they create a cascade of new particles, including pions, which quickly decay into high-energy neutrinos. Neutrinos are ghostly particles that rarely interact with anything, allowing them to escape the dense environment of the galaxy and travel in a straight line to Earth, carrying a direct message about the violence that created them.

The team built a detailed model to simulate how these shocks behave in two different types of environments: the clearer regions where optical tidal disruption events occur, and the dense, dusty regions where the infrared-only events happen. They found that in the clearer environments, the shock wave expands quickly but runs out of fuel before it can produce many neutrinos. The density of the target gas is too low, so most of the speeding protons escape without ever hitting anything. However, the story changes dramatically in the dusty, infrared-only environments. Here, the surrounding material is packed much more tightly. As the shock wave moves through this dense gas, it heats up and then cools down rapidly, causing the gas to compress even further. This compression acts like a magnifying glass for the collisions, packing the target protons so tightly that the speeding cosmic rays are almost guaranteed to hit them.

In these dense, dusty scenarios, the efficiency of neutrino production jumps significantly. The model suggests that the process allows the interaction efficiency to approach the calorimetric regime, meaning the source becomes highly effective at converting energy into neutrinos. The researchers calculated that if these infrared-only events happen frequently enough, they could account for a noticeable slice of the diffuse background of high-energy neutrinos that detectors like IceCube have been observing for years. Specifically, they estimate that these hidden events could contribute up to approximately 10% of the observed diffuse neutrino flux at energies around 100 tera-electronvolts, a level that is significant enough to be tested.

Despite this promising potential, the study also highlights why finding these signals from a single event is nearly impossible. The neutrino emission from any one tidal disruption event is faint and spreads out over many years, making it too weak to stand out against the background noise of other cosmic rays. Instead, the researchers propose a strategy of "stacking," where astronomers combine data from many known events to look for a collective signal. They simulated how future neutrino telescopes, including a proposed massive detector called HUNT, would perform this search. Their results indicate that by focusing on nearby events and looking over several years, a next-generation observatory could reach a level of sensitivity where it could confirm or rule out this theory with high confidence.

The paper also carefully considered other ways these events might produce neutrinos, such as protons colliding with light particles from the star's own radiation. They found that for the slower, non-relativistic winds typical of these events, the protons do not reach the high speeds needed to trigger these light-based collisions efficiently. This rules out the idea that the light from the event itself is the primary driver of neutrino production, reinforcing the conclusion that the collision with the surrounding gas is the key mechanism. The study concludes that while the bright, visible flashes of tidal disruption events are inefficient neutrino factories, the hidden, dusty ones are likely powerful sources. As new telescopes come online to map the sky in both visible and infrared light, we will soon have the catalogs needed to test this idea and determine if the universe's most obscured explosions are indeed the source of some of its most energetic particles.

Technical Summary: High-energy neutrinos from shocked circumnuclear material around optically-bright and infrared-only tidal disruption events

Problem Statement
Tidal disruption events (TDEs) are known to launch sub-relativistic outflows that interact with the surrounding circumnuclear material (CNM), driving shocks capable of accelerating cosmic rays (CRs). While previous studies have explored neutrino production in TDEs via relativistic jets or hidden winds, the potential for high-energy neutrino emission from non-relativistic outflows interacting with dense CNM remains underexplored. Recent observations have identified a population of luminous infrared (IR) transients with weak or absent optical counterparts, interpreted as TDEs embedded in dusty, obscured nuclear environments. These environments are expected to possess significantly higher gas densities than those surrounding optically bright TDEs. The central problem addressed is whether these dense, obscured environments can serve as efficient hadronuclear ($pp$) neutrino sources and how their contribution compares to that of optically bright TDEs in the context of the diffuse astrophysical neutrino flux.

Methodology
The authors develop a semi-analytical model to compute shock dynamics, CR acceleration, and neutrino production for two distinct TDE populations: optically bright and IR-only.

Shock Dynamics: The model treats the interaction between a TDE-driven outflow and the CNM as a forward shock propagating into the ambient medium. The CNM density profile is parameterized as a power law in the inner region transitioning to a constant density at larger radii. For IR-only TDEs, the outer constant density ( $n_{env}$ ) is set to $10^6 \text{ cm}^{-3}$ , significantly higher than the $100 \text{ cm}^{-3}$ adopted for optical TDEs.
Radiative Cooling: A critical component of the methodology is the treatment of radiative cooling in the dense IR-only environment. The authors calculate the free-free cooling timescale ( $t_{ff}$ ) versus the dynamical timescale ( $t_{dyn}$ ). When $t_{ff} < t_{dyn}$ , the shock transitions to a radiative regime. The model accounts for the resulting compression of the shocked gas downstream, enhancing the target density for $pp$ interactions while simultaneously modifying the shock deceleration and maximum proton energy.
CR Acceleration and Neutrino Production: The model assumes diffusive shock acceleration (Bohm diffusion) and calculates the maximum proton energy ( $\varepsilon_p^{max}$ ) limited by escape, synchrotron cooling, and the finite duration of the acceleration environment (cooling time). The steady-state proton distribution is used to compute the neutrino spectra from $pp$ interactions, distinguishing between the immediate post-shock density (for acceleration constraints) and the radiatively compressed density (for interaction targets).
Diffuse Flux and Stacking: The authors integrate the single-source neutrino yields over cosmological time to estimate the contribution to the all-sky diffuse flux, considering both "negative evolution" and "star-formation-rate (SFR) tracing" scenarios for the TDE rate density. They also forecast detection prospects using an electromagnetically informed stacking strategy for current and future neutrino observatories (IceCube, IceCube-Gen2, KM3NeT, and HUNT), optimizing for source redshift ( $z_{max}$ ) and search window duration ( $\delta t_{search}$ ).

Key Contributions and Results

Efficiency of Neutrino Production: The study finds a stark contrast between the two populations. Optically bright TDEs remain optically thin to $pp$ interactions throughout their evolution, resulting in low neutrino yields. In contrast, the dense environments of IR-only TDEs allow the $pp$ interaction efficiency to approach the calorimetric regime (efficiency $\sim 1$ ), particularly after the onset of radiative cooling which compresses the target material.
Diffuse Flux Contribution: Despite adopting a local occurrence rate for IR-only TDEs that is a factor of three lower than that of optical TDEs, the IR-only population dominates the predicted diffuse neutrino flux due to their higher per-event yield. The authors estimate that IR-only TDEs could contribute approximately 1% to 5% of the observed diffuse neutrino intensity at 100 TeV, potentially reaching $\sim 10\%$ when accounting for rate uncertainties and strong population evolution. Optical TDEs contribute negligibly ( $< 0.03\%$ ).
Spectral Evolution: In the IR-only model, the transition to the radiative regime leads to rapid shock deceleration, causing the maximum proton energy to decrease sharply at late times ( $\varepsilon_p^{max} \propto t^{-3}$ ). This shifts the high-energy cutoff of the neutrino spectrum to lower energies, even as the total interaction efficiency increases.
Detection Prospects: Individual IR-only TDEs are unlikely to be detected as point sources due to low event counts. However, the authors demonstrate that stacking searches using EM-selected catalogs are viable. They forecast that a future $\sim 30 \text{ km}^3$ neutrino observatory (e.g., HUNT) could achieve a $3\sigma$ detection significance for IR-only TDEs within a redshift of $z \lesssim 0.4$ using multi-year search windows. Current detectors (IceCube, KM3NeT) remain below $3\sigma$ significance for the fiducial parameters.
Photohadronic Interactions: The authors evaluate and rule out photohadronic ( $p\gamma$ ) interactions as a dominant channel. They find that the maximum proton energies achieved in their sub-relativistic shock model are generally insufficient to overcome the threshold for $\Delta$ -resonance production with characteristic IR dust-reprocessed photons or primary optical/UV photons.

Significance
The paper claims that the outflow-CNM interaction scenario provides a concrete, conservative target for high-energy neutrino searches that does not rely on the rare occurrence of relativistic jets or specific viewing angles. The identification of a population of IR-only TDEs in dense, obscured environments offers a new pathway to explain a fraction of the diffuse astrophysical neutrino flux. The work emphasizes that the rapidly growing samples of TDEs from wide-field optical and IR surveys (such as LSST, WFST, Roman, and SPHEREx) will enable population-resolved stacking searches. These searches can test the hypothesis that dense, obscured nuclear environments are efficient high-energy neutrino factories, potentially linking the physical conditions of TDE circumnuclear environments to their high-energy neutrino signatures. The authors note that their results are optimistic benchmarks dependent on specific density profiles and that detailed hydrodynamical treatments of the radiative shock structure and CR transport are required for future refinement.

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