[Paper Review] High-energy Neutrinos from Stellar Explosions in Active Galactic Nuclei Accretion Disks

Jin-Ping Zhu, Kai Wang+1 more|arXiv (Cornell University)|Jul 13, 2021

Astrophysics and Cosmic Phenomena86 references26 citations

TL;DR

This paper proposes that high-energy neutrinos are efficiently produced when ejecta from stellar explosions in active galactic nucleus (AGN) accretion disks shock-heat and interact with dense disk material. Using shock dynamics and particle acceleration models, the authors show these events could contribute up to ~10% of the observed diffuse high-energy neutrino background, making AGN stellar explosions promising multimessenger sources.

ABSTRACT

Some catastrophic stellar explosions, such as supernovae (SNe), compact binary coalescences, and micro-tidal disruption events, are believed to be embedded in the accretion disks of active galactic nuclei (AGN). We show high-energy neutrinos can be produced efficiently through $pp$-interactions between shock-accelerated cosmic rays and AGN disk materials shortly after the explosion ejecta shock breaks out of the disk. AGN stellar explosions are ideal targets for joint neutrino and electromagnetic (EM) multimessenger observations. Future EM follow-up observations of neutrino bursts can help us search for yet-discovered AGN stellar explosions. We suggest that AGN stellar explosions could potentially be important astrophysical neutrino sources. The contribution from AGN stellar explosions to the observed diffuse neutrino background depends on the uncertain local event rate densities of these events in AGN disks. By considering thermonuclear SNe, core-collaspe SNe, gamma-ray burst associated SNe, kilonovae, and choked GRBs in AGN disks with known theoretical local event rate densities, we show that these events may contribute to $\lesssim10\%$ of the observed diffuse neutrino background.

Motivation & Objective

• To investigate the potential of AGN stellar explosions as sources of high-energy neutrinos.
• To model neutrino production mechanisms in AGN disk environments, particularly via shock-accelerated cosmic rays.
• To estimate the contribution of various AGN stellar explosion types to the observed diffuse high-energy neutrino background.
• To assess the feasibility of joint neutrino and electromagnetic multimessenger observations of these events.

Proposed method

• Modeling the radial density and scale height of AGN accretion disks using the Sirko & Goodman (2003) disk structure, with a Gaussian vertical density profile.
• Simulating shock dynamics using the velocity law vs(h) ≈ (E₀ / (Mₑⱼ + M(h)))¹ᐟ² × (ρd(h)/ρ₀)⁻ᵘ, where μ ≈ 0.19, to track shock propagation above the disk mid-plane.
• Calculating shock kinetic luminosity as Lₛ = 2πρd vₛ³ h² and breakout time via t(h) = ∫₀ʰ dh / vs(h).
• Applying Fermi acceleration with a power-law spectrum dnp/dϵₚ ∝ ϵₚ⁻ᑫ (q ≈ 2) and estimating maximum proton energy via competition between pp inelastic reactions and adiabatic cooling.
• Estimating neutrino fluence using the diffuse flux integral E²Φν = (c / 4πH₀) ∫₀ᶻᵐᵃˣ R₀f(z)ϵ²νφν(ϵν) / (1 + z)²√[Ωₘ(1 + z)³ + Ωₗ] dz, with cosmological parameters from Planck.
• Simulating 1×10⁶ events per explosion type to estimate detection rates and diffuse flux contributions under various redshift evolution models.

Experimental results

Research questions

• RQ1 Can high-energy neutrinos be efficiently produced in AGN accretion disks via interactions between shock-accelerated cosmic rays and disk material?
• RQ2 What is the expected neutrino fluence and duration for different types of AGN stellar explosions (e.g., SNe Ia, CCSNe, kilonovae, GRBs)?
• RQ3 How do the predicted neutrino fluxes from AGN stellar explosions compare to the observed diffuse high-energy neutrino background?
• RQ4 What role do disk winds, BLR clouds, or cavities in the disk play in prolonging neutrino emission and enhancing flux?

Key findings

• High-energy neutrinos are efficiently produced via pp interactions between shock-accelerated cosmic rays and dense AGN disk material, with maximum proton energies reaching ~94 TeV (for pp-limited case) and ~1.7×10⁵ TeV (for adiabatic-limited case).
• The maximum neutrino energy is limited by the competition between proton acceleration and cooling via pp inelastic reactions or adiabatic losses, with the latter yielding higher energy limits at larger shock heights.
• For a 10⁷ M⊙ SMBH at r = 10³ rS, the maximum neutrino energy is ≈9.4×10³ TeV, while at r = 10⁴ rS it reaches ≈1.7×10³ TeV, depending on disk density and shock velocity.
• The diffuse neutrino fluence from AGN stellar explosions is estimated to contribute ≤10% of the observed astrophysical diffuse neutrino background, with SNe Ia and LGRBs being the dominant contributors among the studied types.
• Detection rate simulations show that SNe Ia, LGRBs, and CCSNe could produce up to ~0.2, ~0.2, and ~0.03 detections per year, respectively, in IceCube-like detectors.
• The presence of disk winds, BLR clouds, or low-density cavities could enhance neutrino fluence and extend emission duration, potentially increasing the total diffuse contribution beyond current estimates.