Neutrino Emission from Gamma-ray Burst Jet Propagating inside the Cavity within Active Galactic Nucleus Accretion Disks

Short gamma-ray bursts (sGRBs) from the merger of binary compact objects (BCOs) could occur in the accretion disks of the active galactic nucleus (AGN). Before merging, the BCO will inevitably form a low-density cavity. The sGRB jet will interact with the AGN disk photons during its propagation through the cavity, leading to unique electromagnetic and neutrino signatures. In this work, we investigate the influence of the AGN disk photon field on neutrino emission within the internal dissipation regions of a two-component sGRB jet (a narrow core and a wide wing). We find that, due to the strong AGN disk photon field, the neutrino flux at high-energy part (e.g., PeV to EeV) will be suppressed, while the relatively lower-energy part (e.g., TeV to PeV) will be enhanced. Such a conclusion can enhance the constraints on GRB parameters (e.g., baryonic loading factor and bulk Lorentz factor) based on the future detection or non-detection of high-energy neutrinos from GRBs. Besides, the two-component jet can display two-bump structure at higher and lower energy in the neutrino spectrum. Therefore, the joint observations of electromagnetic and neutrinos emission can help us identify the sGRB jet and its structure in the AGN disk.

💡 Research Summary

The manuscript investigates the neutrino emission from short gamma‑ray burst (sGRB) jets that are launched inside the accretion disk of an active galactic nucleus (AGN). Prior to the merger of a binary compact object (BCO), strong winds from the binary carve a low‑density cavity in the AGN disk. The authors assume that the sGRB jet propagates through this cavity and interacts with the intense photon field of the surrounding AGN disk. They model the jet as a two‑component (core‑wing) structure, with a fast, narrow core and a slower, wider sheath, each characterized by its own bulk Lorentz factor, isotropic kinetic luminosity, and opening angle.

The paper first describes two AGN disk models (Sirko & Goodman 2003, Thompson et al. 2005) and computes the radial profiles of effective temperature and scale height for three super‑massive black‑hole masses (10⁶, 10⁷, 10⁸ M⊙). The local photon field is approximated by a black‑body spectrum whose temperature follows the disk profile. Within the jet’s internal dissipation region (at radius r_diss) the authors adopt the empirical Band function for the jet’s own photon spectrum and solve the steady‑state electron continuity equation, including adiabatic, synchrotron, synchrotron‑self‑Compton (SSC), and external inverse‑Compton (EIC) cooling. The EIC component explicitly accounts for scattering of AGN disk photons by jet electrons.

Protons are assumed to follow a power‑law distribution (∝ E⁻²) with a baryonic loading factor ε_p. Their maximum energy is set by balancing acceleration (t′_acc = E′/(e c B′)) against cooling, which includes dynamical escape, synchrotron, and photomeson (pγ) losses. The photomeson interaction rate is computed by integrating over the combined photon field (Band + scattered disk photons) and includes the full cross‑section and inelasticity as a function of the photon energy in the proton rest frame. The authors also evaluate γγ opacity for high‑energy photons using the AGN disk photon density.

Key results: (1) The intense AGN disk photon field dramatically increases the cooling of both electrons (via EIC) and protons (via pγ), but the effect is energy‑dependent. For ultra‑high‑energy protons (producing PeV–EeV neutrinos) the target photon density at the required center‑of‑mass energy is reduced by γγ attenuation of the high‑energy jet photons, leading to a suppression of the neutrino flux in the PeV–EeV band. (2) Conversely, for lower‑energy protons (producing TeV–PeV neutrinos) the abundant soft disk photons provide an efficient pγ target, enhancing the neutrino flux in this band. (3) Because the core and wing have different Lorentz factors and luminosities, each component generates its own neutrino “bump”: the core dominates the suppressed high‑energy region, while the wing yields an enhanced intermediate‑energy bump. The combined spectrum therefore exhibits a characteristic two‑bump structure. (4) Parameter scans show that larger SMBH masses (hence hotter, thicker disks) increase the suppression of the high‑energy neutrinos, whereas smaller SMBHs or thinner disks allow a higher PeV flux.

The authors discuss observational implications for IceCube, IceCube‑Gen2, KM3NeT, and Baikal‑GVD. Detection of a two‑bump neutrino spectrum coincident with an sGRB located in an AGN disk would provide strong evidence for (i) a structured jet, (ii) the presence of a low‑density cavity, and (iii) the role of AGN disk photons in shaping high‑energy particle emission. Non‑detection, especially of the high‑energy component, would tighten constraints on the baryonic loading factor and bulk Lorentz factor of the jet.

In summary, the paper presents the first comprehensive quantitative treatment of how AGN disk photon fields modify neutrino production inside sGRB jets, highlighting a novel spectral signature (high‑energy suppression plus intermediate‑energy enhancement) that can be probed with upcoming multimessenger facilities.