Multi Messenger Study of GRB 221009A with VHE Gamma-ray and Neutrino Afterglow from a Gaussian Structured Jet

TL;DR

This work develops a Gaussian structured-jet external forward-shock model to explain the GeV–TeV afterglow of GRB 221009A and simultaneously estimate the accompanying $p\gamma$ neutrino flux. By fitting broadband VHE data with an off-axis structured jet in a uniform ISM, the authors extract best-fit geometric and microphysical parameters, demonstrating that $\epsilon_e\gg\epsilon_B$ yields SSC-dominated TeV emission and that mildly off-axis viewing can reproduce the observed light curves and SEDs. They compute neutrino fluxes for on-axis and off-axis geometries, incorporate flavor oscillations, and compare against projected sensitivities of IceCube Gen2 and GRAND200k, finding the single-burst flux well below detectability yet identifying parameter regimes that could yield measurable events with future instruments. The study highlights the crucial role of jet angular structure in multi-messenger GRB signals and suggests that only exceptionally bright and nearby bursts with high baryon loading are likely to produce detectable neutrinos, with CTA and next-generation neutrino detectors offering the most stringent tests of these models.

Abstract

Recent detections of very-high-energy (VHE; $\gtrsim 100$~GeV) emission from GRB afterglows, notably the unprecedented brightness of GRB~221009A observed by LHAASO, reveal emission components beyond the standard electron synchrotron model. Multi-TeV photons motivate synchrotron self-Compton and possibly hadronic contributions, while the non-detection of coincident neutrinos by IceCube/KM3NeT/GRAND200k constrains the microphysical parameters, jet kinetic energy and ambient medium density. We model the VHE afterglow of GRB 221009A with an external forward shock from a Gaussian structured jet in a uniform density medium. This angular structure reproduces the extreme TeV output at an off-axis angle but without demanding large energies as in a top-hat jet. We also compute the corresponding $pγ$ neutrino flux in the PeV-EeV energies and derive a time-integrated upper limit based on the effective areas of IceCube Gen2 and GRAND200k, providing the contributions of individual GRBs to the neutrino events. The predicted neutrino flux for GRB 221009A with model parameters inferred from multi-wavelength spectral energy distribution lies below the sensitivities of these detectors. Even under highly optimistic microphysical conditions, our correlation analysis infers that the events from this GRB are of order $\sim 0.1$ for upcoming GRAND200k. We also compare neutrino fluxes for on-axis and off-axis viewing geometries and find that jet orientation alone can introduce nearly an order of magnitude variation in the signal. Thus, our studies imply that a GRB both brighter and closer than GRB 221009A would be crucial for any neutrino detections by upcoming neutrino telescopes. Future GRB detections by the CTA will provide important constraints on their geometry, radiation mechanisms, and any potential associated neutrino signals.

Figures (10)

• Figure 1: Normalized angular profiles of jet energy per unit solid angle, $\varepsilon(\theta)/\varepsilon_c$ (black solid line), and initial velocity, $\Gamma_0(\theta)\beta_0(\theta)/\eta_c$ (red dashed line), for a Gaussian structured jet. For example, the jet profile is shown as a function of the polar angle $\theta$ for a jet core angle $\theta_c = 6^{\circ}$, jet half-opening angle $\theta_j = 25^{\circ}$, initial velocity $\eta_{c}=300$ and total kinetic energy $E_k = 1\times10^{53}~\mathrm{erg}$. Vertical dotted and dashed lines mark $\theta_c$ and $\theta_j$, respectively.
• Figure 2: Corner plot showing posterior distributions of afterglow model parameters for GRB 221009A. All the contours enclosing median values and $\pm1 \sigma$ uncertainties. Thin reference lines denote the truth values.
• Figure 3: Broadband SEDs of GRB 221009A at three time intervals derived from the structured-jet afterglow model. Top: $T^{*}+[22,100]$ s; middle: $T^{*}+[100,674]$ s; bottom: $T^{*}+[674,1774]$ s, where $T^{*}=T_{0}+226$ s. Red diamonds denote AGILE–GRID GeV data, orange circles show LHAASO intrinsic TeV fluxes, and blue squares include EBL attenuation. The solid purple curves represent the intrinsic model spectra, with the low-energy hump from synchrotron emission and the high-energy hump from SSC radiation. The dashed grey lines show spectra corrected for EBL attenuation, while dash-dotted black lines indicate the total synchrotron + SSC emission.
• Figure 4: Afterglow light curves of GRB 221009A in the GeV and TeV bands modeled with the Gaussian structured-jet framework. The orange curve represents the TeV LC, fitted with LHAASO TeV data for 0.3–5 TeV, and the blue curve shows the GeV LC fit to AGILE–GRID GeV data of 50 MeV–3 GeV. Purple circles correspond to the observed LHAASO TeV data, while dark-red squares denote AGILE–GRID GeV data (scaled by $\times 10^{-1}$).
• Figure 5: Neutrino flux for a GRB at $z=0.151$ in two viewing geometries. The upper (purple) band corresponds to the on-axis case ($\theta_{v}<\theta_{c}$; here $\theta_{c}=6^{\circ}$, $\theta_{v}=3^{\circ}$), while the lower (yellow) band shows the off-axis case ($\theta_{v}>\theta_{c}$; $\theta_{v}=13^{\circ}$). Within each band, all three neutrino flavor fluxes at Earth after oscillation are plotted for the three observer time epochs $t > t_{dec} \in \{30,\,300,\,3000\}\,\mathrm{s}$: $\nu_{\mu}+\bar{\nu}_{\mu}$ (dash–dotted curve), $\nu_{e}+\bar{\nu}_{e}$ (dashed curve), and $\nu_{\tau}+\bar{\nu}_{\tau}$ (solid curve). Model parameters: $E_{k}=3.1\times10^{53}\,\mathrm{erg}$ (implying $E_{k,\mathrm{iso}}^{\rm on}\sim 1\times10^{56}\,\mathrm{erg}$ and $E_{k,\mathrm{iso}}^{\rm off}\sim 1\times10^{55}\,\mathrm{erg}$), $\epsilon_{e}=\epsilon_{B}=0.1$, $\epsilon_{p}=1.0$, $k=2.5$.