Multi-Wavelength Afterglows as Diagnostic Probes of Dense Circumburst Medium in GRBs

TL;DR

This work explores how GRBs embedded in dense circumburst environments, particularly AGN disks, imprint distinctive multi-wavelength afterglow signatures. By combining analytical treatments of critical frequencies with numerical jet simulations that self-consistently evolve the electron distribution and include SSC, SSA, and pair-absorption, it shows that dense media drive early SSC-dominated X-rays, attenuate high-energy photons via $\gamma\gamma$ absorption to roughly $\lesssim$10 GeV, and create jet-break patterns that depend on the jet opening angle. A case study of GRB 191019A demonstrates that a dense environment with $n \sim 5\times10^4$ cm$^{-3}$ can reproduce observed X-ray/optical trends, supporting the AGN-disk scenario though alternative explanations remain viable. These results provide concrete observational diagnostics for identifying GRBs in dense environments and guide future multi-wavelength follow-up campaigns.

Abstract

Gamma-ray bursts (GRBs) are generally believed to occur in environments where the surrounding medium is either a uniform interstellar medium (ISM) or, in some cases, a dense stellar wind from a massive progenitor. Recently, GRB 191019A has been proposed to originate within the accretion disk of an active galactic nucleus (AGN), suggesting that some GRBs may occur in extremely dense environments, although this interpretation remains under debate. This scenario has drawn considerable attention, as AGN disks are promising sites that can host progenitors of both long and short GRBs, and whose dense, gas-rich environment could significantly influence jet propagation and afterglow emission. Yet, our theoretical understanding of the resulting afterglow signatures in such environments is limited, and further systematic exploration is required. In this study, we investigate how multi-wavelength afterglow light curves can be utilized as diagnostic tools to probe the nature of the circumburst environment. Our results show that in dense environments, GRB afterglows exhibit distinct frequency-dependent behaviors. For jets with large opening angles, the X-ray light curve displays a shallow decay or bump due to a transition from synchrotron to SSC dominance, while the optical and high-energy (GeV) light curves follow typical power-law decays. On the other hand, for small opening angles, the light curves exhibit wavelength-dependent jet breaks: the GeV and optical bands break simultaneously, while the X-ray break is delayed as the SSC component gradually compensates for the fading synchrotron component. These signatures provide potential diagnostics of GRBs occurring in dense media such as AGN disks.

Figures (6)

• Figure 1: Temporal evolution and density dependence of the synchrotron critical frequencies. Upper row: frequency evolution with time observed $T$ for different ambient densities. The secondary y-axis on the right shows the evolution of the bulk Lorentz factor $\gamma$, indicated by purple dashed lines. Note that the analytical evolution of the synchrotron critical frequencies is only valid for $\gamma\gg1$ and breaks down when $\gamma$ approaches unity. Lower row: frequency dependence of density n at different times. The adopted parameters include $E_{\rm iso}=1\times 10^{51}$ erg, $\epsilon_B=0.001$, $\epsilon_e=0.1$, $p=2.3$.
• Figure 2: Analytical afterglow light curves in the X-ray and GeV bands, including the synchrotron component and the SSC component.
• Figure 3: Evolution of the maximum photon energy from the GRB afterlow emission. The solid line and the dashed line are for $E_{\rm iso}$=10$^{51}$ erg and $E_{\rm iso}$=10$^{53}$ erg , respectively. When a parameter varies, the others are fixed at their typical values, as taken in Fig. \ref{['evolu_nu']}. Note that the break in the curves marks the transition from the synchrotron-dominated to the SSC-dominated regime.
• Figure 4: Comparison of light curves for different emission components (SSC, synchrotron, and total of the two components) in 1 keV and 1 GeV bands under varying jet half opening angle $\theta_0$, ambient density $n$, and the isotropic equivalent energy $E_{\rm iso}$. Fixed parameters: $\epsilon_B = 0.001$, $\epsilon_e = 0.1$$p=2.3$.
• Figure 5: Comparison of total light curves (including the synchrotron and the SSC components) in 2 eV (the optical band), 1 keV, and 1 GeV under varying parameters. In panel A, the jet breaks in the GeV band (dashed lines) are explicitly marked. Fixed parameters are the same to Fig. \ref{['fig:comp_ssc_syn_comp']} .