SSC Radiation in the ICMART Model: Spectral Simulations and Application to the Record-Breaking GRB 221009A

TL;DR

This work computes the SSC spectrum within the ICMART framework using the ICMARTpy code, linking magnetization $σ_0$, reconnection-driven energy dissipation, and microphysical parameters to the broadband SED of GRBs. The results show a robust two-component spectrum (synchrotron and SSC) shaped by the Y parameter, Klein–Nishina suppression, and the evolving comoving magnetic field $B''_e$, with $B''_e ∝ \sqrt{k}\,σ_0$ and $γ_{e,m} ∝ \sqrt{σ_0}/f_e$. A key finding is a positive $Y$–$σ_0$ relation, in contrast to internal-shock expectations, implying that larger magnetization enhances SSC, up to observational constraints. When applied to GRB 221009A, the model favors $σ_0 \le 20$ to reconcile MeV and TeV measurements, while some intervals resist a pure non-thermal fit, suggesting either thermal contributions or time-averaging effects. The study demonstrates the diagnostic power of simultaneous MeV–TeV data for probing magnetic dissipation and particle acceleration in GRB jets and motivates future multi-wavelength campaigns.

Abstract

This paper presents simulations of the synchrotron self-Compton (SSC) spectrum within the Internal-Collision-induced Magnetic Reconnection and Turbulence (ICMART) model. We investigate how key parameters like the magnetization $σ_0$ shape the broadband spectral energy distribution by regulating the electron distribution and magnetic field strength. The overall spectrum typically comprises two components: synchrotron radiation peaking at $E_{\rm p}$ with a low-energy spectral index $α$ between -1 and -1.5, and an SSC component peaking at $E_{\rm ssc}$. At high energies, Klein-Nishina suppression causes an exponential cutoff. The flux ratio Y between these components is critical: when Y is small, the SSC peak can be suppressed. Spectral features of the synchrotron component reveal the underlying physical conditions: harder spectra with $α\sim-1$ indicate a large Y parameter and strong KN suppression. We find a positive correlation between Y and $σ_0$, contrasting with internal shock model predictions. Applied to GRB 221009A, our model suggests $σ_0\leq20$ can reproduce the MeV-TeV observations. This study underscores the value of combined MeV-TeV observations in probing GRB emission mechanisms.

Figures (3)

• Figure 1: (a)The solid line represents the total spectrum, the dashed line and the dotted line represents the synchrotron component and the SSC component respectively. Parameters set up: $M_\text{bulk}= 10^{34}\rm g$, $\Gamma_0=84$, $\sigma_0=60$, $f_\text{p}=0.04$, $k=5\times10^{-11}$, $f_\text{e}=0.9$, $L'_0=6\times10^{10} \text{cm}$, $v'_\text{in}=1.5\times10^{9} \text{cm/s}$, $R_0=10^{14} \text{cm}$, $b=30$, $l=0.2$, each reconnection within the event has a distinct position and orientation characterized by $\theta \in (0, 2/\Gamma_0)$ and $\phi \in (0, \pi/2)$, the observation time is settled at the peak luminosity time; (b)Spectrums with different shapes.
• Figure 2: Spectrums with different $\gamma_{\rm e,m}$ and ${B"}_{\rm e}$.
• Figure 3: Solid lines with different shades of browns represent different $\sigma_0$ listed in the label. The blue dashed-dotted lines are the fitting functions of the observed MeV spectrums and the green dotted line is the fitting function of the observed TeV spectrums in $T_0+[240, 250] \rm s$.