From X-rays to High-Energy Gamma-rays: A Comprehensive Multi-Wavelength Study of Early Gamma-Ray Burst Afterglows

TL;DR

This work presents a time-resolved, multiwavelength analysis of 31 GRBs with simultaneous $0.3\,\mathrm{keV}$ to $300\,\mathrm{GeV}$ data from Swift and Fermi, testing synchrotron and SSC emission from forward shocks. By combining LAT, XRT, and BAT observations, the authors constrain the afterglow spectral evolution and compare fluxes and indices across energy bands, finding a wind-like circumburst medium with a low magnetic-energy fraction $\epsilon_{B} \sim 10^{-4}$ provides the best match to both X-ray and GeV trends. Using the LeMoC SSC modeling framework, they explore 36 benchmark configurations and identify a preferred parameter set with $p\approx 2.2$, $\epsilon_{e}=0.1$, $\epsilon_{B}=10^{-4}$, and a wind environment with $A_{*}\approx 0.1$, which reproduces observed fluxes, spectral indices, and flux-flux correlations, and makes testable predictions for TeV emission. The results refine microphysical parameter estimates and reinforce the role of SSC processes in GRB afterglows, with implications for VHE detectability by CTA-era observations.

Abstract

Gamma-ray Bursts (GRBs) generate powerful relativistic jets that inject a large amount of energy into their surrounding environment, producing blast waves that accelerate particles to high energies. The GRB afterglow radiation provides a powerful means to investigate the microphysics of relativistic shocks and to probe the medium surrounding the progenitor of the burst. In this study, we present a comprehensive multiwavelength analysis of 31 GRBs observed between 2008 and 2024 from the Neil Gehrels Swift Observatory (X-ray Telescope and Burst Alert Telescope) and the Fermi Large Area Telescope, covering photon energies from 0.3 keV to 300 GeV. Our goal is to characterize the broadband spectral properties of GRB afterglows in soft X-rays, hard X-rays, and high-energy gamma rays. We investigate correlations between spectral shape and energy output across different parts of the spectrum. The observed emission is modeled using a forward shock scenario that includes both synchrotron and synchrotron self-Compton (SSC) radiation losses. The results favor an SSC-dominated radiation model in a wind-like medium, consistent with expectations for long-duration GRBs. Crucially, this work provides new benchmarks for the microphysical parameters governing the emission, particularly indicating a notably low magnetic energy fraction, which refines previous estimates. By modeling broadband data, this study offers one of the most detailed SSC analyses in a wind-like environment to date. Notably, our results naturally account for the comparable energy output observed in both the soft X-ray and TeV bands, consistent with the previously detected TeV-GRBs.

Figures (14)

• Figure 1: Venn diagram showing the number of GRBs detected by XRT, BAT, and LAT during the period from August 2008 to August 2024. The overlaps represent GRBs detected by multiple instruments. Specifically, 31 GRBs were jointly detected in at least one time interval by all three instruments, as evidenced in the pink overlap region.
• Figure 2: Distribution of time-bin centers relative to BAT trigger time. The histograms show the distribution of time bins centers with respect to the BAT trigger time. Time bins exhibiting consistent temporal decay behavior in afterglow are shown in blue, while those containing X-ray flares are shown in gray.
• Figure 3: Comparison between X-ray and HE gamma-ray fluxes. Plot (a) and (b) presents comparison of X-ray and HE gamma-ray fluxes. (a): Flux in 0.3–10 keV (XRT) vs. 0.1–10 GeV (LAT). (b): Flux in 0.3–150 keV (XRT+BAT) vs. 0.1–10 GeV (LAT). Plot (c) and (d) shows comparison of X-ray and HE gamma-ray fluxes normalized with GBM fluence (0.01-1MeV). (c): Ratio between flux and fluence in 0.3–10 keV (XRT) vs. 0.1–10 GeV (LAT). (d): Ratio between flux and fluence in 0.3–150 keV (XRT+BAT) vs. 0.1–10 GeV (LAT). Each GRB has assigned unique color consistent throughout the paper. The dashed line indicates equality line. Both panels show LAT detections (data points with flux uncertainity, $\text{TS} > 20$) and upper limits (downward triangle). The right panel has fewer bins than left panel due to limited sensitivity of BAT (see Tab. \ref{['tab:timeresolved']} and Sect. \ref{['sec:flux_flux']} for details).
• Figure 4: X-ray vs HE gamma-rays photon indices. Left: Comparison of spectral indices between X-rays (0.3–10 keV, x-axis) and GeV gamma-rays (0.1–10 GeV, y-axis). Right: Comparison of spectral indices between X-rays (0.3–10 keV, x-axis) and GeV gamma-rays (0.1 GeV–$E_{\text{max}}$, y-axis). Both the plot includes three lines: one at LAT index = $-2$, another at XRT index = $-2$, and the equality line (X-ray index = GeV index). Data points with error bars indicate significant LAT detections ($\text{TS} > 20$), while upper limits (calculated in 0.1-10 GeV) are shown with downward arrows ($\text{TS} < 20$). For more details, see Tab. \ref{['tab:timeresolved']} and Sect. \ref{['section:Index comparison']}.
• Figure 5: The relation between the luminosity (L$_{\rm iso}$) and the prompt emission isotropic energy (E$_{\rm iso}$) in rest frame of GRBs. The solid pink and magenta lines represent the trends reported by 2014MNRAS.443.3578N and 2019ApJ...878...52A, respectively, for the ratio L$_\gamma/E_{\rm iso}$. The shaded cyan band indicates the $1\sigma$ confidence region for the corresponding trend in X-rays (L$_{\rm X}/E_{\rm iso}$) reported in 2012MNRAS.425..506D, where $L_{\rm X}$ is calculated in the 2--10 keV band. The magenta and blue band represents the simulated GeV and X-ray emission for the SSC parameters $p=2.2$, $\epsilon_{\rm e} = 0.1$, $\epsilon_{\rm B} = 10^{-4}$ in wind medium with A$_{*}$ = 0.1.