Photospheric emission from GRB 211211A altered by a strong radiation-mediated shock

TL;DR

This study demonstrates that the broad, two-break prompt spectra of GRB 211211A can be robustly explained by photospheric emission with strong radiation-mediated shocks below the photosphere, modeled via the Kompaneets RMS Approximation (KRA). By fitting time-resolved spectra with the KRA and incorporating a measured Lorentz factor, the authors derive RMS parameters indicating a strong, mildly relativistic shock at moderate optical depth, with a relatively cold upstream and a high photon-to-proton ratio, compatible with a pair-loaded jet scenario. The KRA fits significantly outperform the conventional Band function across all time bins, linking spectral evolution to evolving RMS properties and challenging the notion that MeV-band curvature alone reveals the emission mechanism. This work underscores that broad-band spectral information, together with physically grounded RMS modeling, is crucial for diagnosing GRB prompt emission mechanisms and cautions against relying solely on the MeV spectral shape for mechanism inferences.

Abstract

Gamma-ray burst (GRB) spectra are typically non-thermal, with many including two spectral breaks suggestive of optically-thin emission. However, the emitted spectrum from a GRB photosphere, which includes prior dissipation of energy by radiation-mediated shocks (RMSs), can also produce such spectral features. Here, we analyze the bright GRB 211211A using the Kompaneets RMS Approximation (KRA). We find that the KRA can fit the time-resolved spectra well, significantly better than the traditionally used Band function in all studied time bins. The analysis of GRB 211211A reveals a jet with a typical Lorentz factor ($Γ\sim 300$), and a strong RMS (upstream dimensionless specific momentum, $γ_u β_u \sim 3$) occurring at a moderate optical depth ($τ\sim 35$) in a relatively cold upstream ($θ_u = k_{\rm B} T_u / m_e c^2 \sim 10^{-4}$). We conclude that broad GRB spectra that exhibit two breaks can also be well explained by photospheric emission. This implies that, {in such cases}, the spectral shape in the MeV-band alone is not enough to determine the emission mechanism during the prompt phase in GRBs.

Figures (9)

• Figure 1: The Fermi GBM light curve of GRB 211211A from $T_0 - 1$ s to $T_0 + 20$ s. The red, blue, and green colors represent the precursor, the main emission, and the late/extended emission, respectively. The black histogram shows the binned light curve used in the analysis.
• Figure 2: A schematic of a typical immediate downstream spectrum in $\nu F_\nu$ representation. The two breaks are shown with their respective parameter dependencies. Here, $\theta_{ u }$ is the upstream temperature and $u_u$ is the dimensionless specific momentum in the RMS model.
• Figure 3: The measured values of $\Gamma$, using the method outlined in § \ref{['sec:gamma']}. The black dots show the best-fit values with the 68% confidence interval displayed by the error bars. The best fit values are used for the analysis in § \ref{['sec:results_spectra']}. The red line shows the constant value of $\Gamma = 100$ used for comparison in § \ref{['sec:results_RMS']}. The light blue histogram is the binned light curve, with values on the y-axis to the right.
• Figure 4: Time evolution of the spectra from the Bayesian inference. The darkest red is the first time bin at 1-2 s and the color brightens for each bin until the brightest red for the last bin at 11-12 s. Each color shows a set of $\sim 100$ spectra from the posterior distributions. The background shades show the energy ranges of Fermi GBM NaI (purple shade) and BGO (pink shade) detectors.
• Figure 5: Corner plot of the fitted KRA parameters at 7-8 s. The center dotted line is the median value and the left and right represent the 68% confidence interval.