Constrains on the physics of the prompt emission from a distant and energetic gamma-ray burst GRB 220101A

TL;DR

This study constrains the emission region of a distant, energetic gamma-ray burst using multi-band observations, favoring a proton-synchrotron model over electron-synchrotron for the prompt emission mechanism.

Contribution

It provides new constraints on the dissipation site and emission mechanisms of GRB 220101A, especially favoring proton-synchrotron models based on spectral and energetic analysis.

Findings

Constrained the Lorentz factor to 700-1160.

Estimated the emission radius to ~4.5 x 10^{13} cm.

Favored proton-synchrotron over electron-synchrotron models.

Abstract

The emission region of $\rm \gamma$-ray bursts (GRBs) is poorly constrained. The uncertainty on the size of the dissipation site spans over 4 orders of magnitude ($\rm 10^{12}-10^{17}$ cm) depending on the unknown energy composition of the GRB jets. The joint multi-band analysis from soft X-rays to high energies (up to $\rm \sim$ 1 GeV) of one of the most energetic and distant GRB 220101A (z = 4.618) allows us for an accurate distinction between prompt and early afterglow emissions. The enormous amount of energy released by GRB 220101A ($\rm E_{iso} \approx 3 \times10^{54}$ erg) and the spectral cutoff at $\rm E_{cutoff} = 85_{-26}^{+16}$ MeV observed in the prompt emission spectrum constrains the parameter space of GRB dissipation site. We put stringent constraints on the prompt emission site, requiring $\rm 700<\Gamma_0<1160 $ and $\rm R_\gamma \sim 4.5 \times 10^{13}$ cm. Our findings further highlights the difficulty of finding a simple self consistent picture in the electron-synchrotron scenario, favoring instead a proton-synchrotron model, which is also consistent with the observed spectral shape. Deeper measurements of the time variability of GRBs together with accurate high-energy observations (MeV-GeV) would unveil the nature of the prompt emission.