Dissecting the radiation mechanism of short GRB~160821B through multi-wavelength modelling

TL;DR

The paper investigates the origin of VHE emission in the short GRB 160821B by performing multi-wavelength spectral modeling with a modified NAIMA single-zone forward-shock framework, incorporating synchrotron, SSC, and EC processes and using MCMC for robust parameter estimation. By comparing ISM and wind density profiles, it finds the wind medium to better reproduce the 1.7–4 h broadband SED, with EC contributions negligible relative to SSC. The resulting fit yields a high initial Lorentz factor $\Gamma_0$ (≈273 for wind) and a low $\epsilon_B$, situating GRB 160821B in the low-$E_{k,iso}$, high-density wind regime and highlighting SSC as the dominant VHE mechanism in this event. The work suggests that VHE emission in short GRBs may generally arise from SSC in wind environments and emphasizes the need for more VHE detections (e.g., with CTA) to constrain particle acceleration and emission sites across the short GRB population.

Abstract

GRB~160821B is the only short GRB detected to date at very high energy (VHE, $\gtrsim 100$ GeV). At a redshift $z=0.161$, it was detected by MAGIC telescopes approximately four hours since the trigger. VHE dataset was complied with the datasets of other wavelengths in between the timescale of 1.7 to 4 hours to construct the broadband spectral energy distribution (SED). In previous studies of GRB~160821B, synchrotron and external Compton (EC) model could explain the VHE emission better than the synchrotron and synchrotron self-Compton (SSC) model. Although, these fits were mostly eyeballing data without any optimisation. Our model includes the combination of synchrotron, SSC, and EC models with Markov Chain Monte Carlo (MCMC) techniques. Our analysis reveals that the EC contribution is negligible in comparison with the SSC and our model explains the VHE data well for the wind medium. We found that GRB~160821B is the least energetic VHE GRB and it occurred in high density wind medium which is quiet unusual for a short GRB. But like other long-duration VHE GRBs, GRB~160821B occurred in a poorly magnetised medium. As there is no statistical study on afterglow modelling of short GRB sample, we compare the inferred properties of GRB~160821B with other VHE GRBs. It stands out distinctively in the $E_{k, \rm iso}$ - $ε_B$ parameter space and lies outside the 3-$σ$ region of the correlation. In future, more VHE detections of short GRBs, in the CTA era, will provide crucial insights into the emission sites, radiation mechanisms, and particle acceleration, as well as their connection to long GRBs.

Figures (5)

• Figure 1: SED fitting of GRB 160821B in the ISM medium. The MCMC $1\sigma$-confidence band is shown over the model for the best-fit parameters mentioned in Table \ref{['tab:model_parameters']}.
• Figure 2: SED fitting of GRB 160821B for the wind medium. The MCMC $1\sigma$-confidence band is shown over the model for the best-fit parameters tabulated in Table \ref{['tab:model_parameters']}.
• Figure 3: Corner plot for the ISM medium.
• Figure 4: Corner plot for the wind medium.
• Figure 5: Correlation plot between $\epsilon_B$ and $E_{\rm k,iso}$, for the high energetic GRB sample. The diamonds and circles represent VHE detected long-durations and other high energetic long-durations mentioned in Cenko2011Aksulu2022. The shaded regions represent $3\sigma$ and $1\sigma$ confidence interval respectively.