Evaluating the bulk Lorentz factors of outflow material: lessons learned from the extremely-energetic outburst GRB 160625B

TL;DR

This study estimates the bulk Lorentz factors of outflow material in the energetic GRB 160625B by analyzing its prompt emission spectra, highlighting the importance of spectral models in accurate Lorentz factor determination and discussing the central engine nature.

Contribution

The paper compares different spectral models to estimate Lorentz factors in GRB 160625B, emphasizing the implications of thermal and cutoff components on outflow dynamics and central engine insights.

Findings

Thermal-radiation model yields Lorentz factors consistent with known correlations.

Spectral cutoff models suggest lower Lorentz factors, conflicting with pair opacity constraints.

Spectral cutoffs likely relate to particle acceleration, not Lorentz factors.

Abstract

GRB 160625B is an extremely-bright outburst with well-monitored afterglow emission. The geometry-corrected energy is high up to $\sim 5.2\times10^{52}$ erg or even $\sim 8\times 10^{52}$ erg, rendering it the most energetic GRB prompt emission recorded so far. We analyzed the time-resolved spectra of the prompt emission and found that in some intervals there were likely thermal-radiation components and the high energy emission were characterized by significant cutoff. The bulk Lorentz factors of the outflow material are estimated accordingly. We found out that the Lorentz factors derived in the thermal-radiation model are consistent with the luminosity-Lorentz factor correlation found in other bursts as well as in GRB 090902B for the time-resolved thermal-radiation components. While the spectral cutoff model yields much lower Lorentz factors that are in tension with the constraints set by the electron pair Compoton scattering process. We then suggest that these spectral cutoffs are more likely related to the particle acceleration process and that one should be careful in estimating the Lorentz factors if the spectrum cuts at a rather low energy (e.g., $\sim$ tens MeV). The nature of the central engine has also been discussed and a stellar-mass black hole is likely favored.