GRB 190114C: Fireball Energy Budget and Radiative Efficiency Revisited

TL;DR

This study conducts a detailed spectral analysis of GRB 190114C, revealing high radiative efficiency and consistent central engine properties, enhancing understanding of jet composition and energy conversion in gamma-ray bursts.

Contribution

It provides a comprehensive temporal and spectral analysis of GRB 190114C, measuring fireball parameters and radiative efficiency with high precision, and demonstrates high efficiency and consistent engine properties.

Findings

High radiative efficiency (~36-41%) suggests efficient energy conversion.

Thermal components dominate early pulses, nonthermal spectra extend to afterglow.

Consistent efficiency across pulses implies stable central engine properties.

Abstract

The jet composition of gamma-ray bursts (GRBs), as well as how efficiently the jet converts its energy to radiation, are long-standing problems in GRB physics. Here, we reported a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its high fluence ($\sim$ 4.4$\times$10$^{-4}$ erg cm$^{-2}$) allows us to conduct the time-resolved spectral analysis in great detail and study their variations down to a very short time-scale ($\sim$0.1 s) while preserving a high significance. Its prompt emission consists of three well-separated pulses. The first two main pulses ($P_1$ and $P_2$) exhibit independently strong thermal components, starting from the third pulse ($P_3$) and extending to the entire afterglow, the spectra are all nonthermal, the synchrotron plus Compton upscattering model well interprets the observation. By combining the thermal ($P_1$ and $P_2$) and the nonthermal ($P_3$) observations based on two different scenarios (global and pulse properties) and following the method described in Zhang et al., we measure the fireball parameters and GRB radiative efficiency with little uncertainties for this GRB. A relevantly high GRB radiative efficiency is obtained based on both the global and pulse properties, suggesting that if GRBs are powered by fireballs, the efficiency can sometimes be high. More interestingly, though the observed parameters are individually different (e.g., the amount of mass loading $M$), the radiative efficiency obtained from $P_1$ ($\eta_\gamma=36.0\pm6.5\%$) and $P_2$ ($\eta_\gamma=41.1\pm1.9\%$) is roughly the same, which implies that the central engine of the same GRB has some common properties.