A Cosmological Fireball with Thirty-Percent Gamma-Ray Radiative Efficiency

Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. The composition of the jets is, however, subject to debate\cite{Peer2015,Zhang2018}. Whereas the traditional model invokes a relativistic matter-dominated fireball with a bright photosphere emission component\cite{Meszaros2000}, the lack of the detection of such a component in some GRBs\cite{Abdo2009} has led to the conclusion that GRB jets may be Poynting-flux-dominated\cite{Zhang2009}. Furthermore, how efficiently the jet converts its energy to radiation is poorly constrained. A definitive diagnosis of the GRB jet composition and measurement of GRB radiative efficiency requires high-quality prompt emission and afterglow data, which has not been possible with the sparse observations in the past. Here we report a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its fluence is one of the highest of all GRBs detected so far, which allows us to perform a high-significance study on the prompt emission spectral properties and their variations down to a very short timescale of about 0.1 s. We identify a clear thermal component during the first two prompt emission episodes, which is fully consistent with the prediction of the fireball photosphere model. The third episode of the prompt emission is consistent with synchrotron radiation from the deceleration of the fireball. This allows us to directly dissect the fireball energy budget in a parameter-independent manner\cite{Zhang2021} and robustly measure a nearly $30\%$ radiative efficiency for this GRB. The afterglow microphysics parameters can be also well constrained from the data. GRB 190114C, therefore, exhibits the evolution of a textbook-version relativistic fireball, suggesting that fireballs can indeed power at least some GRBs with high efficiency.