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

10691 Stockholm, Sweden 5 School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK 6 ICRANet, Piazza della Repubblica 10, I-65122 Pescara, Italy 7 Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville,AL,USA 8 ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. How efficiently the jet converts its energy to radiation is a long-standing problem and it is poorly constrained. The standard model invokes a relativistic fireball with a bright photosphere emission component. A definitive diagnosis of GRB radiation components and measurement of GRB radiative efficiency require prompt emission and afterglow data with high-resolution and wide-band coverage in time and energy. 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 highresolution study of the prompt emission spectral properties and their temporal evolution down to a timescale of about 0.1 s. We observe that each of the initial pulses has a thermal component contributing ∼ 20% of the total energy, the corresponding temperature and the inferred Lorentz factor of the photosphere evolve following broken power-law shapes. From the observation of the non-thermal spectra and the light-curve, the onset of afterglow corresponding to the deceleration of the fireball is considered at ∼ 6~s. By incorporating the thermal and the non-thermal observations, as well as the photosphere and the synchrotron radiative mechanisms, we can directly derive the fireball energy budget with little dependence on hypothetical parameters and to measure a ∼ 16% radiative efficiency for this GRB. With the fireball energy budget derived, the afterglow microphysics parameters can also be constrained directly from the data.