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GRB 221009A is the brightest gamma-ray burst ever detected since thediscovery of this kind of energetic explosions. However, an accuratemeasurement of the prompt emission properties of this burst is very challengingdue to its exceptional brightness. With joint observations of\textit{Insight}-HXMT and GECAM-C, we made an unprecedentedly accuratemeasurement of the emission during the first $\sim$1800 s of GRB 221009A,including its precursor, main emission (ME, which dominates the burst in flux),flaring emission and early afterglow, in the hard X-ray to soft gamma-ray bandfrom $\sim$ 10 keV to $\sim$ 6 MeV. Based on the GECAM-C unsaturated data ofthe ME, we measure a record-breaking isotropic equivalent energy ($E_{\rmiso}$) of $\bf \sim 1.5 \times 10^{55}$ erg, which is about eight times thetotal rest-mass energy of the Sun. The early afterglow data require asignificant jet break between 650 s and 1100 s, most likely at $\sim950$ s fromthe afterglow starting time $T_{AG}$, which corresponds to a jet opening angleof $\sim {0.7^\circ} \ (\eta_\gamma n)^{1/8}$, where $n$ is the ambient mediumdensity in units of $\rm cm^{-3}$ and $\eta_\gamma$ is the ratio between$\gamma$-ray energy and afterglow kinetic energy. The beaming-corrected total$\gamma$-ray energy $E_{\gamma}$ is $\sim 1.15 \times10^{51} \ (\eta_\gamman)^{1/4}$ erg, which is typical for long GRBs. These results suggest that thisGRB may have a special central engine, which could launch and collimate a verynarrowly beamed jet with an ordinary energy budget, leading to exceptionallyluminous gamma-ray radiation per unit solid angle. Alternatively, more GRBsmight have such a narrow and bright beam, which are missed by an unfavorableviewing angle or have been detected without distance measurement.

Insight-HXMT and GECAM-C observations of the brightest-of-all-time GRB 221009A