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As the brightest Gamma-Ray burst (GRB) ever detected, GRB 221009A may offer achance that reveals some interesting features which are hidden in those burststhat are not so bright. There seems a very weak emission with a flux of$10^{-8}\sim10^{-7}$ erg cm$^{-2}$ s$^{-1}$ between the first pulse ($T_0\simT_0+50$~s, $T_0$ is the trigger time) and the main burst (appears from$T_0+180$ s). Thus the gap time between them is not really quiescent, and thefirst pulse could be taken as an unconventional precursor, which may provide apeculiar case study for the GRB-precursor phenomena. A two-stage collapsarscenario is proposed as the most likely origin for this burst. In this model,the jet for the precursor is produced during the initial core-collapse phase,and should be weak enough not to disrupt the star when it breaks out of theenvelope, so that the fallback accretion process and the forming of the diskcould continue. We present an approach in which the duration and flux bothprovide constraints on the luminosity ($L_{\rm j}$) and the Lorentz factor atthe breakout time ($\Gamma_{\rm b}$) of this weak jet. The estimated $L_{\rmj}\lesssim 10^{49}$ erg s$^{-1}$ and $\Gamma_{\rm b}$ has an order of ten,which are well consistent with the theoretical prediction. Besides, the weakemission in the gap time could be interpreted as a MHD outflow due to amagnetically driven wind during the period from the proto-neutron star phase toforming the accretion disk in this scenario.

GRB 221009A with an unconventional precursor: a typical two-stage collapsar scenario?