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The central engine of gamma-ray bursts (GRBs) is believed to be a hot anddense disk with hyperaccretion onto a few solar-mass black hole. We investigatewhere the magnetorotational instability (MRI) actively operates in thehyperaccretion disk, which can cause angular momentum transport in the disk.The inner region of hyperaccretion disks can be neutrino opaque, and theenergy- and momentum-transport by neutrinos could affect the growth of the MRIsignificantly. Assuming reasonable disk models and a weak magnetic field $B\lesssim 10^{14} \rm{G}$, it is found that the MRI is strongly suppressed bythe neutrino viscosity in the inner region of hyperaccretion disks. On theother hand, the MRI can drive active MHD turbulence in the outerneutrino-transparent region regardless of the field strength. This suggeststhat the baryonic matter is accumulated into the inner dead zone where the MRIgrows inactively and the angular momentum transport is inefficient. When thedead zone gains a large amount of mass and becomes gravitationally unstable,intense mass accretion onto the central black hole would occur episodicallythrough the gravitational torque. This process can be a physical mechanism ofthe short-term variability in the prompt emission of GRBs. Finally, the originof flaring activities in the X-ray afterglow is predicted in the context of ourepisodic accretion scenario.Comment: 11pages, 4figures. Accepted for publication in the Astrophysical  Journa

astrophysical journal/the astrophysical journal

Dead Zone Formation and Nonsteady Hyperaccretion in Collapsar Disks: A Possible Origin of Short‐Term Variability in the Prompt Emission of Gamma‐Ray Bursts