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To understand why supercritical accretion is feasible onto a neutron star, wecarefully examine the accretion flow dynamics by 2.5-dimensional generalrelativistic (GR) radiation magnetohydrodynamic (RMHD) simulations, comparingthe cases of accretion onto a non-magnetized neutron star (NS) and that onto ablack hole (BH). Supercritical BH accretion is relatively easy, since BH canswallow excess radiation energy, so that radiation flux can be inward in itsvicinity. This mechanism can never work for NS which has a solid surface. Infact, we find that the radiation force is always outward. Instead, we foundsignificant reduction in the mass accretion rate due to strongradiation-pressure driven outflow. The radiation flux $F_\mathrm{rad}$ isself-regulated such that the radiation force balances with the sum of gravityand centrifugal forces. Even when the radiation energy density much exceedsthat expected from the Eddington luminosity $E_\mathrm{rad} \simeqF_\mathrm{rad}\tau/c> 10^2 L_\mathrm{Edd}/(4\pi r^2 c)$, the radiation flux isalways kept below the certain value which makes it possible not to blow all thegas away from the disk. These effects make supercritical accretion feasible. Wealso find that a settling region, where accretion is significantly deceleratedby radiation cushion, is formed around the NS surface. In the settling region,the radiation temperature and mass density roughly follow $T_\mathrm{rad}\propto r^{-1}$ and $\rho \propto r^{-3}$, respectively. No settling regionappears around the BH so that matter can be directly swallowed by the BH withsupersonic speed.

Supercritical Accretion onto a Non-Magnetized Neutron Star: Why is it Feasible?