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

To understand why supercritical accretion is feasible onto a neutron star (NS), we carefully examine the accretion flow dynamics by 2.5-dimensional general relativistic radiation magnetohydrodynamic (RMHD) simulations, comparing the cases of accretion onto a non-magnetized NS and that onto a black hole (BH). Supercritical BH accretion is relatively easy, since BHs can swallow excess radiation energy, so that radiation flux can be inward in its vicinity. This mechanism can never work for an NS, which has a solid surface. In fact, we find that the radiation force is always outward. Instead, we found significant reduction in the mass accretion rate due to strong radiation-pressure-driven outflow. The radiation flux F _rad is self-regulated such that the radiation force balances with the sum of gravity and centrifugal forces. Even when the radiation energy density greatly exceeds that expected from the Eddington luminosity {E}_{rad}≃ {F}_{rad}τ /c> {10}²{L}_{Edd}/(4π {r}²c), the radiation flux is always kept below a certain value, which makes it possible not to blow all the gas away from the disk. These effects make supercritical accretion feasible. We also find that a settling region, where accretion is significantly decelerated by a radiation cushion, is formed around the NS surface. In the settling region, the radiation temperature and mass density roughly follow {T}_{rad}\propto {r}^-1 and ρ \propto {r}^-3, respectively. No settling region appears around the BH, so matter can be directly swallowed by the BH with supersonic speed.