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

Abstract

To understand why supercritical accretion is feasible onto a neutron star, we carefully examine the accretion flow dynamics by 2.5-dimensional general relativistic (GR) radiation magnetohydrodynamic (RMHD) simulations, comparing the cases of accretion onto a non-magnetized neutron star (NS) and that onto a black hole (BH). Supercritical BH accretion is relatively easy, since BH can swallow excess radiation energy, so that radiation flux can be inward in its vicinity. This mechanism can never work for 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 much exceeds that expected from the Eddington luminosity E rad ≃ F rad τ /c > 10 2 L Edd /(4πr 2 c), the radiation flux is always kept below the 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 radiation cushion, is formed around the NS surface. In the settling region, the radiation temperature and mass density roughly follow T rad ∝ r −1 and ρ ∝ r −3 , respectively. No settling region appears around the BH so that matter can be directly swallowed by the BH with supersonic speed.

No magnetars in ULXs

“…Numerical simulations, although very impressive, cannot be of much help (even if they discuss the "spherization radius"). Instead of an external thin disc, they have a torus (Takahashi & Ohsuga 2017;Takahashi, Mineshige & Ohsuga 2018;Abarca, Kluźniak & Sadowski 2018). The critical radii are uncertain by no more than factors ∼ 1, so we expect (as here) the results of KLK17 will remain robust in comparison with both observations and future numerical simulations.…”

Ultraluminous X-Ray Sources

“…The last work shows that supercritical accretion is possible even on a neutron star that has no magnetic field at all. Based on the MHD calculations, Takahashi et al (2018) showed that in this case an accretion disk should have a structure generally similar to that described in the Introduction of this review (i.e., a conical funnel with H ∼ R filled with rarefied gas moving at relativistic velocities), but having a more powerful wind. Near the surface of a neutron star, the gas stops and a "cushion" consisting of matter and radiation is formed.…”

“…In addition to models with strong magnetic fields, another class of models have been proposed the presence of magnetar fields is not required to explain the ULXP phenomenon, and large observed luminosities is attributed to high collimation (King andLasota, 2016, 2019 (Takahashi et al, 2018). The last work shows that supercritical accretion is possible even on a neutron star that has no magnetic field at all.…”

“…Near the surface of a neutron star, the gas stops and a "cushion" consisting of matter and radiation is formed. The excess of the Eddington luminosity is achieved as a result of the fact that the radiation pressure is balanced by the sum of the centrifugal and gravitational forces (Takahashi et al, 2018).…”

Long-term X-ray spectral evolution of ultraluminous X-ray sources: implications on the accretion flow geometry and the nature of the accretor

“…Lastly, in the case of accretion onto NS magnetic poles, the accretion column is expected to be responsible for the emission at high energies (Walton et al 2018a). Unfortunately, simulated spectra from super-critically accreting NSs from numerical simulations are still under study (e.g., Takahashi & Ohsuga 2017;Takahashi et al 2018) and it is currently hard to know how the emission from the accretion column reacts to changes in the mass-transfer rate and magnetic field strength. Nevertheless, if the accretion column is responsible for the stable emission observed in these sources, it is still unclear why we observe this difference in terms of stability between PULXs and sources like NGC 1313 X-1 and Holmberg IX X-1.…”