Two-temperature radiative hot accretion flow around neutron stars

Abstract

Numerical simulations of radiative two-temperature hot accretion flows (HAFs) around Neutron stars (NSs) are performed. We assume that all of the energy carried by the HAF around a NS will be thermalized and radiated out at the surface of the NS. The thermal photons will propagate outwards radially and cool the HAF vis Comptonization. We define $\dot{m}$ as the mass accretion rate at the surface of the central object in unit of Eddington accretion rate (${\dot{M}}_{\mathrm{Edd}}=10L_{\mathrm{Edd}}/c^2$, with $L_{\mathrm{Edd}}$ and $c$ being Eddington luminosity and speed of light, respectively). When $\dot{m}$ is lower than $\sim 10^{-4}$, the cooling of the HAF is not important and outflows are very strong. When $\dot{m}>\sim 10^{-3}$, cooling becomes important and outflows are significantly weak. In the range $10^{-4}<\dot{m}<10^{-3}$, the HAFs transients from a strong outflow phase to a very weak outflow phase with increase of $\dot{m}$. The properties of the HAF around a NS are also compared to those of the HAF around a BH. We find that with a similar $\dot{m}$, the dynamical properties of the HAF around a NS are quite similar as those of the HAF around a BH. However, the emitted spectrum of a HAF around a NS can be quite different from that of a HAF around a BH due to the presence of a thermal soft X-ray component coming from the surface of the NS.

arXiv comment12 pages, 4 figures, accepted by ApJ

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