Hot Accretion Flow around Neutron Stars

Abstract

Abstract We perform for the first time hydrodynamic simulations to study the properties of hot accretion flow (HAF) around a neutron star (NS). The energy carried by the HAF will eventually be radiated out at the surface of the NS. The emitted photons can propagate inside the HAF and cool the HAF via Comptonization. We find that the Compton cooling can affect the properties of the HAF around an NS significantly. We define the Eddington accretion rate as , with L Edd and c being the Eddington luminosity and the speed of light, respectively. We define as the mass accretion rate at the NS surface in units of . When , Compton cooling can effectively cool the HAF and suppress wind. Therefore, the mass accretion rate is almost a constant with radius. The density profile is ρ ∝ r −1.4. When , the Compton cooling effects become weaker, wind becomes stronger, and accretion rate is proportional to r 0.3–0.5. Consequently, the density profile becomes flatter, . When , the Compton cooling effects can be neglected. We find that with the same accretion rate, the temperature of the HAF around an NS is significantly lower than that of the HAF around a black hole (BH). Also, the Compton y-parameter of the HAF around an NS is significantly smaller than that of the HAF around a BH. This result predicts that the HAF around an NS will produce a softer spectrum compared to the HAF around a BH, which is consistent with observations.