On the state of low luminous accreting neutron stars

Observational appearance of a neutron star in the subsonic propeller state which is a companion of a wind-fed mass-exchange close binary system is discussed. During the subsonic propeller state (which was first introduced by Davies et al. \cite{dfp79}) the neutron star magnetosphere is surrounded by a spherical quasi-static plasma envelope, which is extended from the magnetospheric boundary up to the star accretion radius. The energy input to the envelope due to the propeller action by the neutron star exceeds the radiative losses and the plasma temperature in the envelope is of the order of the free-fall temperature. Under this condition the magnetospheric boundary is interchange stable. Nevertheless, I find that the rate of plasma penetration from the envelope into the magnetic field of the neutron star due to diffusion and magnetic field line reconnection processes is large enough for the accretion power to dominate the spindown power. I show that the accretion luminosity of the neutron star in the subsonic propeller state is L_a =~ 5 x 10³⁰ /10³³ M₁₅ erg s^-1, where dot M₁₅ is the strength of the normal companion stellar wind which is parametrized in terms of the maximum possible mass accretion rate onto the neutron star magnetosphere. On this basis I suggest that neutron stars in the subsonic propeller state are expected to be observed as low luminous accretion-powered pulsars. The magnetospheric radius of the neutron star in this state is determined by the strength of the stellar wind, dot M_c, while the accretion luminosity is determined by the rate of plasma penetration into the star magnetosphere, dot M_a, which is dot M_a << dot M_c. That is why the classification of the neutron star state in these objects using the steady accretion model (i.e. setting dot M_a = dot M_c) can lead to a mistaken conclusion.