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 dis cussed. During the subsonic propeller state (which was first introduced by Davies et al. 1979) the neutron star magnetosphere is surrounded by a spher ical quasi-static plasma envelope, which is extended from the magnetospheri c boundary up to the star accretion radius. The energy input to the envelop e due to the propeller action by the neutron star exceeds the radiative los ses and the plasma temperature in the envelope is of the order of the free- fall temperature. Under this condition the magnetospheric boundary is inter change stable. Nevertheless, I find that the rate of plasma penetration fro m the envelope into the magnetic field of the neutron star due to diffusion and magnetic field line reconnection processes is large enough for the acc retion power to dominate the spindown power. I show that the accretion lumi nosity of the neutron star in the subsonic propeller state is L(a)similar o r equal to5 x 10(30) divided by 10(33) M-15 erg s(-1), where M-15 is the st rength of the normal companion stellar wind which is parametrized in terms of the maximum possible mass accretion rate onto the neutron star magnetosp here. 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, M-c, while the accretion luminosity i s determined by the rate of plasma penetration into the star magnetosphere, M-a, which is M(a)much less thanM(c). That is why the classification of th e neutron star state in these objects using the steady accretion model (i.e . setting _ M-a=M-c) can lead to a mistaken conclusion.