Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
N.R. Ikhsanov:
On the state of low luminous accreting
neutron stars
Astronomy and Astrophysics 375, 944-949 (2001)
Abstract.
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. 1979) 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 = 5x10**{30} - 10**{33}
(dM/dt)_15 erg/s, where dM/dt_15 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, (dM/dt)_c, while the accretion luminosity is determined
by the rate of plasma penetration into the star magnetosphere,
(dM/dt)_a, which is (dM/dt)_a << (dM/dt)_c. That is why the
classification of the neutron star state in these objects using the
steady accretion model (i.e. setting (dM/dt)_a = (dM/dt)_c) can lead to
a mistaken conclusion.
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