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MPIfR Optical & Infrared Interferometry Group

N.R. Ikhsanov:

Propeller vs. Accretor State of Low Luminous Accreting Neutron Stars

Astronomische Gesellschaft Abstract Series (AGM 18, MS0803)
Five Days of Creation: Astronomy with Large Telescopes from Ground and Space
Annual Scientific Meeting of the Astronomische Gesellschaft at the Joint European and National Meeting JENAM 2001 of the European Astronomical Society and the Astronomische Gesellschaft, Sep 10-15, 2001, Munich, Germany

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 this state 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 of the star is interchange stable. Nevertheless, I find that the rate of plasma penetration from the envelope into the magnetosphere due to diffusion and magnetic field lines 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 La ~= 5 × 1030 div 1033 M15 erg s-1, where {Mdot}15 is the strength of the normal companion stellar wind. 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, {Mdot}c, while the accretion luminosity is determined by the rate of plasma penetration into the star magnetosphere, {Mdot}a, which is {Mdot}a << {Mdot}c. That is why, the classification of the neutron star state in these objects using the steady accretion model (i.e. setting {Mdot}a = {Mdot}c) can lead to the mistaken conclusion that the star is in the centrifugal inhibition regime.

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