Astronomy and Astrophysics, volume 617A, 126-126 (2018/9-1)
Magnetospheric radius of an inclined rotator in the magnetically threaded disk model.
BOZZO E., ASCENZI S., DUCCI L., PAPITTO A., BURDERI L. and STELLA L.
Abstract (from CDS):
The estimate of the magnetospheric radius in a disk-fed neutron star X-ray binary is a long standing problem in high energy astrophysics. We have reviewed the magnetospheric radius calculations in the so-called magnetically threaded disk model, comparing the simplified approach originally proposed by Ghosh & Lamb (1979ApJ...232..259G
) with the revised version proposed by Wang (1987A&A...183..257W
), Wang (1995ApJ...449L.153W
), and Wang (1997ApJ...475L.135W
). We show that for a given set of fixed parameters (assuming also a comparable screening factor of the neutron star magnetic field by the currents induced on the disk surface) the revised magnetically threaded disk model predicts a magnetospheric radius that is significantly smaller than that derived from the Ghosh & Lamb (1979ApJ...232..259G
) treatment. For a fixed value of the neutron star magnetic field and a wide range of mass accretion rates, the inclusion of a large inclination angle between the neutron star rotation and magnetic field axes (χ≥60°) leads to a further decrease of the magnetospheric radius. To illustrate the relevance of these calculations, we consider, as an example, the case of the transitional pulsars. During the so-called high mode of their sub-luminous accretion disk state, these sources have shown X-ray pulsations interpreted as due to accretion at an unprecedented low luminosity level compared to other neutron stars in X-ray binaries. In the context of the magnetic threaded disk model, we show that accretion at luminosities of ∼1033
erg/s (and thus accretion-driven X-ray pulsations) can be more easily explained when the prescription of the magnetospheric radius provided by Wang (1997ApJ...475L.135W
) is used. This avoids the need to invoke very strong propeller outflows in the transitional pulsars, as proposed in other literature works.
© ESO 2018
stars: neutron - X-rays: binaries
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