Subsonic propellers in a strong wind as anomalous X-ray pulsars

The appearance of subsonic propellers situated in a strong wind is discussed. We show that it is similar to the appearance of anomalous X-ray pulsars (AXPs) provided the mass and the magnetic moment of neutron stars are 1.4 solar masses, and 2E+30 G cm^3, respectively, and the strength of the wind is M_c = 3E+17 g/s. Under these conditions, the spin periods of subsonic propellers are limited within the range of 5-15 s, and the expected spin-down rates are close to 7E-11 s/s. The mass accretion rate onto the stellar surface is limited to the rate of plasma penetration into its magnetosphere at the boundary. As this process is governed by the reconnection of the field lines, the accretion rate onto the stellar surface constitutes 1-2% of M_c. In this case the X-ray luminosity of the objects under consideration can be evaluated as 4E+35 erg/s. The model predicts the existence of at least two spatially separated sources of the X-ray emission: hot spots at the stellar surface, and the hot atmosphere surrounding the magnetosphere of the star. The ages of the subsonic propellers under the conditions of interest are limited to 10^5 yr.Comment: 5 pages, 1 figure, accepted for publication in A&

Complex Topology of the Magnetic Field in Strong Flares

We report the "5+1" dynamical classification of the most frequently observed topologies of the magnetic field in sunspot groups associated with powerful flares.Comment: 2 pages, 1 figure, Proceedings of IAU Symp. 223, Volume 2004, page 25

Supersonic propeller spindown of neutron stars in wind-fed mass-exchange close binaries

The supersonic propeller spindown of a neutron star moving in a strong stellar wind of its massive companion is discussed. I show that the supersonic propeller model presented by Davies & Pringle (1981}) is self-consistent if the strength of the stellar wind of the normal companion is \dot{M}_{\rm c} \la 2.2 10^{18} (M_{\rm ns}/M_{\sun}) V_8 g/s. Under these conditions the model can be used for the interpretation of the long-period pulsars in Be/X-ray transients. The spin history of the neutron star in the long period Be/X-ray transient A0535+26 is considered.Comment: 3 pages, published in A&A Letters 381, L61 (2002

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 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 5*10**{30} - 10**{33} (dM/dt)_{15} erg/s, where dM/dt 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.Comment: 6 pages, accepted for publication in A&

On the circularly polarized optical emission from AE Aquarii

The reported nightly mean value of the circular polarization of optical emission observed from the close binary system AE Aqr is 0.06% (+-) 0.01%. We discuss a possibility that the observed polarized radiation is emitted mainly by the white dwarf or its vicinity. We demonstrate that this hypothesis is rather unlikely since the contribution of the white dwarf to the optical radiation of the system is too small. This indicates that the polarimetric data on AE Aqr cannot be used for the evaluation of the surface magnetic field strength of the white dwarf in this system.Comment: 4 pages, 3 figures, accepted for publication in Astron. & Astrophy

On the mass transfer in AE Aquarii

The observed properties of the close binary AE Aqr indicate that the mass transfer in this system operates via the Roche lobe overflow mechanism, but the material transferred from the normal companion is neither accreted onto the surface of the white dwarf nor stored in a disk around its magnetosphere. As previously shown, such a situation can be realized if the white dwarf operates as a propeller. At the same time, the efficiency of the propeller action by the white dwarf is insufficient to explain the rapid braking of the white dwarf, which implies that the spin-down power is in excess of the bolometric luminosity of the system. To avoid this problem we have simulated the mass-transfer process in AE Aqr assuming that the observed braking of the white dwarf is governed by a pulsar-like spin-down mechanism. We show that the expected H_alpha Doppler tomogram in this case resembles the tomogram observed from the system. We find that the agreement between the simulated and the observed tomograms is rather good provided the mean value of the mass-transfer rate ~5x10^16 g/s. Three spatially separated sources of H_alpha emission can be distinguished within this approach. The structure of the tomogram depends on the relative contributions of these sources to the H_alpha emission and is expected to vary from night to night.Comment: 12 pages, 3 figures (6 eps files). Published in A&A. The paper with high resolution images can be downloaded from http://urania.it.nuigalway.ie/papers/ae_aqr.ps.g

On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud

We explore the possibility to explain the properties of the Be/X-ray pulsars observed in the Small Magellanic Cloud within the magnetic levitation accretion scenario. This implies that their X-ray emission is powered by a wind-fed accretion onto a neutron star (NS) which captures matter from a magnetized stellar wind. The NS in this case is accreting matter from a non-keplerian magnetically levitating disc (ML-disc) which is surrounding its magnetosphere. This allows us to explain the observed periods of the pulsars in terms of spin equilibrium without the need of invoking dipole magnetic fields outside the usual range ~ 10^11- 10^13 G inferred from cyclotron features of Galactic high mass X-ray binaries. We find that the equilibrium period of a NS, under certain conditions, depends strongly on the magnetization of the stellar wind of its massive companion and, correspondingly, on the magnetic field of the massive companion itself. This may help to explain why similar NSs in binaries with similar properties rotate with different periods yielding a large scatter of periods of the accretion-powered pulsar observed in SMC and our galaxy.Comment: 6 pages, 1 figure, Published in MNRAS 454, 3760-3765 (2015

Can the rapid braking of the white dwarf in AE Aquarii be explained in terms of the gravitational wave emitter mechanism?

The spin-down power of the white dwarf in the close binary AE Aquarii significantly exceeds the bolometric luminosity of the system. The interpretation of this phenomenon in terms of the gravitational-wave emitter mechanism has been recently suggested by Choi & Yi. The basic assumption of their interpretation is that the spatially limited blobs or mounds of the mass \delta m ~ 10^{-3} M_sun, are present at the magnetic poles of the white dwarf. We show that the mounds of this mass can be confined by the magnetic field of the white dwarf only if the dipole magnetic moment of the star exceeds 4x10^{37} G cm^3. Under these conditions, however, the magnetodipole losses of the white dwarf would exceed the evaluated spin-down power 6 orders of magnitude. On this basis we discard a possibility that the observed rapid braking of the white dwarf in AE Aquarii can be explained in terms of the mechanism proposed by Choi & Yi.Comment: 6 pages, published in ApJ, 576, L5