227 research outputs found
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
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