176 research outputs found
Electron-positron pairs in hot plasma of accretion column in bright X-ray pulsars
The luminosity of X-ray pulsars powered by accretion onto magnetized neutron
stars covers a wide range over a few orders of magnitude. The brightest X-ray
pulsars recently discovered as pulsating ultraluminous X-ray sources reach
accretion luminosity above which exceeds the
Eddington value more than by a factor of ten. Most of the energy is released
within small regions in the vicinity of magnetic poles of accreting neutron
star - in accretion columns. Because of the extreme energy release within a
small volume accretion columns of bright X-ray pulsars are ones of the hottest
places in the Universe, where the internal temperature can exceed 100 keV.
Under these conditions, the processes of creation and annihilation of
electron-positron pairs can be influential but have been largely neglected in
theoretical models of accretion columns. In this letter, we investigate
properties of a gas of electron-positron pairs under physical conditions
typical for accretion columns. We argue that the process of pairs creation can
crucially influence both the dynamics of the accretion process and internal
structure of accretion column limiting its internal temperature, dropping the
local Eddington flux and increasing the gas pressure.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter
Bright X-ray pulsars as sources of MeV neutrinos in the sky
High mass accretion rate onto strongly magnetised neutron stars results in
the appearance of accretion columns supported by the radiation pressure and
confined by the strong magnetic field of a star. At mass accretion rates above
, accretion columns are expected to be
advective. Under such conditions, a noticeable part of the total energy release
can be carried away by neutrinos of a MeV energy range. Relying on a simple
model of the neutrino luminosity of accreting strongly magnetised neutron
stars, we estimate the neutrino energy fluxes expected from six ULX pulsars
known up to date and three brightest Be X-ray transits hosting magnetised
neutron stars. Despite the large neutrino luminosity expected in ULX pulsars,
the neutrino energy flux from the Be X-ray transients of our Galaxy, SMC and
LMC is dominant. However, the neutrino flux from the brightest X-ray transients
is estimated to be below the isotropic background by two orders of magnitude at
least, which makes impossible direct registration of neutrino emission from
accreting strongly magnetised neutron stars nowadays.Comment: 7 pages, 6 figures, submitted to MNRA
Importance of electron-positron pairs on the maximum possible luminosity of the accretion columns in ULXs
One of the models explaining the high luminosity of pulsing ultra-luminous
X-ray sources (pULXs) was suggested by Mushtukov et al. (2015). They showed
that the accretion columns on the surfaces of highly magnetized neutron stars
can be very luminous due to opacity reduction in the high magnetic field.
However, a strong magnetic field leads also to amplification of the
electron-positron pairs creation. Therefore, increasing of the electron and
positron number densities compensates the cross-section reduction, and the
electron scattering opacity does not decrease with the magnetic field
magnification. As a result, the maximum possible luminosity of the accretion
column does not increase with the magnetic field. It ranges between 10 erg s depending only slightly on the magnetic field strength.Comment: 3 pages, 2 figures, subm. to Proc. IAU Symp. 363, poster presentatio
Competition between Vortex and βSβ-like States in Laterally Confined Magnetic Trilayers
AbstractWe report on the magnetization reversal and micromagnetic configurations of Co(10nm)/Pd(0.8nm)/Co(10nm) and Co (20nm) nanodisks studied as a function of the disk diameter. Using magneto-optical Kerr effect (MOKE) we show that the magnetic fields of vortex nucleation and annihilation decrease while the nanodisk diameter increases. We have discovered that in an array of trilayer nanodisks with diameters D = 200nm, direct exchange through pinholes and interlayer indirect ferromagnetic exchange coupling promote the formation of a non-uniform βSβ-like state with opposed magnetic moments in adjacent Co layers. In larger trilayers nanodisks (D = 400 - 800nm) the vortex state, like in single layer nanodisk, is formed
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