134 research outputs found
Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state
Spectra of the spreading layers on the neutron star surface are calculated on
the basis of the Inogamov-Sunyaev model taking into account general relativity
correction to the surface gravity and considering various chemical composition
of the accreting matter. Local (at a given latitude) spectra are similar to the
X-ray burst spectra and are described by a diluted black body. Total spreading
layer spectra are integrated accounting for the light bending, gravitational
redshift, and the relativistic Doppler effect and aberration. They depend
slightly on the inclination angle and on the luminosity. These spectra also can
be fitted by a diluted black body with the color temperature depending mainly
on a neutron star compactness. Owing to the fact that the flux from the
spreading layer is close to the critical Eddington, we can put constraints on a
neutron star radius without the need to know precisely the emitting region area
or the distance to the source. The boundary layer spectra observed in the
luminous low-mass X-ray binaries, and described by a black body of color
temperature Tc=2.4+-0.1 keV, restrict the neutron star radii to R=14.8+- 1.5 km
(for a 1.4-Msun star and solar composition of the accreting matter), which
corresponds to the hard equation of state.Comment: 13 pages, 13 figures, MNRAS, in pres
Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems
Magnetic fields are important for accretion disc structure. Magnetic fields
in a disc system may be transported with the accreted matter. They can be
associated with either the central body and/or jet, and be fossil or dynamo
excited in situ. We consider dynamo excitation of magnetic fields in accretion
discs of accreting binary systems in an attempt to clarify possible
configurations of dynamo generated magnetic fields. We first model the entire
disc with realistic radial extent and thickness using an alpha-quenching
non-linearity. We then study the simultaneous effect of feedback from the
Lorentz force from the dynamo-generated field. We perform numerical simulations
in the framework of a relatively simple mean-field model which allows the
generation of global magnetic configurations. We explore a range of
possibilities for the dynamo number, and find quadrupolar-type solutions with
irregular temporal oscillations that might be compared to observed rapid
luminosity fluctuations. The dipolar symmetry models with have
lobes of strong toroidal field adjacent to the rotation axis that could be
relevant to jet launching phenomena. We have explored and extended the
solutions known for thin accretion discs.Comment: 13 pages, 14 figure
Observational appearance of rapidly rotating neutron stars: X-ray bursts, cooling tail method, and radius determination
Neutron stars (NSs) in low-mass X-ray binaries rotate at frequencies high
enough to significantly deviate from sphericity ( 200--600 Hz). We
investigate the effects of rapid rotation on the observational appearance of a
NS. We propose analytical formulae relating gravitational mass and equatorial
radius of the rapidly rotating NS to the mass and radius of a
non-rotating NS of the same baryonic mass using accurate fully relativistic
computations. We compute spectra from an oblate rotating NS observed at
different inclination angles using the modified oblate Schwarzschild (MOS)
approximation, where light bending is computed in Schwarzschild metric, but
frame dragging and quadrupole moment of a NS are approximately accounted for in
the photon redshift calculations. We generalize the cooling tail method to the
case of a rapidly rotating NS to obtain the most probable values of and
of the corresponding non-rotating NS with the same baryonic mass. We
approximate the local spectra from the NS surface by a diluted blackbody using
previously computed NS atmosphere models. We show that the NS radius could be
overestimated by 3--3.5 km for face-on stars of km rotating at
700 Hz if the version of the cooling tail method for a non-rotating
NS is used. We apply the method to an X-ray burst observed from the NS rotating
at 532 Hz in SAX J1810.82609. The resulting radius of the
non-rotating NS (assuming ) becomes km if it is
viewed at inclination i=60 deg and km for a face-on view, which
are smaller by 0.6 and 1.2 km than the radius obtained using standard cooling
tail method ignoring rotation. The corresponding equatorial radii of these
rapidly rotating NSs are 12.3 km (for i=60 deg) and 11.6\,km
(for i=0 deg).Comment: 17 pages, 16 figures, accepted for publication in Astronomy and
Astrophysic
The Boundary Layer in compact binaries
Disk accretion onto stars leads to the formation of a Boundary Layer (BL)
near the stellar surface where the disk makes contact with the star. Albeit a
large fraction of the total luminosity of the system originates from this tiny
layer connecting the accretion disk and the accreting object, its structure has
not been fully understood yet. It is the aim of this work, to obtain more
insight into the Boundary Layer around the white dwarf in compact binary
systems. There are still many uncertainties concerning the extent and
temperature of the BL and the rotation rate of the white dwarf. We perform
numerical hydrodynamical simulations, where the problem is treated in a
one-dimensional, radial approximation (slim disk). The turbulence is described
by the alpha parameter viscosity. We include both cooling from the disk
surfaces and radial radiation transport. The radiation energy is treated in the
one-temperature approximation. For a given M_dot our results show a strong
dependence on the stellar mass and rotation rate. Both, the midplane and the
effective temperature rise considerably with increasing stellar mass or
decreasing stellar rotation rate. Our simulations further show, that the
radiation energy and pressure are indeed important in the BL. However, some
models show a low optical depth in the BL, making it necessary to find a better
representation for optically thin regions. The combination of a high mass and a
small radius, such as in white dwarfs, can lead to an enormous energy release
in the BL, provided the WD rotates slowly. Since the radial extent of BLs is
typically very small (about 0.02 to 0.05 R_star), this leads to surface
temperatures of a few hundred thousand Kelvin. All of our models showed
subsonic infall velocities with Mach numbers of < 0.4 at most.Comment: 13 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
On the spreading layer emission in luminous accreting neutron stars
Emission of the neutron star surface potentially contains information about
its size and thus of vital importance for high energy astrophysics. In spite of
the wealth of data on the emission of luminous accreting neutron stars, the
emission of their surfaces is hard to disentangle from their time averaged
spectra. A recent X-ray transient source XTE J1701-462 has provided a unique
dataset covering the largest ever observed luminosity range for a single
source. In this paper, we extract the spectrum of the boundary layer between
the inner part of the accretion disc and the neutron star surface with the help
of maximally spectral model-independent method. We show compelling evidences
that the energy spectrum of the boundary layer stays virtually the same over
factor of 20 variations of the source luminosity. It is rather wide and cannot
be described by a single temperature blackbody spectrum, probably because of
the inhomogeneity of the boundary layer and a spread in the colour temperature.
The observed maximum colour temperature of the boundary/spreading layer
emission of kT~2.4-2.6 keV is very close to the maximum observed colour
temperature in the photospheric radius expansion X-ray bursts, which is set by
the limiting Eddington flux at the neutron star surface. Observed stability of
the boundary layer spectrum and its maximum colour temperature strongly
supports theoretical models of the boundary/spreading layers on surfaces of
luminous accreting neutron stars, which assume the presence of a region
emitting at the local Eddington limit. Variations in the luminosity in that
case lead to changes in the size of this region, but affect less the spectral
shape. Elaboration of this model will provide solid theoretical grounds for
measurements of the neutron star sizes using the emission of the
boundary/spreading layers of luminous accreting neutron stars.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Models of neutron star atmospheres enriched with nuclear burning ashes
Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear
(type-I) X-ray bursts, which are powered by unstable nuclear burning of helium
and/or hydrogen into heavier elements deep in the NS "ocean". In some cases the
burning ashes may rise from the burning depths up to the NS photosphere by
convection, leading to the appearance of the metal absorption edges in the
spectra, which then force the emergent X-ray burst spectra to shift toward
lower energies. These effects may have a substantial impact on the color
correction factor and the dilution factor , the parameters of the
diluted blackbody model that is commonly used
to describe the emergent spectra from NSs. The aim of this paper is to quantify
how much the metal enrichment can change these factors. We have developed a new
NS atmosphere modeling code, which has a few important improvements compared to
our previous code required by inclusion of the metals. The opacities and the
internal partition functions (used in the ionization fraction calculations) are
now taken into account for all atomic species. In addition, the code is now
parallelized to counter the increased computational load. We compute a detailed
grid of atmosphere models with different exotic chemical compositions that
mimic the presence of the burning ashes. From the emerging model spectra we
compute the color correction factors and the dilution factors that
can then be compared to the observations. We find that the metals may change
by up to about 40%, which is enough to explain the scatter seen in the
blackbody radius measurements. The presented models open up the possibility for
determining NS mass and radii more accurately, and may also act as a tool to
probe the nuclear burning mechanisms of X-ray bursts.Comment: 14 pages, 7 figures, to be published in A&
Optically thick envelopes around ULXs powered by accreating neutron stars
Magnetized neutron stars power at least some ultra-luminous X-ray sources.
The accretion flow in these cases is interrupted at the magnetospheric radius
and then reaches the surface of a neutron star following magnetic field lines.
Accreting matter moving along magnetic field lines forms the accretion envelope
around the central object. We show that, in case of high mass accretion rates
the envelope becomes closed and optically
thick, which influences the dynamics of the accretion flow and the
observational manifestation of the neutron star hidden behind the envelope.
Particularly, the optically thick accretion envelope results in a multi-color
black-body spectrum originating from the magnetospheric surface. The spectrum
and photon energy flux vary with the viewing angle, which gives rise to
pulsations characterized by high pulsed fraction and typically smooth pulse
profiles. The reprocessing of radiation due to interaction with the envelope
leads to the disappearance of cyclotron scattering features from the spectrum.
We speculate that the super-orbital variability of ultra-luminous X-ray sources
powered by accreting neutron stars can be attributed to precession of the
neutron star due to interaction of magnetic dipole with the accretion disc.Comment: 8 pages, 6 figures, accepted for publication in MNRA
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