1,620 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
CXOU J160103.1-513353: another CCO with a carbon atmosphere?
We report on the analysis of XMM-Newton observations of the central compact
object CXOU J160103.1-513353 located in the center of the non-thermally
emitting supernova remnant (SNR) G330.2+1.0. The X-ray spectrum of the source
is well described with either single-component carbon or two-component hydrogen
atmosphere models. In the latter case, the observed spectrum is dominated by
the emission from a hot component with a temperature ~3.9MK, corresponding to
the emission from a hotspot occupying ~1% of the stellar surface (assuming a
neutron star with mass M = 1.5M, radius of 12 km, and distance of ~5
kpc as determined for the SNR). The statistics of the spectra and obtained
upper limits on the pulsation amplitude expected for a rotating neutron star
with hot spots do not allow us to unambiguously distinguish between these two
scenarios. We discuss, however, that while the non-detection of the pulsations
can be explained by the unfortunate orientation in CXOU J160103.1-513353, this
is not the case when the entire sample of similar objects is considered. We
therefore conclude that the carbon atmosphere scenario is more plausible.Comment: accepted in A&
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
Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars
The aim of this work is to study the possible effects and biases on the
radius constraints for rotation-powered millisecond pulsars when using Thomson
approximation to describe electron scattering in the atmosphere models, instead
of using exact formulation for Compton scattering. We compare the differences
between the two models in the energy spectrum and angular distribution of the
emitted radiation. We also analyse a self-generated synthetic phase-resolved
energy spectrum, based on Compton atmosphere and the most X-ray luminous
rotation-powered millisecond pulsars observed by the Neutron star Interior
Composition ExploreR (NICER). We derive constraints for the neutron star
parameters using both the Compton and Thomson models. The results show that the
method works by reproducing the correct parameters with the Compton model.
However, biases are found in size and the temperature of the emitting hot spot,
when using the Thomson model. The constraints on the radius are still not
significantly changed, and therefore the Thomson model seems to be adequate if
we are interested only in the radius measurements using NICER.Comment: 6 pages, 9 figures, published in A&
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
Absorption features in the spectra of X-ray bursting neutron stars
The discovery of photospheric absorption lines in XMM-Newton spectra of the
X-ray bursting neutron star in EXO0748-676 by Cottam and collaborators allows
us to constrain the neutron star mass-radius ratio from the measured
gravitational redshift. A radius of R=9-12km for a plausible mass range of
M=1.4-1.8Msun was derived by these authors. It has been claimed that the
absorption features stem from gravitationally redshifted (z=0.35) n=2-3 lines
of H- and He-like iron. We investigate this identification and search for
alternatives. We compute LTE and non-LTE neutron-star model atmospheres and
detailed synthetic spectra for a wide range of effective temperatures
(effective temperatures of 1 - 20MK) and different chemical compositions.
We are unable to confirm the identification of the absorption features in the
X-ray spectrum of EXO0748-676 as n=2-3 lines of H- and He-like iron (Fe XXVI
and Fe XXV). These are subordinate lines that are predicted by our models to be
too weak at any effective temperature. It is more likely that the strongest
feature is from the n=2-3 resonance transition in Fe XXIV with a redshift of
z=0.24. Adopting this value yields a larger neutron star radius, namely
R=12-15km for the mass range M=1.4-1.8Msun, favoring a stiff equation-of-state
and excluding mass-radius relations based on exotic matter. Combined with an
estimate of the stellar radius R>12.5km from the work of Oezel and
collaborators, the z=0.24 value provides a minimum neutron-star mass of
M>1.48Msun, instead of M>1.9Msun, when assuming z=0.35.Comment: 8 pages, 17 figure
Model atmospheres of X-ray bursting neutron stars
We present an extended set of model atmospheres and emergent spectra of X-ray
bursting neutron stars in low mass X-ray binaries. Compton scattering is taken
into account. The models were computed in LTE approximation for six different
chemical compositions: pure hydrogen and pure helium atmospheres, and
atmospheres with a solar mix of hydrogen and helium and various heavy elements
abundances: Z = 1, 0.3, 0.1, and 0.01 Z_sun, for three values of gravity, log g
=14.0, 14.3, and 14.6 and for 20 values of relative luminosity l = L/L_Edd in
the range 0.001 - 0.98. The emergent spectra of all models are fitted by
diluted blackbody spectra in the observed RXTE/PCA band 3 - 20 keV and the
corresponding values of color correction factors f_c are presented. We also
show how to use these dependencies to estimate the neutron star's basic
parameters.Comment: 2 pages, 1 figure, conference "Astrophysics of Neutron Stars - 2010"
in honor of M. Ali Alpar, Izmir, Turke
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
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