1,095 research outputs found
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
Probing the possibility of hotspots on the central neutron star in HESS J1731-347
The X-ray spectra of the neutron stars located in the centers of supernova
remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models.
These fits yield plausible neutron star sizes for the known or estimated
distances to these supernova remnants. The evidence in favor of the presence of
a pure carbon envelope at the neutron star surface is rather indirect and is
based on the assumption that the emission is generated uniformly by the entire
stellar surface. Although this assumption is supported by the absence of
pulsations, the observational upper limit on the pulsed fraction is not very
stringent. In an attempt to quantify this evidence, we investigate the
possibility that the observed spectrum of the neutron star in HESS J1731-347 is
a combination of the spectra produced in a hydrogen atmosphere of the hotspots
and of the cooler remaining part of the neutron star surface. The lack of
pulsations in this case has to be explained either by a sufficiently small
angle between the neutron star spin axis and the line of sight, or by a
sufficiently small angular distance between the hotspots and the neutron star
rotation poles. As the observed flux from a non-uniformly emitting neutron star
depends on the angular distribution of the radiation emerging from the
atmosphere, we have computed two new grids of pure carbon and pure hydrogen
atmosphere model spectra accounting for Compton scattering. Using new hydrogen
models, we have evaluated the probability of a geometry that leads to a pulsed
fraction below the observed upper limit to be about 8.2 %. Such a geometry thus
seems to be rather improbable but cannot be excluded at this stage.Comment: 8 pages, 14 figures. Accepted for publication in A&
Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921
We analyze the spectral and timing properties of IGR J17498-2921 and the
characteristics of X-ray bursts to constrain the physical processes responsible
for the X-ray production in this class of sources. The broad-band average
spectrum is well-described by thermal Comptonization with an electron
temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson
optical depth \taut ~ 1 in a slab geometry. The slab area corresponds to a
black body radius of R_bb ~9 km. During the outburst, the spectrum stays
remarkably stable with plasma and soft seed photon temperatures and scattering
optical depth that are constant within the errors. This behavior has been
interpreted as indicating that the X-ray emission originates above the neutron
star (NS) surface in a hot slab (either the heated NS surface or the accretion
shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a
period of 2.5 milliseconds up to ~65 keV. The pulsed fraction is consistent
with being constant, i.e. energy independent and has a typical value of 6-7%.
The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV
at a rather small value of ~ -60\mu s with those observed in other accreting
pulsars. The short burst profiles indicate that there is a hydrogen-poor
material at ignition, which suggests either that the accreted material is
hydrogen-deficient, or that the CNO metallicity is up to a factor of about two
times solar. However, the variation in the burst recurrence time as a function
of \dot{m} (inferred from the X-ray flux) is much smaller than predicted by
helium-ignition models.Comment: 9 pages, 8 figures, accepted for publication in A&A. arXiv admin
note: text overlap with arXiv:1012.022
Radiation mechanisms and geometry of Cygnus X-1 in the soft state
We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition
from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in
1996 May and June. The spectra consist of a dominant soft component below ~2
keV and a power-law-like continuum extending to at least ~800 keV. We interpret
them as emission from an optically-thick, cold accretion disc and from an
optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total
available power is dissipated in the corona. We model the soft component by
multi-colour blackbody disc emission taking into account the torque-free
inner-boundary condition. If the disc extends down to the minimum stable orbit,
the ASCA/RXTE data yield the most probable black hole mass of about 10 solar
masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft
state in the upper part of the stable, gas-pressure dominated, accretion-disc
solution branch. The spectrum of the corona is well modelled by repeated
Compton scattering of seed photons from the disc off electrons with a hybrid,
thermal/non-thermal distribution. The electron distribution can be
characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50
keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The
compactness of the corona is between 2 and 7, and a presence of a significant
population of electron-positron pairs is ruled out. We find strong signatures
of Compton reflection from a cold and ionized medium, presumably an accretion
disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum
is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted
to MNRA
Searching for X-ray sources in nearby late-type galaxies with low star formation rates
Late type non-starburst galaxies have been shown to contain X-ray emitting
objects, some being ultraluminous X-ray sources. We report on XMM-Newton
observations of 11 nearby, late-type galaxies previously observed with the
Hubble Space Telescope (HST) in order to find such objects. We found 18 X-ray
sources in or near the optical extent of the galaxies, most being point-like.
If associated with the corresponding galaxies, the source luminosities range
from erg s to erg s. We
found one ultraluminous X-ray source, which is in the galaxy IC 5052, and one
source coincident with the galaxy IC 4662 with a blackbody temperature of
keV that could be a quasi-soft source or a quiescent neutron
star X-ray binary in the Milky Way. One X-ray source, XMMU J205206.0691316,
is extended and coincident with a galaxy cluster visible on an HST image. The
X-ray spectrum of the cluster reveals a redshift of and a
temperature of 3.60.4 keV. The redshift was mainly determined by a cluster
of Fe XXIV lines between the observed energy range keV.Comment: 8 pages, to appear in MNRA
Traditional and new sources of grain protein in the healthy and sustainable Nordic diet
Cereal foods provide carbohydrates and dietary fibre, but also protein. To support the goals of sustainable development, cereal grain proteins should be more efficiently used to replace animal proteins. In the Nordic countries, wheat is the major source of cereal protein, followed by rye, oats, and barley. Although oats have been traditionally consumed as many staple foods in the Nordic countries and new oat-based food concepts are emerging, the potential of oats as a healthy and sustainable protein source is still underused. Oat protein is high in nutritional quality, and oats also contain unique phytochemicals and dietary fibres with proven health effects. Therefore, utilization of traditional wholegrain oat products to replace animal protein sources would increase both health-supporting components and cereal diversity in Nordic diets. While novel oat-based meat, milk, and dairy analogues do not contribute as much to dietary fibre, vitamin and mineral intake as corresponding whole grain products, they provide valuable and effective means to reduce animal protein intake, and thus, the environmental burden
ASTEROID SIZING BY RADIOGALAXY OCCULTATION AT 5 GHZ
Stellar occultations by asteroids observed at visual wavelengths have been an important tool for studying the size and shape of asteroids and for revising the orbital parameters of asteroids. At radio frequencies, a shadow of an asteroid on the Earth is dominated by diffraction effects. Here, we show, for the first time, that a single observation of an occultation of a compact radio source at a frequency of 5 GHz can be used to derive the effective size of the occulting object and to derive the distance between the observer and the center of the occultation path on the Earth. The derived diameter of the occulting object, asteroid (115) Thyra, is 75 +/- 6 km. The observed occultation profile shows features that cannot be explained by diffraction of a single asteroid.Peer reviewe
ON THE GEOMETRY OF THE X-RAY EMITTING REGION IN SEYFERT GALAXIES
For the first time, detailed radiative transfer calculations of Comptonized
X-ray and gamma-ray radiation in a hot pair plasma above a cold accretion disk
are performed using two independent codes and methods. The simulations include
both energy and pair balance as well as reprocessing of the X- and gamma-rays
by the cold disk. We study both plane-parallel coronae as well as active
dissipation regions having shapes of hemispheres and pill boxes located on the
disk surface. It is shown, contrary to earlier claims, that plane-parallel
coronae in pair balance have difficulties in selfconsistently reproducing the
ranges of 2-20 keV spectral slopes, high energy cutoffs, and compactnesses
inferred from observations of type 1 Seyfert galaxies. Instead, the
observations are consistent with the X-rays coming from a number of individual
active regions located on the surface of the disk.
A number of effects such as anisotropic Compton scattering, the reflection
hump, feedback to the soft photon source by reprocessing, and an active region
in pair equilibrium all conspire to produce the observed ranges of X-ray
slopes, high energy cutoffs, and compactnesses. The spread in spectral X-ray
slopes can be due to a spread in the properties of the active regions such as
their compactnesses and their elevations above the disk surface. Simplified
models invoking isotropic Comptonization in spherical clouds are no longer
sufficient when interpreting the data.Comment: 9 pages, 3 postscript figures, figures can be obtained from the
authors via e-mail: [email protected]
Accretion heated atmospheres of X-ray bursting neutron stars
Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in
low-mass X-ray binaries take place during hard persistent states of the
systems. Spectral evolution of these bursts is well described by the atmosphere
model of a passively cooling neutron star when the burst luminosity is high
enough. The observed spectral evolution deviates from the model predictions
when the burst luminosity drops below a critical value of 20-70% of the maximum
luminosity. We suggest that these deviations are induced by the additional
heating of the accreted particles. We present a method for computation of the
neutron star atmosphere models heated by accreted particles assuming that their
energy is released via Coulomb interactions with electrons. We compute the
temperature structures and the emergent spectra of the atmospheres of various
chemical compositions and investigate the dependence of the results on the
other model parameters. We show that the heated atmosphere develops the hot
(20--100 keV) corona-like surface layer cooled by Compton scattering, and the
deeper, almost isothermal optically thick region with a temperature of a few
keV. The emergent spectra deviate strongly from those of undisturbed neutron
star atmospheres, with the main differences being the presence of a high-energy
tail and a strong excess in the low-energy part of the spectrum. They also lack
the iron absorption edge, which is visible in the spectra of undisturbed
low-luminosity atmospheres with solar chemical composition. Using the computed
spectra, we obtained the dependences of the dilution and color-correction
factors as functions of relative luminosities for pure helium and solar
abundance atmospheres. We show that the helium model atmosphere heated by
accretion corresponding to 5% of the Eddington luminosity describes well the
late stages of the X-ray bursts in 4U 1820-30.Comment: 14 pages, 19 figures, published version, some misprints were fixe
Accreting millisecond X-ray pulsars: 10 years of INTEGRAL observations
During the last 10 years, INTEGRAL made a unique contribution to the study of
accreting millisecond X-ray pulsars (AMXPs), discovering three of the 14
sources now known of this class. Besides increasing the number of known AMXPs,
INTEGRAL also carried out observations of these objects above 20 keV,
substantially advancing our understanding of their behaviour. We present here a
review of all the AMXPs observed with INTEGRAL and discuss the physical
interpretation of their behaviour in the X-ray domain. We focus in particular
on the lightcurve profile during outburst, as well as the timing, spectral, and
thermonuclear type-I X-ray bursts properties.Comment: 8 pages, 8 figures. Proceedings of "An INTEGRAL view of the
high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October
15-19, 2012, Paris, Franc
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