953 research outputs found
Mass and radius estimation for the neutron star in X-ray burster 4U 1820-30
We present a new method for determining masses and radii of neutron stars
residing in thermo-nuclear X-ray burst sources. To illustrate this method we
apply it to a burst from the source 4U 1820-30 recorded by the Rossi X-Ray
Timing Explorer. Fits of the observed X-ray spectra to grids of Comptonised
model atmospheres yield estimates for the mass and radius of the neutron star,
M=1.3 \pm 0.6 M_sol and R=11^+3_-2 km, respectively.Comment: MNRAS in prin
On the Relevance of Compton Scattering for the Soft X-ray Spectra of Hot DA White Dwarfs
We re-examine the effects of Compton scattering on the emergent spectra of
hot DA white dwarfs in the soft X-ray range. Earlier studies have implied that
sensitive X-ray observations at wavelengths \AA might be capable
of probing the flux deficits predicted by the redistribution of
electron-scattered X-ray photons toward longer wavelengths. We adopt two
independent numerical approaches to the inclusion of Compton scattering in the
computation of pure hydrogen atmospheres in hydrostatic equilibrium. One
employs the Kompaneets diffusion approximation formalism, while the other uses
the cross-sections and redistribution functions of Guilbert. Models and
emergent spectra are computed for stellar parameters representative of HZ 43
and Sirius B, and for models with an effective temperature K. The differences between emergent spectra computed for Compton and
Thomson scattering cases are completely negligible in the case of both HZ 43
and Sirius B models, and are also negligible for all practical purposes for
models with temperatures as high as K. Models of the
soft X-ray flux from these stars are instead dominated by uncertainties in
their fundamental parameters.Comment: 7 pages, 5 figures, accepted for publication in A&
About the global magnetic fields of stars
We present a review of observations of the stellar longitudinal (effective)
magnetic field () and its properties. This paper also discusses
contemporary views on the origin, evolution and structure of .Comment: Plenary report, The Gamov International Astronomical Conference, XIII
ODESSA, 19-25 August, 2013, Odessa, Ukrain
Theoretical UBVRI colors of iron core white dwarfs
We explore photometric properties of hypothetical iron core white dwarfs and
compute their expected colors in UBVRI Johnson broadband system. Atmospheres of
iron core WDs in this paper consist of pure iron covered by a pure hydrogen
layer of an arbitrary column mass. LTE model atmospheres and theoretical
spectra are calculated on the basis of Los Alamos TOPS opacities and the
equation of state from the OPAL project, suitable for nonideal Fe and H gases.
We have also computed UBVRI colors of the models and determined an area on the
B-V vs. U-B and U-B vs. V-I planes, occupied by both pure Fe, and pure H model
atmospheres of WD stars. Finally, we search for iron core white dwarf
candidates in the available literature.Comment: 13 pages, 12 figures, Astronomy & Astrophysics (2003) in prin
Disk emission and atmospheric absorption lines in black hole candidate 4U 1630-472
We re-analyzed SUZAKU data of the black hole candidate 4U 1630-472 being in
the high/soft state. We show that the continuum X-ray spectrum of 4U 1630-472
with iron absorption lines can be satisfactorily modeled by the spectrum from
an accretion disk atmosphere. Absorption lines of highly ionized iron
originating in hot accretion disk atmosphere can be an alternative or
complementary explanation to the wind model usually favored for these type of
sources. We model continuum and line spectra using a single model. Absorption
lines of highly ionized iron can origin in upper parts of the disk atmosphere
which is intrinsically hot due to high disk temperature. Iron line profiles
computed with natural, thermal and pressure broadenings match very well
observations. We showed that the accretion disk atmosphere can effectively
produce iron absorption lines observed in 4U 1630-472 spectrum. Absorption line
arising in accretion disk atmosphere is the important part of the observed line
profile, even if there are also other mechanisms responsible for the absorption
features. Nevertheless, the wind theory can be an artifact of the fitting
procedure, when the continuum and lines are fitted as separate model
components.Comment: 10 pages, 4 figures, accepted by Astronomy and Astrophysic
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