3,507 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
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
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
Super-Eddington fluxes during thermonuclear X-ray bursts
It has been known for nearly three decades that the energy spectra of
thermonuclear X-ray bursts are often well-fit by Planck functions with
temperatures so high that they imply a super-Eddington radiative flux at the
emitting surface, even during portions of bursts when there is no evidence of
photospheric radius expansion. This apparent inconsistency is usually set aside
by assuming that the flux is actually sub-Eddington and that the fitted
temperature is so high because the spectrum has been distorted by the
energy-dependent opacity of the atmosphere. Here we show that the spectra
predicted by currently available conventional atmosphere models appear
incompatible with the highest-precision measurements of burst spectra made
using the Rossi X-ray Timing Explorer, such as during the 4U 1820-30 superburst
and a long burst from GX 17+2. In contrast, these measurements are well-fit by
Bose-Einstein spectra with high temperatures and modest chemical potentials.
Such spectra are very similar to Planck spectra. They imply surface radiative
fluxes more than a factor of three larger than the Eddington flux. We find that
segments of many other bursts from many sources are well-fit by similar
Bose-Einstein spectra, suggesting that the radiative flux at the emitting
surface also exceeds the Eddington flux during these segments. We suggest that
burst spectra can closely approximate Bose-Einstein spectra and have fluxes
that exceed the Eddington flux because they are formed by Comptonization in an
extended, low-density radiating gas supported by the outward radiation force
and confined by a tangled magnetic field.Comment: 5 pages, 1 figure. Analyzed additional data, adjusted text, figure,
and references following referee response. Accepted for publication in
ApJLetter
Dark-state suppression and optimization of laser cooling and fluorescence in a trapped alkaline-earth-metal single ion
We study the formation and destabilization of dark states in a single trapped
88Sr+ ion caused by the cooling and repumping laser fields required for Doppler
cooling and fluorescence detection of the ion. By numerically solving the
time-dependent density matrix equations for the eight-level system consisting
of the sublevels of the 5s 2S1/2, 5p 2P1/2, and 4d 2D3/2 states, we analyze the
different types of dark states and how to prevent them in order to maximize the
scattering rate, which is crucial for both the cooling and the detection of the
ion. The influence of the laser linewidths and ion motion on the scattering
rate and the dark resonances is studied. The calculations are then compared
with experimental results obtained with an endcap ion trap system located at
the National Research Council of Canada and found to be in good agreement. The
results are applicable also to other alkaline earth ions and isotopes without
hyperfine structure
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