604 research outputs found
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
Ultra Luminous X-ray sources - new distance indicators?
In this paper we fit the NuSTAR and XMM-Newton data of three sources:
NGC7793~P13, NGC5907~ULX1, and Circinus~ULX5. Our single model contains
emission form non-spherical system: neutron star plus accretion disk directed
towards observer. We obtained the very good fit with the reduced per
degree of freedom equal 1.08 for P13, 1.01 for ULX1, and 1.14 for ULX5.The
normalization of our model constrains the distance to the source. The resulting
distances are , and
~Mpc for P13, ULX1 and ULX5 respectively. The distances
to P13 and ULX5 are in perfect agreement with previous distance measurements to
their host galaxies. Our results confirm that P13, ULX1 and ULX5 may contain
central hot neutron star. When the outgoing emission is computed by integration
over the emitting surface and successfully fitted to the data, then the
resulting model normalization is the direct distance indicator.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Broadband, unpolarized repumping and clearout light sources for Sr single-ion clocks
Future transportable optical clocks require compact and reliable light
sources. Here, broadband, unpolarized repumper and state clearout sources for
Sr+ single-ion optical clocks are reported. These turn-key devices require no
frequency stabilization nor external modulators. They are fiber based,
inexpensive, and compact. Key characteristics for clock operation are
presented, including optical spectra, induced light shifts and required
extinction ratios. Tests with an operating single-ion standard show a clearout
efficiency of 100%. Compared to a laser-based repumper, the achievable
fluorescence rates for ion detection are a few tens of per cent lower. The
resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the
ion system. Similar repumper light sources could be made for Ca+ (866 nm) and
Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure
Model atmospheres and X-ray spectra of iron-rich bursting neutron stars. II. Iron rich Comptonized Spectra
This paper presents the set of plane-parallel model atmosphere equations for
a very hot neutron star (X-ray burst source). The model equations assume both
hydrostatic and radiative equilibrium, and the equation of state of an ideal
gas in local thermodynamic equilibrium (LTE). The equation of radiative
transfer includes terms describing Compton scattering of photons on free
electrons in fully relativistic thermal motion, for photon energies approaching
m_e *c^2. Model equations take into account many bound-free and free-free
energy-dependent opacities of hydrogen, helium, and the iron ions, and also a
dozen bound-bound opacities for the highest ions of iron. We solve model
equations by partial linearisation and the technique of variable Eddington
factors. Large grid of H-He-Fe model atmospheres of X-ray burst sources has
been computed for 10^7 < T_eff < 3*10^7 K, a wide range of surface gravity, and
various iron abundances. We demonstrate that the spectra of X-ray bursters with
iron present in the accreting matter differ significantly from pure H-He
spectra (published in an earlier paper), and also from blackbody spectra.
Comptonized spectra with significant iron abundance are generally closer to
blackbody spectra than spectra of H-He atmospheres. The ratio of color to
effective temperatures in our grid always remains in the range 1.2 < T_c/T_eff
< 1.85. The present grid of model atmospheres and theoretical X-ray spectra
will be used to determine the effective temperatures, radii and M/R ratios of
bursting neutron stars from observational data.Comment: A&A in prin
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