754 research outputs found
Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions
We present a complete set of analytic fits to the non-relativistic
photoionization cross sections for the ground states of atoms and ions of
elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization
thresholds, the fits are based on the Opacity Project theoretical cross
sections interpolated and smoothed over resonances. At higher energies, the
fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections.Comment: 24 pages including Postscript figures and tables, uses aaspp4.sty,
accepted for publication in Astrophysical Journal. Misprint in Eq.(1) is
correcte
High-Resolution Spectroscopy from 3050 to 10000 A of the HDF-S QSO J2233-606 with UVES at the ESO VLT
We report on high-resolution observations () of the Hubble
Deep Field South QSO J2233-606 obtained with the VLT UV-Visual Echelle
Spectrograph (UVES). We present spectral data for the wavelength region \AA. The ratio of the final spectrum is about 50 per
resolution element at 4000 \AA, 90 at 5000 \AA, 80 at 6000 \AA, 40 at 8000 \AA.
Redshifts, column densities and Doppler widths of the absorption features have
been determined with Voigt-profile fitting. A total of 621 lines have been
measured. In particular 270 Ly-alpha lines, 41 Ly-beta and 24 systems
containing metal lines have been identified. Together with other data in the
literature, the present spectrum confirms that the evolution of the number
density of Ly-alpha lines with \huno has an upturn at .Comment: 34 pages Latex, with 3 PostScript figures. Astronomical Journal, in
press. A few revised upper limit
Atomic Data for Astrophysics. I. Radiative Recombination Rates for H-like, He-like, Li-like, and Na-like Ions over a Broad Range of Temperature
We present new calculations and analytic fits to the rates of radiative recombination towards H-like, He-like, Li-like and Na-like ions of all elements from H through Zn (Z=30). The fits are valid over a wide range of temperature, from 3 K to 109 K
High-Resolution X-ray Spectroscopy of the Interstellar Medium: Structure at the Oxygen Absorption Edge
(Abbrev.) We present high-resolution spectroscopy of the oxygen K-shell
interstellar absorption edge in 7 X-ray binaries using the HETGS onboard
Chandra. Using the brightest sources as templates, we found a best-fit model of
2 absorption edges and 5 Gaussian absorption lines. All of these features can
be explained by the recent predictions of K-shell absorption from neutral and
ionized atomic oxygen. We identify the K alpha and K beta absorption lines from
neutral oxygen, as well as the S=3/2 absorption edge. The expected S=1/2 edge
is not detected in these data due to overlap with instrumental features. We
also identify the K alpha absorption lines from singly and doubly ionized
oxygen. The OI K alpha absorption line is used as a benchmark with which to
adjust the absolute wavelength scale for theoretical predictions of the
absorption cross-sections. We find that shifts of 30-50 mA are required,
consistent with differences previously noticed from comparisons of the theory
with laboratory measurements. Significant oxygen features from dust or
molecular components, as suggested in previous studies, are not required by our
HETGS spectra. With these spectra, we can begin to measure the large-scale
properties of the ISM. We place a limit on the velocity dispersion of the
neutral lines of <200 km s^{-1}, consistent with measurements at other
wavelengths. We also make the first measurement of the oxygen ionization
fractions in the ISM. We constrain the interstellar ratio of OII/OI to ~0.1 and
the ratio of OIII/OI to <0.1.Comment: 12 pages, 8 figures, accepted for publication in the Astrophysical
Journal (Vol. 612, September 1 issue
Scattering by Interstellar Dust Grains. II. X-Rays
Scattering and absorption of X-rays by interstellar dust is calculated for a
model consisting of carbonaceous grains and amorphous silicate grains. The
calculations employ realistic dielectric functions with structure near X-ray
absorption edges, with resulting features in absorption, scattering, and
extinction.
Differential scattering cross sections are calculated for energies between
0.3 and 10 keV. The median scattering angle is given as a function of energy,
and simple but accurate approximations are found for the X-ray scattering
properties of the dust mixture, as well as for the angular distribution of the
scattered X-ray halo for dust with simple spatial distributions. Observational
estimates of the X-ray scattering optical depth are compared to model
predictions. Observations of X-ray halos to test interstellar dust grain models
are best carried out using extragalactic point sources.Comment: ApJ, accepted. 27 pages, 12 figures. Much of this material was
previously presented in astro-ph/0304060v1,v2,v3 but has been separated into
the present article following recommendation by the refere
The Observability of Metal Lines Associated with the Lyman-alpha Forest
We develop a prescription for characterizing the strengths of metal lines
associated with Lyman-alpha forest absorbers (LYFAs) of a given neutral
hydrogen column density N_HI and metallicity [Fe/H]. This Line Observability
Index (LOX) is line-specific and translates, for weak lines, into a measure of
the equivalent width. It can be evaluated quickly for thousands of transitions
within the framework of a given model of the Lyman-alpha forest, providing a
ranking of the lines in terms of their strengths and enabling model builders to
select the lines that should be detectable in observed spectra of a given
resolution and signal-to-noise ratio. We compute the LOX for a large number of
elements and transitions in two cosmological models of the Lyman-alpha forest
at z=3 derived from a hydrodynamic simulation of structure formation, and we
discuss how the LOX depends on redshift and on model parameters such as the
mean baryonic density and radiation field. We find that the OVI (1032,1038)
doublet is the best probe of the metallicity in low column density LYFAs N_{HI}
\approx 10^{14.5} cm^{-2}). Metallicities down to [O/H] \sim -3 ([Fe/H] \sim
-3.5 with the assumed [O/Fe] ratio) yield OVI absorption features that should
be detectable in current high-quality spectra, provided that the expected
position of the OVI feature is not contaminated by HI absorption. The strongest
transitions in lower ionisation states of oxygen are OV(630), OIV(788), and
OIII(833), and are likely to be detected with next generation UV instruments.
Of the lines with rest wavelengths \lambda_r > 1216, which can potentially be
observed redwards of the \lya forest, the CIV(1548,1551) doublet is expected to
dominate in all LYFAs, regardless of the value of N_HI.Comment: Substantially revised version: larger line database, additional
cosmological model analyzed. Accepted for Ap
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
Modeling of the Interaction of GRB Prompt Emission with the Circumburst Medium
We present methodology and results of numerical modeling of the interaction
of GRB prompt emission with the circumburst medium using a modified version of
the multi-group radiation hydrocode STELLA. The modification includes the
nonstationary photoionization, the photoionization heating and the Compton
heating along with the hydrodynamics and radiation transfer. The lightcurves
and spectra of the outcoming gamma-ray, X-ray and optical emission are
calculated for a set of models (shells) of the circumburst environment, which
differ in dimensions, density, density profile, composition, temperature. In
some cases total bolometric and optical luminosities can reach 10^47 and 10^43
erg/s respectively. These effects can be responsible for irregularities which
are seen on lightcurves of some GRB's X-ray and optical afterglows.Comment: 27 pages, 16 colour figures, this version is translated by authors,
so it differs from that, which is published in Astronomy Letter
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