153 research outputs found
Second birth rates across Europe: interactions between women’s level of education and child care enrolment
Fertility differences in Europe are to a large extent due to parity progression after the first child. We therefore use data from the third round of the European Social Survey to investigate second-birth rates in 23 countries. Focusing on the role of education level and child care availability, we argue that child care provision is an important determinant of the opportunity cost of parity progression, particularly for highly educated women. We find that in countries where the highly educated have lower second birth rates than the less educated, total fertility tends to be low, and vice versa. In addition, the effect of the timing of the first child appears to be mediated by education level and child care availability: in countries where large proportions of young children attend formal child care, the more highly educated exhibit much higher second-birth rates, while child care availability does not affect parity progression for the less educated.
The formation of the coronal flow/ADAF
We develop a new method to describe the accretion flow in the corona above a
thin disk around a black hole in vertical and radial extent. The model is based
on the same physics as the earlier one-zone model, but now modified including
inflow and outflow of mass, energy and angular momentum from and towards
neighboring zones. We determine the radially extended coronal flow for
different mass flow rates in the cool disk resulting in the truncation of the
thin disk at different distance from the black hole. Our computations show how
the accretion flow gradually changes to a pure vertically extended coronal or
advection-dominated accretion flow (ADAF). Different regimes of solutions are
discussed. For some cases wind loss causes an essential reduction of the mass
flow.Comment: 8 pages, 4 figures, accepted for publication in A&
The change from accretion via a thin disk to a coronal flow: dependence on the viscosity of the hot gas
We study the transition from the geometrically thin disk to the hot coronal
flow for accretion onto black holes. The efficiency of evaporation determines
the truncation of the geometrically thin disk as a function of the black hole
mass and the mass flow rate in the outer disk. The physics of the evaporation
was already described in detail in earlier work (Meyer et al. 2000b). We show
now that the value of the viscosity parameter for the coronal gas has a strong
influence on the evaporation efficiency. For smaller values of the viscosity
evaporation is less efficient. For a given mass flow rate from outside the
geometrically thin disk then extends farther inward. Spectral transitions
between soft and hard states are then expected for different mass flow rates in
the outer disk. The physics is the same for the cases of stellar and
supermassive black holes systems.Comment: 6 pages, 1 figure, accepted for publication in A&
Thermal instability in X-ray photoionized media in Active Galactic Nuclei: Influence on the gas structure and spectral features
A photoionized gas in thermal equilibrium can display a thermal instability,
with 3 or more solutions in the multi-branch region of the S-shape curve giving
the temperature versus the radiation-to-gas-pressure ratio. Many studies have
been devoted to this curve and to its dependence on different parameters,
always in the optically thin case. The purpose of our study is the thermal
instability in optically thick, stratified media, in total pressure
equilibrium. We have developped a new algorithm to select the hot/cold stable
solution, and thereof to compute a fully consistent photoionization model. We
have implemented it in the TITAN code and computed a set of models encompassing
the range of conditions valid for the Warm Absorber in Active Galactic Nuclei.
We have demonstrated that the thermal instability problem is quite different in
thin or thick media. In thick media the spectral distribution changes as the
radiation progresses inside the ionized gas. This has observational
implications in the emitted/absorbed spectra, ionization states, and
variability. However impossible to know what solution the plasma will adopt
when attaining the multi-solutions regime, we expect the emitted/absorbed
spectrum to be intermediate between those resulting from pure cold and hot
models. Large spectral fluctuations corresponding to the onset of a cold/hot
solution could be observed in timescales of the order of the dynamical time. A
strong turbulence implying supersonic velocities should permanently exist in
the multi-branch region of thick, stratified, pressure equilibrium media.Comment: LaTeX file: 18 pages, including 14 figures. Accepted for publication
in Astronomy & Astrophysic
Role of emission angular directionality in spin determination of accreting black holes with broad iron line
Spin of an accreting black hole can be determined by spectroscopy of the
emission and absorption features produced in the inner regions of an accretion
disc. We discuss the method employing the relativistic line profiles of iron in
the X-ray domain, where the emergent spectrum is blurred by general
relativistic effects. Precision of spectra fitting procedure could be
compromised by inappropriate account of the angular distribution of the disc
emission. Often a unique profile is assumed, invariable over the entire range
of radii in the disc and energy in the spectral band. We study how sensitive
the spin determination is to the assumptions about the intrinsic angular
distribution of the emitted photons. We find that the uncertainty of the
directional emission distribution translates to 20% uncertainty in
determination of the marginally stable orbit. By assuming a rotating black hole
in the centre of an accretion disc, we perform radiation transfer computations
of an X-ray irradiated disc atmosphere to determine the directionality of
outgoing X-rays in the 2-10 keV energy band. We implemented the simulation
results as a new extension to the KY software package for X-ray spectra fitting
of relativistic accretion disc models. Although the parameter space is rather
complex, leading to a rich variety of possible outcomes, we find that on
average the isotropic directionality reproduces our model data to the best
precision. Our results also suggest that an improper usage of limb darkening
can partly mimic a steeper profile of radial emissivity. We demonstrate these
results on the case of XMM-Newton observation of the Seyfert galaxy
MCG-6-30-15, for which we construct confidence levels of chi squared
statistics, and on the simulated data for the future X-ray IXO mission.Comment: 18 pages, 18 figures, accepted to Astronomy and Astrophysic
Reprocessing of X-rays in AGN. I. Plane parallel geometry -- test of pressure equilibrium
We present a model of the vertical stratification and the spectra of an
irradiated medium under the assumption of constant pressure. Such a solution
has properties intermediate between constant density models and hydrostatic
equilibrium models, and it may represent a flattened configuration of gas
clumps accreting onto the central black hole. Such a medium develops a hot
skin, thicker than hydrostatic models, but thinner than constant density
models, under comparable irradiation. The range of theoretical values of the
alpha_ox index is comparable to those from hydrostatic models and both are
close to the observed values for Seyfert galaxies but lower than in quasars.
The amount of X-ray Compton reflection is consistent with the observed range.
The characteristic property of the model is a frequently multicomponent iron K
alpha line.Comment: accepted for publication in Astronomy and Astrophysic
Flare-induced fountains and buried flares in AGN
We discuss the local physical changes at the surface of an AGN accretion disk
after the onset of a magnetic flare. The X-ray irradiation by a flare creates a
hot spot at the disk surface where the plasma both heats up and expands in the
vertical direction in order to regain the hydrostatic equilibrium. Assuming
that the magnetic loop causing the flare is anchored deeply within the disk
interior, we derive analytical estimates for the vertical dimension H_hot and
the optical depth tau_es of the heated atmosphere as a function of the position
within the spot. We perform computations for various values of the accretion
rate dm/dt, the fraction f_cor of radiation dissipated within the disk corona,
and the covering factor f_cover of the disk surface with flare-illuminated
patches. It turns out that generally we can distinguish three characteristic
radial zones within the disk showing a qualitatively different behavior of the
heated material. In the innermost regions of the disk (inner zone) the
expansion of the disk material is restricted by strong gravitational forces.
Further out, the flare source, initially above the disk, soon becomes embedded
by the expanding disk atmosphere. At these intermediate disk radii (middle
zone) the material is optically thick thus greatly modifying the observed
radiation by multiple Compton scattering. We show exemplary spectra models
obtained from Monte Carlo simulations illustrating the trends. In the outermost
regions of the disk (outer zone) the expanding material is optically thin and
its influence on the observed spectra is smaller but pressure gradients in
radial directions should cause the development of a fountain-like dynamical
structure around the flare source. We discuss the observational consequences of
our results.Comment: 12 pages, 14 figures, accepted by Astronomy & Astrophysic
X-ray He-like ions diagnostics: New Computations for Photoionized Plasmas: I. preliminary considerations
Using the new version of the photoionization code Titan designed for
plane-parallel photoionized thick hot media, which is unprecedented from the
point of view of line transfer, we have undertaken a study of the influence of
different parameters on the He-like and H-like emission of a medium
photoionized by an X-ray source. We explain why in modelling the emitting
medium it is important to solve in a self-consistent way the thermal and
ionization equilibria and to take into account the interconnection between the
different ions. We give the equivalent widths of the sum of the He-like
triplets and the triplet intensity ratios and , for the most important
He-like ions, for a range of density, column density, and ionization parameter,
in the case of constant density media. We show that the line intensities from a
given ion can be accounted for, either by small values of both the column
density and of the ionization parameter, or by large values of both quantities,
and it is necessary to take into account several ions to disentangle these
possibilities. We show also that a "pure recombination spectrum" almost never
exists in a photoionized medium: either it is thin, and resonance lines are
formed by radiative excitation, or it is thick, and free-bound absorption
destroys the resonance photons as they undergo resonant diffusion.Comment: 19 pages, 14 figures, accepted in A &
Constraints for the accretion disk evaporation rate in AGN from the existence of the Broad Line Region
We analyze the consequences of the hypothesis that the formation of the Broad
Line Region is intrinsically connected with the existence of the cold accretion
disk. We assume that the Broad Line Region radius is well estimated by the
formula of Kaspi et al. (2000). We consider three models of the disappearance
of the inner disk which limit the existence of the Broad Line Region: (i)
classical ADAF approach, i.e. the inner hot flow develops whenever it can exist
(ii) disk evaporation model of Meyer & Meyer-Hofmeister (2002) (iii)
generalized disk evaporation model of Rozanska & Czerny (2000b). For each of
the models, we determine the minimum value of the Eddington ratio and the
maximum value of the broad line widths as functions of the viscosity parameter
alpha and the magnetic field parameter beta. We compare the predicted parameter
space with observations of several AGN. Weak dependence of the maximum value of
the FWHM and minimum value of the Eddington ratio on the black hole mass in our
sample is noticeable. It seems to favor the description of the cold disk/hot
inner flow transition as in the classical ADAF approach than with the model of
disk evaporation due to conduction between the disk and accreting corona.Comment: A&A, 428, 39 (2004
- …