124 research outputs found
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
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 &
The LOFT perspective on neutron star thermonuclear bursts
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of thermonuclear X-ray bursts on
accreting neutron stars. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
Evolution of the X-ray spectrum in the flare model of Active Galactic Nuclei
Nayakshin & Kazanas (2002) have considered the time-dependent illumination of
an accretion disc in Active Galactic Nuclei, in the lamppost model. We extend
their study to the flare model, which postulates the release of a large X-ray
flux above a small region of the accretion disc. A fundamental difference with
the lamppost model is that the region of the disc below the flare is not
illuminated before the onset of the flare.
A few test models show that the spectrum which follows immediately the
increase in continuum flux should display the characteristics of a highly
illuminated but dense gas, i.e. very intense X-ray emission lines and
ionization edges in the soft X-ray range. The behaviour of the iron line is
different in the case of a "moderate" and a ``strong'' flare: for a moderate
flare, the spectrum displays a neutral component of the Fe K line at
6.4 keV, gradually leading to more highly ionized lines. For a strong flare,
the lines are already emitted by FeXXV (around 6.7 keV) after the onset, and
have an equivalent width of several hundreds of eV. We find that the observed
correlations between , , and the X-ray flux, are well accounted by a
combination of flares having not achieved pressure equilibrium, strongly
suggesting that the observed spectrum is dominated by regions in non-pressure
equilibrium, typical of the onset of the flares. Finally a flare being confined
to a small region of the disc, the spectral lines should be narrow (except for
a weak Compton broadening), Doppler shifted, and moving.Comment: 14 pages, 13 figures, accepted in A & A, english corrected versio
Leaving the ISCO: the inner edge of a black-hole accretion disk at various luminosities
The "radiation inner edge" of an accretion disk is defined as the inner
boundary of the region from which most of the luminosity emerges. Similarly,
the "reflection edge" is the smallest radius capable of producing a significant
X-ray reflection of the fluorescent iron line. For black hole accretion disks
with very sub-Eddington luminosities these and all other "inner edges" locate
at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner
edge of the black hole accretion disk. However, even for moderate luminosities,
there is no such unique inner edge as differently defined edges locate at
different places. Several of them are significantly closer to the black hole
than ISCO. The differences grow with the increasing luminosity. For nearly
Eddington luminosities, they are so huge that the notion of the inner edge
losses all practical significance.Comment: 12 pages, 15 figures, submitted to A&
The structure and radiation spectra of illuminated accretion disks in AGN. II. Flare/spot model of X-ray variability
We discuss a model of X-ray variability of active galactic nuclei (AGN). We
consider multiple spots which originate on the surface of an accretion disk
following intense irradiation by coronal flares. The spots move with the disk
around the central black hole and eventually decay while new spots continuously
emerge. We construct time sequences of the spectra of the spotted disk and
compute the corresponding energy-dependent fractional variability amplitude. We
explore the dependence on the disk inclination and other model parameters. AGN
seen at higher inclination with respect to the observer, such as Seyfert 2
galaxies, are expected to have fractional variability amplitude of the direct
emission by a factor of a few higher than objects seen face on, such as the
Seyfert 1s.Comment: Astronomy and Astrophysics (in press
Universal spectral shape of high accretion rate AGN
The spectra of quasars and NLS1 galaxies show surprising similarity in their
spectral shape. They seem to scale only with the accretion rate. This is in
contradiction with the simple expectations from the standard disk model which
predicts lower disk temperature for higher black hole mass. Here we consider
two mechanisms modifying the disk spectrum: the irradiation of the outer disk
due to the scattering of the flux by the extended ionized medium (warm absorber
and the development of the warm Comptonizing disk skin under the effect of the
radiation pressure instability. Those two mechanisms seem to lead to a spectrum
which indeed roughly scales, as observed, only with the accretion rate. The
scenario applies only to objects with relatively high luminosity to the
Eddington luminosity ratio for which disk evaporation is inefficient.Comment: 14 pages, 14 figures, 1 table, accepted for publication in A&
Characterization of the emitting and absorbing media around the nucleus of the active galaxy UGC11763 using XMM-Newton data
Aims. The detailed analysis of all data taken by the XMM-Newton satellite of
UGC11763 to characterize the different components that are emitting and
absorbing radiation in the vicinity of the active nucleus.
Methods. The continuum emission was studied through the EPIC spectra taking
profit of the spectral range of these cameras. The high resolution RGS spectra
were analyzed in order to characterize the absorbing features and the emission
line features that arise in the spectra of this source.
Results. A power law with a photon index \Gamma = 1.72^{+0.03}_{-0.01}
accounts for the continuum emission of this source in the hard X-rays from 10
down to 1 keV. At lower energies, a black body model with kT= 0.100\pm 0.003
keV provides a good description of the observed soft excess. The absorption
signatures in the spectra of UGC11763 are consistent with the presence of a two
phase ionized material (log U=1.65^{+0.07}_{-0.08}; 2.6\pm 0.1 and log N_{H} =
21.2\pm 0.2; 21.51\pm 0.01 cm^{-2}, respectively) in the line of sight. The
physical conditions found are consistent with the two phases being in pressure
equilibrium. The low ionization component is more ionized than typically found
for warm absorbers in other Seyfert 1 galaxies. There are also signatures of
some emission lines: Ovii He(r), Ovii He(f), a blend of the
Neix He triplet and Fexviii at \lambda 17.5 \AA.Comment: 11 pages, 10 figures, accepted to be published by A&
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