125 research outputs found
The structure and radiation spectra of illuminated accretion discs in AGN. I. Moderate illumination
We present detailed computations of the vertical structure of an accretion
disc illuminated by hard X-ray radiation with the code {\sc titan-noar}
suitable for Compton thick media. The energy generated via accretion is
dissipated partially in the cold disc as well as in the X-ray source. We study
the differences between the case where the X-ray source is in the form of a
lamp post above the accretion disc and the case of a heavy corona. We consider
radiative heating via Comptonization together with heating via photo-absorption
on numerous heavy elements as carbon, oxygen, silicon, iron. The transfer in
lines is precisely calculated. A better description of the heating/cooling
through the inclusion of line transfer, a correct description of the
temperature in the deeper layers, a correct description of the entire disc
vertical structure, as well as the study of the possible coronal pressure
effect, constitute an improvement in comparison to previous works. We show that
exact calculations of hydrostatic equilibrium and determination of the disc
thickness has a crucial impact on the optical depth of the hot illuminated
zone. We assume a moderate illumination where the viscous flux equals the X-ray
radiation flux. A highly ionized skin is created in the lamp post model, with
the outgoing spectrum containing many emission lines and ionization edges in
emission or absorption in the soft X-ray domain, as well as an iron line at
keV consisting of a blend of low ionization line from the deepest
layers and hydrogen and helium like resonance line from the upper layers, and
almost no absorption edge, contrary to the case of a slab of constant density.A
full heavy corona completely suppresses the highly ionized zone on the top of
the accretion disc and in such case the spectrum is featureless.Comment: 16 pages, 20 figures, corrected two sentences, accepted by MNRA
Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region
We explore the conditions for the thermal instability to operate in the
mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold
media are known to coexist. The photoionisation Cloudy calculations are
performed for different physical states of plasma. We neglect the dynamics of
the material and concentrate on the study of the parameter ranges where the
thermal instability may operate, taking into account the past history of Sgr A*
bolometric luminosity. We show that the thermal instability does not operate at
the present very low level of the Sgr A* activity. However, Sgr A* was much
more luminous in the past. For the highest luminosity states the two-phase
medium can be created up to 1.4 pc from the centre. The presence of dust grains
tends to suppress the instability, but the dust is destroyed in the presence of
strong radiation field and hot plasma. The clumpiness is thus induced in the
high activity period, and the cooling/heating timescales are long enough to
preserve later the past multi-phase structure. The instability enhances the
clumpiness of the mini-spiral medium and creates a possibility of episodes of
enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the
range of masses and sizes of clouds; under the conditions of Sgr A*, the likely
values come out - for the cloud typical mass.Comment: Accepted for publication in MNRAS, 10 pages, 7 figure
Antibiotic consumption in surgical site infections treatment in Polish hospitals according to data from the program of active surveillance of hospital acquired infections
Measurements of accretion disc corona size in LMXB: consequences for Comptonization and LMXB models
We present results of measurements of the radial extent of the accretion disc
corona in low mass X-ray binaries. These results prove conclusively the
extended nature of the ADC, with radial extent varying from 20,000 km in the
faintest sources to 700,000 km in the brightest, a substantial fraction of the
accretion disc radius, typically 15%. This result rules out the Eastern model
for LMXB which is extensively used, in which the Comptonizing region is a small
central region. The ADC size depends strongly on the 1 - 30 keV source
luminosity via a simple relationship r_ADC = L^{0.88 +/- 0.16} (99% confidence)
close to a simple proportionality. We also present limited evidence that the
ADC size agrees with the Compton radius r_C, or maximum radius for hydrostatic
equilibrium. The results are consistent with models in which an extended ADC is
formed by illumination of the disc by the central source. The dependence on
luminosity may reflect the known decrease of coronal temperature as the source
luminosity increases leading to an increase of r_C. The extended nature of the
ADC means that the seed photons for Comptonization must consist of emission
from the disc to the same radial extent as the corona, providing copious
supplies of soft seed photons. We demonstrate the importance of the size of the
ADC to the correct description of Comptonization, and derive the Comptonized
spectrum of a LMXB based on thermal Comptonization of these seed photons and
show that this differs fundamentally from that of the Eastern model which
assumes a cut-off below 1 keV. Finally, we argue that our results are
inconsistent with the assumption often made that the X-ray emission of
accreting Black Holes and Neutron Stars has a common mechanism depending on the
properties of the accretion flow only.Comment: 9 pages, 5 figures, MNRAS in press - final versio
Vertical dissipation profiles and the photosphere location in thin and slim accretion disks
We calculate optically thick but geometrically thin (and slim) accretion disk
models and perform a ray-tracing of photons (in the Kerr geometry) to calculate
the observed disk spectra. Previously, it was a common practice to ray-trace
photons assuming that they are emitted from the Kerr geometry equatorial plane,
z = 0. We show that the spectra calculated with this assumption differ from
these calculated under the assumption that photons are emitted from the actual
surface of the disc, z = H(r). This implies that a knowledge of the location of
the thin disks effective photosphere is relevant for calculating the spectra.
In this paper we investigate, in terms of a simple toy model, a possible
influence of the (unknown, and therefore ad hoc assumed) vertical dissipation
profiles on the vertical structure of the disk and thus on the location of the
effective photosphere, and on the observed spectra. For disks with moderate and
high mass accretion rates (\dot m>0.01\dot m_C) we find that the photosphere
location in the inner, radiation pressure dominated, disk region (where most of
the radiation comes from) does not depend on the dissipation profile and
therefore emerging disk spectra are insensitive to the choice of the
dissipation function. For lower accretion rates the photosphere location
depends on the assumed vertical dissipation profile down to the disk inner
edge, but the dependence is very weak and thus of minor importance. We conclude
that the spectra of optically thick accretion disks around black holes should
be calculated with the ray-tracing from the effective photosphere and that,
fortunately, the choice of a particular vertical dissipation profile does not
substantially influence the calculated spectrum.Comment: 7 pages, 7 figure
EXOPOLYSACCHARIDE-PRODUCING LACTIC ACID BACTERIA – HEALTH-PROMOTING PROPERTIES AND APPLICATION IN THE DAIRY INDUSTRY
Absorption spectrum of the quasar HS1603+3820 I. Observations and data analysis
We present the analysis of multi-wavelength observations of bright quasar
HS1603+3820: the optical data taken with the MMT and Keck telescopes, and X-ray
data obtained with the Chandra X-ray Observatory. The optical spectra contain a
very large number of absorption lines from numerous heavy elements. We derived
X-ray properties of HS1603. The quasar has the optical-to-X-ray slope index
alpha_ox of 1.70, which is on the high end of the typical range for radio quiet
QSOs. We found 49 individual heavy element absorption clouds, which can be
grouped into eleven distinct systems. We determined column densities and
redshifts of the individual components. Absorbers from the associated system
which is likely spatially closest to the QSO show large CIV to HI column
density ratio, reaching ca.20.Comment: Accepted for publication in A&A, 17 pages, 11 figures, 5 table
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
- …