84 research outputs found
Variability of the H-beta line profiles as an indicator of orbiting bright spots in accretion disks of quasars: a case study of 3C 390.3
Here we show that in the case when double peaked emission lines originate
from outer parts of accretion disk, their variability could be caused by
perturbations in the disk emissivity. In order to test this hypothesis, we
introduced a model of disk perturbing region in the form of a single bright
spot (or flare) by a modification of the power law disk emissivity in
appropriate way. The disk emission was then analyzed using numerical
simulations based on ray-tracing method in Kerr metric and the corresponding
simulated line profiles were obtained. We applied this model to the observed
H-beta line profiles of 3C 390.3 (observed in the period 1995-1999), and
estimated the parameters of both, accretion disk and perturbing region. Our
results show that two large amplitude outbursts of the H-beta line observed in
3C 390.3 could be explained by successive occurrences of two bright spots on
approaching side of the disk. These bright spots are either moving, originating
in the inner regions of the disk and spiralling outwards by crossing small
distances during the period of several years, or stationary. In both cases,
their widths increase with time, indicating that they most likely decay.Comment: 14 pages, 5 figures, 1 table. Accepted for publication in Ap
Using hierarchical octrees in Monte Carlo radiative transfer simulations
A crucial aspect of 3D Monte Carlo radiative transfer is the choice of the
spatial grid used to partition the dusty medium. We critically investigate the
use of octree grids in Monte Carlo dust radiative transfer, with two different
octree construction algorithms (regular and barycentric subdivision) and three
different octree traversal algorithms (top-down, neighbour list, and the
bookkeeping method). In general, regular octree grids need higher levels of
subdivision compared to the barycentric grids for a fixed maximum cell mass
threshold criterion. The total number of grid cells, however, depends on the
geometry of the model. Surprisingly, regular octree grid simulations turn out
to be 10 to 20% more efficient in run time than the barycentric grid
simulations, even for those cases where the latter contain fewer grid cells
than the former. Furthermore, we find that storing neighbour lists for each
cell in an octree, ordered according to decreasing overlap area, is worth the
additional memory and implementation overhead: using neighbour lists can cut
down the grid traversal by 20% compared to the traditional top-down method. In
conclusion, the combination of a regular node subdivision and the neighbour
list method results in the most efficient octree structure for Monte Carlo
radiative transfer simulations.Comment: 6 pages, 1 figure, accepted for publication in Astronomy and
Astrophysic
Spectroscopy and spectropolarimetry of AGN: from observations to modelling
Active galactic nuclei (AGN) are one of the most luminous objects in the
Universe, emitting powerful continuum and line emission across all wavelength
bands. They represent an important link in the investigations of the galaxy
evolution and cosmology. The resolving of the AGN inner structure is still a
difficult task with current instruments, therefore the spectroscopy and
spectropolarimetry are crucial tools to investigate these objects and their
components, such as the properties of the supermassive black hole, the broad
line region, and the dusty torus. In this review, we present the results of the
project "Astrophysical spectroscopy of extragalactic objects", from the
observations, data processing and analysis, to the modelling of different
regions in AGN.Comment: Proceedings of the Serbian Astronomical Conference 201
Photo-centric variability of quasars caused by variations in their inner structure: Consequences on Gaia measurements
We study the photocenter position variability due to variations in the quasar
inner structure. We consider variability in the accretion disk emissivity and
torus structure variability due to different illumination by the central
source. We discuss possible detection of these effects by Gaia. Observations of
the photocenter variability in two AGNs, SDSS J121855+020002 and SDSS
J162011+1724327 have been reported and discussed. With investigation of the
variations in the quasar inner structure we explore how much this effect can
affect the position determination and whether it can be (or not) detected with
Gaia mission. We used (a) a model of a relativistic disk, including the
perturbation that can increase brightness of a part of the disk, and
consequently offset the photocenter position, and (b) a model of a dusty torus
which absorbs and re-emits the incoming radiation from accretion disk. We
estimated the value of the photocenter offset due to these two effects. We
found that perturbations in the inner structure can significantly offset the
photocenter. It depends on the characteristics of perturbation and accretion
disk and structure of the torus. In the case of two considered QSOs the
observed photocenter offsets cannot be explained by variations in the accretion
disk and other effects should be considered. We discussed possibility of
exploding stars very close to the AGN source, and also possibility that there
are two variable sources in the center of these two AGNs that may indicate a
binary super-massive black hole system on a kpc (pc) scale. The Gaia mission
seems to be very perspective, not only for astrometry, but also for exploring
the inner structure of AGNs. We conclude that variations in the quasar inner
structure can affect the observed photocenter (up to several mas). There is a
chance to observe such effect in the case of bright and low-redshifted QSOs.Comment: 12 pages, 8 figures, 3 tables. Accepted for publication in Astronomy
and Astrophysics. Language improved, typos correcte
BAT AGN Spectroscopic Survey - XIX. Type 1 versus type 2 AGN dichotomy from the point of view of ionized outflows
We present a detailed study of ionized outflows in a large sample of ∼650 hard X-ray-detected active galactic neuclei (AGNs). Using optical spectroscopy from the BAT AGN Spectroscopic Survey (BASS), we are able to reveal the faint wings of the [O iii] emission lines associated with outflows covering, for the first time, an unexplored range of low AGN bolometric luminosity at low redshift (z ∼0.05). We test if and how the incidence and velocity of ionized outflow is related to AGN physical parameters: black hole mass, gas column density, Eddington ratio, [O iii], X-ray, and bolometric luminosities. We find a higher occurrence of ionized outflows in type 1.9 (55 per cent) and type 1 AGNs (46 per cent) with respect to type 2 AGNs (24 per cent). While outflows in type 2 AGNs are evenly balanced between blue and red velocity offsets with respect to the [O iii] narrow component, they are almost exclusively blueshifted in type 1 and type 1.9 AGNs. We observe a significant dependence between the outflow occurrence and accretion rate, which becomes relevant at high Eddington ratios. We interpret such behaviour in the framework of covering factor-Eddington ratio dependence. We do not find strong trends of the outflow maximum velocity with AGN physical parameters, as an increase with bolometric luminosity can be only identified when including samples of AGNs at high luminosity and high redshift taken from literature
BAT AGN Spectroscopic Survey – XIX. Type 1 versus type 2 AGN dichotomy from the point of view of ionized outflows
We present a detailed study of ionized outflows in a large sample of ∼650 hard X-ray-detected active galactic neuclei (AGNs). Using optical spectroscopy from the BAT AGN Spectroscopic Survey (BASS), we are able to reveal the faint wings of the [O III] emission lines associated with outflows covering, for the first time, an unexplored range of low AGN bolometric luminosity at low redshift (z ∼0.05). We test if and how the incidence and velocity of ionized outflow is related to AGN physical parameters: black hole mass (M_(BH)), gas column density (N_H), Eddington ratio (λ_(Edd)), [O III], X-ray, and bolometric luminosities. We find a higher occurrence of ionized outflows in type 1.9 (55 per cent) and type 1 AGNs (46 per cent) with respect to type 2 AGNs (24 per cent). While outflows in type 2 AGNs are evenly balanced between blue and red velocity offsets with respect to the [O III] narrow component, they are almost exclusively blueshifted in type 1 and type 1.9 AGNs. We observe a significant dependence between the outflow occurrence and accretion rate, which becomes relevant at high Eddington ratios [log(λ_(Edd)) ≳ −1.7]. We interpret such behaviour in the framework of covering factor-Eddington ratio dependence. We do not find strong trends of the outflow maximum velocity with AGN physical parameters, as an increase with bolometric luminosity can be only identified when including samples of AGNs at high luminosity and high redshift taken from literature
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