217 research outputs found
A Look at What Is (and Isn't) Known About Quasar Broad Line Regions and How Narrow-Line Seyfert 1 Galaxies Fit In
The evidence is reviewed that the Broad Line Region (BLR) probably has two
distinct components located at about the same distance from the central black
hole. One component, BLR II, is optically-thick, low-ionization emission at
least some of which arises from a disc and the other, BLR I, is probably
optically-thin emission from a more spherically symmetric halo or atmosphere.
The high Fe II/H-beta ratios seen in Narrow-Line Seyfert 1 galaxies (NLS1s) are
not due to strong Fe II emission, as is commonly thought, but to unusually weak
Balmer emission, probably caused by higher densities. NLS1s probably differ
from non-NLS1s because of the higher density of gas near the black hole. This
produces a higher accretion rate, a denser BLR, and a view of the central
regions that is more face-on.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Obscuration model of Variability in AGN
There are strong suggestions that the disk-like accretion flow onto massive
black hole in AGN is disrupted in its innermost part (10-100 Rg), possibly due
to the radiation pressure instability. It may form a hot optically thin quasi
spherical (ADAF) flow surrounded by or containing denser clouds due to the
disruption of the disk. Such clouds might be optically thick, with a Thompson
depth of order of 10 or more. Within the frame of this cloud scenario
(Collin-Souffrin et al. 1996, Czerny & Dumont 1998), obscuration events are
expected and the effect would be seen as a variability. We consider expected
random variability due to statistical dispersion in location of clouds along
the line of sight for a constant covering factor. We discuss a simple
analytical toy model which provides us with the estimates of the mean spectral
properties and variability amplitude of AGN, and we support them with radiative
transfer computations done with the use of TITAN code of Dumont, Abrassart &
Collin (1999) and NOAR code of Abrassart (1999).Comment: to appear in Proc. of 5th Compton Symposium on Gamma-Ray Astronomy
and Astrophysic
Evidence for Supermassive Black Holes in Active Galactic Nuclei from Emission-Line Reverberation
Emission-line variability data for Seyfert 1 galaxies provide strong evidence
for the existence of supermassive black holes in the nuclei of these galaxies,
and that the line-emitting gas is moving in the gravitational potential of that
black hole. The time-delayed response of the emission lines to continuum
variations is used to infer the size of the line-emitting region, which is then
combined with measurements of the Doppler widths of the variable line
components to estimate a virial mass. In the case of the best-studied galaxy,
NGC 5548, various emission lines spanning an order of magnitude in distance
from the central source show the expected velocity proportional to inverse
square root of the distance correlation between distance and line width, and
are thus consistent with a single value for the mass. Two other Seyfert
galaxies, NGC 7469 and 3C 390.3, show a similar relationship. We compute the
ratio of luminosity to mass for these three objects and the narrow-line Seyfert
1 galaxy NGC 4051 and find that that the gravitational force on the
line-emitting gas is much stronger than radiation pressure. These results
strongly support the paradigm of gravitationally bound broad emission-line
region clouds.Comment: 10 pages, 2 figures, Accepted for publication in Astrophysical
Journal Letter
AGN's UV and X-ray luminosities in clumpy accretion flows
We consider the fuelling of the central massive black hole in Active Galactic
Nuclei, through an inhomogeneous accretion flow. Performing simple analytical
treatments, we show that shocks between elements (clumps) forming the accretion
flow may account for the UV and X-ray emission in AGNs. In this picture, a
cascade of shocks is expected, where optically thick shocks give rise to
optical/UV emission, while optically thin shocks give rise to X-ray emission.
The resulting blue bump temperature is found to be quite similar in different
AGNs. We obtain that the ratio of X-ray luminosity to UV luminosity is smaller
than unity, and that this ratio is smaller in massive objects compared to less
massive sources. This is in agreement with the observed ratio
and suggests a possible interpretation of the
anticorrelation.Comment: 8 pages, 1 figure, accepted for publication in A&
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
The global structure of thin, stratified "alpha"-discs and the reliability of the one layer approximation
We report the results of a systematic comparison between the vertically
averaged model and the vertically explicit model of steady state, Keplerian,
optically thick "alpha"-discs. The simulations have concerned discs currently
found in three different systems: dwarf novae, young stellar objects and active
galactic nuclei. In each case, we have explored four decades of accretion rates
and almost the whole disc area
(except the narrow region where the vertically averaged model has degenerate
solutions). We find that the one layer approach gives a remarkably good
estimate of the main physical quantities in the disc, and specially the
temperature at the equatorial plane which is accurate to within 30% for cases
considered. The major deviations (by a factor < 4) are observed on the disc
half-thickness. The sensitivity of the results to the "alpha"-parameter value
has been tested for 0.001 < alpha < 0.1 and appears to be weak. This study
suggests that the ``precision'' of the vertically averaged model which is easy
to implement should be sufficient in practice for many astrophysical
applications.Comment: 4 pages, PostScript. Accepted in Astronomy & Astrophysic
A Brief History of AGN
Astronomers knew early in the twentieth century that some galaxies have
emission-line nuclei. However, even the systematic study by Seyfert (1943) was
not enough to launch active galactic nuclei (AGN) as a major topic of
astronomy. The advances in radio astronomy in the 1950s revealed a new universe
of energetic phenomena, and inevitably led to the discovery of quasars. These
discoveries demanded the attention of observers and theorists, and AGN have
been a subject of intense effort ever since. Only a year after the recognition
of the redshifts of 3C 273 and 3C 48 in 1963, the idea of energy production by
accretion onto a black hole was advanced. However, acceptance of this idea came
slowly, encouraged by the discovery of black hole X-ray sources in our Galaxy
and, more recently, supermassive black holes in the center of the Milky Way and
other galaxies. Many questions remain as to the formation and fueling of the
hole, the geometry of the central regions, the detailed emission mechanisms,
the production of jets, and other aspects. The study of AGN will remain a
vigorous part of astronomy for the foreseeable future.Comment: 37 pages, no figures. Uses aaspp4.sty. To be published in
Publications of the Astronomical Society of the Pacific, 1999 Jun
FUSE Observation of the Narrow-line Seyfert 1 Galaxy RE 1034+39
We present analysis from simultaneous FUSE, ASCA, and EUVE observations, as
well as a reanalysis of archival HST spectra, from the extreme Narrow-line
Seyfert 1 Galaxy RE 1034+39 (KUG 1031+398). RE 1034+39 has an unusually hard
spectral energy distribution (SED) that peaks in the soft X-rays. Its emission
lines are unusual in that they can all be modelled as a Lorentzian centered at
the rest wavelength with only a small range in velocity widths. In order to
investigate whether the unusual SED influences the emission line ratios and
equivalent widths, we present three complementary types of photoionization
analysis. The FUSE spectrum was particularly important because it includes the
high-ionization line OVI. First, we use the photoionization code Cloudy and the
SED developed from the coordinated observations to confirm that the emission
lines are consistent with observed hard SED. The best model parameters were an
ionization parameter log(U) ~ -2 and a hydrogen number density log(n_H)=9.75
[cm^-2]. Second, we present a Locally Optimally-emitting Cloud model. This
model produced enhanced OVI as observed, but also yielded far too strong MgII.
Third, we develop a series of semi-empirical SEDs, run Cloudy models, and
compare the results with the measured values using a figure of merit (FOM). The
FOM minimum indicates similar SED and gas properties as were inferred from the
one-zone model using the RE 1034+39 continuum. Furthermore, the FOM increases
sharply toward softer continua, indicating that a hard SED is required by the
data in the context of a one-zone model.Comment: 65 pages 20 figure
Interpreting the Variability of Double-Peaked Emission Lines in Active Galactic Nuclei with Stochastically Perturbed Accretion Disk Models
In an effort to explain the short-timescale variability of the broad,
double-peaked profiles of some active galactic nuclei, we constructed
stochastically perturbed accretion disk models and calculated H alpha line
profile series as the bright spots rotate, shear and decay. We determined the
dependence of the properties of the line profile variability on the spot
properties. We compared the variability of the line profile from the models to
the observed variability of the H alpha line of Arp 102B and 3C 390.3. We find
that spots need to be concentrated in the outer parts of the line emitting
region to reproduce the observed variability properties for Arp 102B. This
rules out spot production by star/disk collisions and favors a scenario where
the radius of marginal self-gravity is within the line emitting region,
creating a sharp increase in the radial spot distribution in the outer parts.
In the case of 3C 390.3, all the families of models that we tested can
reproduce the observed variability for a suitable choice of model parameters.Comment: 27 pages, 8 figures, accepted for publication in Ap
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