137 research outputs found
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
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
Age Dating of a High-Redshift QSO B1422+231 at Z=3.62 and its Cosmological Implications
The observed Fe II(UV+optical)/Mg II lambda lambda 2796,2804 flux ratio from
a gravitationally lensed quasar B1422+231 at z=3.62 is interpreted in terms of
detailed modeling of photoionization and chemical enrichment in the broad-line
region (BLR) of the host galaxy. The delayed iron enrichment by Type Ia
supernovae is used as a cosmic clock. Our standard model, which matches the Fe
II/Mg II ratio, requires the age of 1.5 Gyr for B1422+231 with a lower bound of
1.3 Gyr, which exceeds the expansion age of the Einstein-de Sitter Omega_0=1
universe at a redshift of 3.62 for any value of the Hubble constant in the
currently accepted range, H_0=60-80 km,s^{-1},Mpc^{-1}. This problem of an age
discrepancy at z=3.62 can be unraveled in a low-density Omega_0<0.2 universe,
either with or without a cosmological constant, depending on the allowable
redshift range of galaxy formation. However, whether the cosmological constant
is a required option in modern cosmology awaits a thorough understanding of
line transfer processes in the BLRs.Comment: 7 pages including 3 figures, to appear in ApJ Letter
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
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
Infrared FeII Emission in Narrow-Line Seyfert 1 Galaxies
We obtained 0.8-2.4 micron spectra at a resolution of 320 km/s of four
narrow-line Seyfert 1 galaxies in order to study the near-infrared properties
of these objects. We focus on the analysis of the FeII emission in that region
and the kinematics of the low-ionization broad lines. We found that the 1
micron FeII lines (9997 A, 10501 A, 10863 A and 11126 A) are the strongest FeII
lines in the observed interval. For the first time, primary cascade lines of
FeII arising from the decay of upper levels pumped by Ly-alpha fluorescence are
resolved and identified in active galactic nuclei. Excitation mechanisms
leading to the emission of the 1 micron FeII features are discussed. A
combination of Ly-alpha fluorescence and collisional excitation are found to be
the main contributors. The flux ratio between near-IR FeII lines varies from
object to object, in contrast to what is observed in the optical region. A good
correlation between the 1 micron and optical FeII emission is found. This
suggests that the upper z4Fo and z4Do levels from which the bulk of the optical
lines descend are mainly populated by the transitions leading to the 1 micron
lines. The width and profile shape of FeII 11127, CaII 8642 and OI 8446 are
very similar but significantly narrower than Pa-beta, giving strong
observational support to the hypothesis that the region where FeII, CaII and OI
are produced are co-spatial, interrelated kinematically and most probably
located in the outermost portion of the BLR.Comment: Accepted for publication in ApJ - 35 page
The emission spectrum of the strong Fe II emitter BAL Seyfert 1 galaxy IRAS 07598+6508
The narrow-line Seyfert 1 galaxy IRAS 07598+6508 is known to be a stong Fe II
emitter. The analysis of several high S/N ratio spectra shows that its spectrum
is dominated by a relatively narrow "broad line" region (1 780 km s
FWHM) emitting not only Fe II, but also Ti II and Cr II lines. Although we were
unable to find a completely satisfactory physical model, we got the best
agreement with the observations with collisional rather than radiative models,
with a high density (n=10 cm), a high column density
(N=10 cm) and a microturbulence of 100 km s. This
BLR is qualitatively similar to the one observed in I Zw 1. We have not found
traces in IRAS 07598+6508 of the narrow line regions found in I Zw 1.Comment: 15 pages, 4 figures, accepted by A&
Clues to Quasar Broad Line Region Geometry and Kinematics
We present evidence that the high-velocity CIV lambda 1549 emission line gas
of radio-loud quasars may originate in a disk-like configuration, in close
proximity to the accretion disk often assumed to emit the low-ionization lines.
For a sample of 36 radio-loud z~2 quasars we find the 20--30% peak width to
show significant inverse correlations with the fractional radio core-flux
density, R, the radio axis inclination indicator. Highly inclined systems have
broader line wings, consistent with a high-velocity field perpendicular to the
radio axis. By contrast, the narrow line-core shows no such relation with R, so
the lowest velocity CIV-emitting gas has an inclination independent velocity
field. We propose that this low-velocity gas is located at higher
disk-altitudes than the high-velocity gas. A planar origin of the high-velocity
CIV-emission is consistent with the current results and with an accretion
disk-wind emitting the broad lines. A spherical distribution of randomly
orbiting broad-line clouds and a polar high-ionization outflow are ruled out.Comment: 5 Latex pages, 1 figure, accepted for publication in ApJ Letter
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