130 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
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
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
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
A Chandra Look at Five of the Broadest Double-Peaked Balmer-Line Emitters
We study the 0.5-10keV emission of a sample of five of the broadest
double-peaked Balmer-line emitters with Chandra. The Balmer lines of these
objects originate close (within a few hundred gravitational radii) to the
central black holes of the Active Galactic Nuclei (AGNs), and their
double-peaked profiles suggest an origin in the AGN accretion disk. We find
that four of the five targets can be modeled by simple power-law continua with
photon indices (1.6-1.8) typical of similar luminosity AGNs. One object, SDSS
J0132-0952, shows evidence of ionized intrinsic absorption. The most-luminous
SDSS double-peaked emitter, SDSS J2125-0813, has either an unusual flat
spectrum (~1) or is also highly absorbed. It is the only double-peaked emitter
for which no external illumination is necessary to account for the Balmer line
emission. The strength of the Balmer-line emission in the remaining four
objects suggests that the total line flux likely exceeds the viscous energy
that can be extracted locally from the accretion disk and external illumination
is necessary. All five double-peaked emitters have unusually strong X-ray
emission relative to their UV/optical emission, which is the likely source of
the external illumination necessary for the production of the observed strong
broad lines. On average about 30% of their bolometric luminosities are emitted
between 0.5-10keV. The spectral energy distributions of the five double-peaked
emitters show the big blue bumps characteristic of radiatively efficient
accretion flows. The Balmer line profiles, as well as the optical and X-ray
fluxes of the double-peaked emitters, are highly variable on timescales of
months to years in the AGN rest frame.Comment: 15 pages, 7 figures, accepted by Ap
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&
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