99 research outputs found
Dense, thin clouds and reprocessed radiation in the central regions of Active Galactic Nuclei
The primary radiation generated in the central continuum-forming region of
Active Galactic Nuclei can be reprocessed by very dense, small-scale clouds
that are optically-thin to Thomson scattering. In spite of the extreme
conditions expected to prevail in this innermost, central environment, the
radiative clouds can survive and maintain cool temperatures relative to the
ambient emitting region by means of magnetic confinement. Motivated by these
ideas, we present a detailed quantitative study of such clouds, explicitly
describing the physical properties they can attain under thermal and radiative
equilibrium conditions. We also discuss the thermal stability of the gas in
comparison to that of other reprocessing material thought to reside at larger
distances from the central source. We construct a model to predict the emergent
spectra from a source region containing dense clouds which absorb and reemit
the primary radiation generated therein. Our predicted spectra show the
following two important results: (i) the reprocessed flux emitted at optical/UV
energies is insufficient to account for the blue bump component in the observed
spectra; and (ii) the amount of line radiation that is emitted is at least
comparable to (and in many cases dominates) the continuum radiation. The lines
are extremely broad and tend to accumulate in the extreme ultraviolet, where
they form a peak much more prominent than that which is observed in the
optical/UV. This result is supported by current observations, which indicate
that the spectral energy distribution of radio-quiet AGN may indeed reach a
maximum in the EUV band.Comment: 14 pages, 5 figures, latex, uses epsf and rotate, accepted for
publication in M
Accretion Discs in Blazars
The characteristic properties of blazars (rapid variability, strong
polarization, high brightness) are widely attributed to a powerful relativistic
jet oriented close to our line of sight. Despite the spectral energy
distributions (SEDs) being strongly jet-dominated, a "big blue bump" has been
recently detected in sources known as flat spectrum radio quasars (FSRQs).
These new data provide a unique opportunity to observationally test coupled
jet-disc accretion models in these extreme sources. In particular, as energy
and angular momentum can be extracted by a jet magnetically coupled to the
accretion disc, the thermal disc emission spectrum may be modified from that
predicted by the standard model for disc accretion. We compare the
theoretically predicted jet-modified accretion disc spectra against the new
observations of the "big blue bump" in FSRQs. We find mass accretion rates that
are higher, typically by a factor of two, than predicted by standard accretion
disc theory. Furthermore, our results predict that the high redshift blazars
PKS 0836+710, PKS 2149-307, B2 0743+25 and PKS 0537-286 may be predominantly
powered by a low or moderate spin (a < 0.6) black hole with high mass accretion
rates mdot_a ~ 50 - 200 msol/yr, while 3C 273 harbours a rapidly spinning black
hole (a = 0.97) with mdot_a ~ 20 msol/yr. We also find that the black hole
masses in these high redshift sources must be > 5 * 10^9 msol.Comment: Accepted for publication (17 August 2009) in MNRA
Towards a New Standard Theory for Astrophysical Disk Accretion
We briefly review recent developments in black hole accretion disk theory,
placing new emphasis on the vital role played by magnetohydrodynamic (MHD)
stresses in transporting angular momentum. The apparent universality of
accretion-related outflow phenomena is a strong indicator that vertical
transport of angular momentum by large-scale MHD torques is important and may
even dominate radial transport by small-scale MHD turbulence. This leads to an
enhanced overall rate of angular momentum transport and allows accretion of
matter to proceed at an interesting rate. Furthermore, we argue that when
vertical transport is important, the radial structure of the accretion disk is
modified and this affects the disk emission spectrum. We present a simple model
demonstrating that energetic, magnetically-driven outflows give rise to a disk
spectrum that is dimmer and redder than a standard accretion disk accreting at
the same rate. We briefly discuss the implications of this key result for
accreting black holes in different astrophysical systems.Comment: Accepted for publication as brief review in Mod. Phys. Let.
Physical constraints on the sizes of dense clouds in the central magnetospheres of Active Galactic Nuclei
The range of microphysical and global dynamical timescales in the central
regions of Active Galactic Nuclei (AGN) is sufficiently wide to permit the
existence of multiphase structure. In particular, very dense, cool clouds can
coexist with a hot, magnetically-dominated medium and can thereby efficiently
reprocess the continuum radiation generated in this primary source region. The
strong dynamical forces in this central magnetosphere can give rise to
extremely small clouds. Microphysical processes then determine whether such
clouds can indeed survive, in spite of their extremely contrasting properties
relative to the surrounding environment, for long enough to produce potentially
observable thermal reprocessing signatures. We examine specific physical
constraints on the thicknesses of such reprocessing clouds. Our results are
plotted to show the range of conditions that is representative of the central
regions of AGN. We find a parameter subspace in the extreme high density regime
for which the effects of microphysical diffusion processes can be overcome and
for which cool gas can maintain pressure equilibrium with the ambient
magnetosphere.Comment: 9 pages, LaTeX type, 2 postscript figures, uses rotate.sty and
epsf.sty, accepted for publication in MNRA
A New Equilibrium for Accretion Disks Around Black Holes
Accretion disks around black holes in which the shear stress is proportional
to the total pressure, the accretion rate is more than a small fraction of
Eddington, and the matter is distributed smoothly are both thermally and
viscously unstable in their inner portions. The nonlinear endstate of these
instabilities is uncertain. Here a new inhomogeneous equilibrium is proposed
which is both thermally and viscously stable. In this equilibrium the majority
of the mass is in dense clumps, while a minority reaches temperatures K. The requirements of dynamical and thermal equilibrium completely
determine the parameters of this system, and these are found to be in good
agreement with the parameters derived from observations of accreting black
holes, both in active galactic nuclei and in stellar binary systems.Comment: AAS LaTeX, accepted to Ap. J. Letter
Compton Scattering of Fe K alpha Lines in Magnetic Cataclysmic Variables
Compton scattering of X-rays in the bulk flow of the accretion column in
magnetic cataclysmic variables (mCVs) can significantly shift photon energies.
We present Monte Carlo simulations based on a nonlinear algorithm demonstrating
the effects of Compton scattering on the H-like, He-like and neutral Fe K alpha
lines produced in the post-shock region of the accretion column. The peak line
emissivities of the photons in the post-shock flow are taken into consideration
and frequency shifts due to Doppler effects are also included. We find that
line profiles are most distorted by Compton scattering effects in strongly
magnetized mCVs with a low white dwarf mass and high mass accretion rate and
which are viewed at an oblique angle with respect to the accretion column. The
resulting line profiles are most sensitive to the inclination angle. We have
also explored the effects of modifying the accretion column width and using a
realistic emissivity profile. We find that these do not have a significant
overall effect on the resulting line profiles. A comparison of our simulated
line spectra with high resolution Chandra/HETGS observations of the mCV GK Per
indicates that a wing feature redward of the 6.4 keV line may result from
Compton recoil near the base of the accretion column.Comment: Accepted for publication in MNRAS, 10 pages with 8 figure
Does the Iron K Line of Active Galactic Nuclei Arise from the Cerenkov Line-like Radiation?
When thermal relativistic electrons with isotropic distribution of velocities
move in a gas region, or impinge upon the surface of a cloud that consists of a
dense gas or doped dusts, the Cerenkov effect produces peculiar atomic or ionic
emission lines -- the Cerenkov line-like radiation. This newly recognized
emission mechanism may find wide applications in high-energy astrophysics. In
this paper, we tentatively adopt this new line emission mechanism to discuss
the origin of iron K feature of AGNs. Motivation of this research is
to attempt a solution to a problem encountered by the ``disk-fluorescence
line'' model, i.e. the lack of temporal response of the observed iron
K line flux to the changes of the X-ray continuum flux. If the
Cerenkov line emission is indeed responsible significantly for the iron
K feature, the conventional scenario around the central supermassive
black holes of AGNs would need to be modified to accommodate more energetic,
more violent and much denser environments than previously thought.Comment: 22 pages, 4 figures, 1 table. ApJ in press (December
X-ray Polarization Signatures of Compton Scattering in Magnetic Cataclysmic Variables
Compton scattering within the accretion column of magnetic cataclysmic
variables (mCVs) can induce a net polarization in the X-ray emission. We
investigate this process using Monte Carlo simulations and find that
significant polarization can arise as a result of the stratified flow structure
in the shock-ionized column. We find that the degree of linear polarization can
reach levels up to ~8% for systems with high accretion rates and low
white-dwarf masses, when viewed at large inclination angles with respect to the
accretion column axis. These levels are substantially higher than previously
predicted estimates using an accretion column model with uniform density and
temperature. We also find that for systems with a relatively low-mass white
dwarf accreting at a high accretion rate, the polarization properties may be
insensitive to the magnetic field, since most of the scattering occurs at the
base of the accretion column where the density structure is determined mainly
by bremsstrahlung cooling instead of cyclotron cooling.Comment: 7 pages, 8 figures, accepted by MNRA
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