159 research outputs found
The outflows accelerated by the magnetic fields and radiation force of accretion disks
The inner region of a luminous accretion disk is radiation pressure
dominated. We estimate the surface temperature of a radiation pressure
dominated accretion disk, \Theta=(c_s/r\Omega_K)^2<<(H/r)^2, which is
significantly lower than that of a gas pressure dominated disk, \Theta (H/r)^2.
This means that the outflow can be launched magnetically from the photosphere
of the radiation pressure dominate disk only if the effective potential barrier
along the magnetic field line is extremely shallow or no potential barrier is
present. For the latter case, the slow sonic point in the outflow may probably
be in the disk, which leads to a slow circular dense flow above the disk. This
implies that hot gas (probably in the corona) is necessary for launching a jet
from the radiation pressure dominated disk, which provides a natural
explanation on the observational evidence that the relativistic jets are
related to hot plasma in some X-ray binaries and active galactic nuclei. We
investigate the outflows accelerated from the hot corona above the disk by the
magnetic field and radiation force of the accretion disk. We find that, with
the help of the radiation force, the mass loss rate in the outflow is high,
which leads to a slow outflow. This may be the reason why the jets in
radio-loud narrow-line Seyfert galaxies are in general mild relativistic
compared with those in blazars.Comment: 8 pages, accepted by ApJ, references update
The jet power extracted from a magnetized accretion disc
We consider the power of a relativistic jet accelerated by the magnetic field
of an accretion disc. It is found that the power extracted from the disc is
mainly determined by the field strength and configuration of the field far from
the disc. Comparing with the power extracted from a rotating black hole, we
find that the jet power extracted from a disc can dominate over that from the
rotating black hole. But in some cases, the jet power extracted from a rapidly
rotating hole can be more important than that from the disc even if the
poloidal field threading the hole is not significantly larger than that
threading the inner edge of the disc. The results imply that the radio-loudness
of quasars may be governed by its accretion rate which might be regulated by
the central black hole mass. It is proposed that the different disc field
generation mechanisms might be tested against observations of radio-loud
quasars if their black hole masses are available.Comment: 6 pages, accepted for publication in MNRA
On the disappearance of broad-line region in low-luminosity active galactic nuclei: the role of the outflows from advection dominated accretion flows
The broad-line region (BLR) disappears in many low-luminosity AGNs, the
reason of which is still controversial. The BLRs in AGNs are believed to be
associated with the outflows from the accretion disks. Most of the
low-luminosity AGNs (LLAGNs) contain advection dominated accretion flows
(ADAFs), which are very hot and have a positive Bernoulli parameter. ADAFs are
therefore associated with strong outflows. We estimate the cooling of the
outflows from the ADAFs, and find that the gases in such hot outflows always
cannot be cooled efficiently by bremsstrahlung radiation. The ADAF may co-exist
with the standard disk, i.e., the inner ADAF connects to the outer thin
accretion disk at radius R_tr, in the sources accreting at slightly lower than
the critical rate. For the ADAFs with >0.001 L_edd, a secondary small inner
cold disk is suggested to co-exist with the ADAF due to the condensation
process. We estimate the Compton cooling of the outflow, of which the soft seed
photons either come from the outer cold disk or the secondary inner cold disk.
It is found that the gas in the outflow far from the ADAF may be efficiently
cooled to form BLR clouds due to the soft seed photons emitted from the cold
disks, provided the transition radius of the ADAF to the outer cold disk is
small or/and the secondary small cold disk has a luminosity >0.003L_edd. The
BLR clouds can still be formed in the outflows from the outer cold thin disks,
if the transition radius is not very large. For the sources with <0.001L_edd,
the inner small cold disk is evaporated completely in the ADAF and outer thin
accretion disk may be suppressed by the ADAF, which leads to the disappearance
of the BLR. The physical implications of this scenario on the double-peaked
broad-line emitters are also discussed.Comment: 6 pages, accepted by Ap
Dynamical behaviour of the `beads' along the magnetic field lines near a rotating black hole
The elements of the cold magnetic driven flows behave like beads on the
magnetic field line. The inclination of the field lines at the surface of the
disc plays a crucial role on the nature of the magnetically driven outflow. For
the non-relativistic case, a centrifugally driven outflow of matter from the
disc is possible, if the poloidal component of the magnetic field makes an
angle of less than a critical 60 degrees with the disc surface. The collimated
flows ejected from active galactic nuclei may probably start from the region
near the black hole. We investigate the dynamical behavior of the 'beads' on
the magnetic field line start from the disc near a black hole. It is found that
the critical angle becomes larger than 60 degrees for the rotating black hole
case (close to 90 degrees for a=1), which may imply that the flows are easy
accelerated in the inner edge of the disk surrounding a rotating black hole
On the radio dichotomy of active galactic nuclei
It is still a mystery why only a small fraction of active galactic nuclei
(AGNs) contain relativistic jets. Strong magnetic field is a necessary
ingredient for jet formation, however, the advection of the external field in a
geometrically thin disk is inefficient. The gas with a small angular velocity
may fall from the Bondi radius nearly freely to the circularization
radius , and a thin accretion disk is formed within . We
suggest that the external magnetic field is substantially enhanced in this
region, and the magnetic field at can be sufficiently strong to
drive outflows from the disk if the angular velocity of the gas is low at
. The magnetic field is efficiently dragged in the disk, because
most angular momentum of the disk is removed by the outflows that leads to a
significantly high radial velocity. The strong magnetic field formed in this
way may accelerate jets in the region near the black hole either by the
Blandford-Payne or/and Blandford-Znajek mechanisms. We suggest that the radio
dichotomy of AGNs predominantly originates from the angular velocity of the
circumnuclear gas. An AGN will appear as a radio-loud (RL) one if the angular
velocity of the circumnuclear gas is lower than a critical value at the Bondi
radius, otherwise, it will appear as a radio-quiet (RQ) AGN. This is supported
by the observations that RL nuclei are invariably hosted by core galaxies. Our
model suggests that the mass growth of the black holes in RL quasars is much
faster than that in RQ quasars with the same luminosity, which is consistent
with the fact that the massive black holes in RL quasars are systematically a
few times heavier than those in their RQ counterparts.Comment: accepted by ApJ, references update
The large scale magnetic fields of advection dominated accretion flows
We calculate the advection/diffusion of the large-scale magnetic field
threading an ADAF, and find that the magnetic field can be dragged inward by
the accretion flow efficiently, if the magnetic Prandtl number P~1. This is due
to the large radial velocity of the ADAF. It is found that the magnetic
pressure can be as high as ~50% of the gas pressure in the inner region of the
ADAF close to the black hole horizon, even if the external imposed vertical
field strength is <5% of the gas pressure at the outer radius of the ADAF,
which is caused by the gas plunging rapidly to the black hole within the
marginal stable circular orbit. In the inner region of the ADAF, the flow is
significantly pressured in the vertical direction by the magnetic field, and
therefore its gas pressure can be two orders of magnitude higher than that in
the ADAF without magnetic fields. This means that the magnetic field strength
near the black hole is underestimated by assuming equipartition between
magnetic and gas pressure with the conventional ADAF model. Our results show
that the magnetic field strength of the flow near the black hole horizon can be
more than one order of magnitude higher than that in the ADAF at 6GM/c^2, which
implies the Blandford-Znajek mechanism could be more important than the
Blandford-Payne mechanism for ADAFs. We find that the accretion flow is
decelerated near the black hole by the magnetic field when the external imposed
field is strong enough or the gas pressure of the flow is low at the outer
radius, or both. This corresponds to a critical accretion rate, below which the
accretion flow will be arrested by the magnetic field near the black hole for a
given external imposed field. In this case, the gas may accrete as magnetically
confined blobs diffusing through field lines in the region very close to the
black hole horizon, which is similar to those in compact stars.Comment: 11 pages, accepted by Ap
The relation between black hole masses and Lorentz factors of the jet components in blazars
We explore the relation between black hole mass () and the motion
of the jet components for a sample of blazars. The Very Long Baseline Array
(VLBA) 2cm Survey and its continuation: Monitoring of Jets in active galactic
nuclei (AGNs) with VLBA Experiments (MOJAVE) have observed 278 radio-loud AGNs,
of which 146 blazars have reliable measurements on their apparent velocities of
jet components. We calculate the minimal Lorentz factors for these sources from
their measured apparent velocities, and their black hole masses are estimated
with their broad-line widths. A significant intrinsic correlation is found
between black hole masses and the minimal Lorentz factors of the jet
components, which the Eddington ratio is only weakly correlated with the
minimal Lorentz factor, which may imply that the Blandford-Znajek (BZ)
mechanism may dominate over the Blandford-Payne (BP) mechanism for the jet
acceleration (at least) in blazars.Comment: 18 pages, 5 figures, 3 tables, submitted to ChJA
The large-scale magnetic field of a thin accretion disk with outflows
The large-scale magnetic field threading an accretion disk plays an important
role in launching jets/outflows. The field may probably be advected inwards by
the plasma in the accretion disk from the ambient environment (interstellar
medium or a companion star). It has been suggested that the external field can
be efficiently dragged inwards in a thin disk with magnetic outflows. We
construct a self-consistent global disk-outflow model, in which the large-scale
field is formed by the advection of the external field in the disk. The
outflows are accelerated by this field co-rotating with the disk, which carry
away most angular momentum of the disk and make its structure significantly
different from the conventional viscous disk structure. We find that the
magnetic field strength in the inner region of the disk can be several orders
of magnitude higher than the external field strength for a geometrically thin
disk with , if the ratio of the gas to magnetic pressure
at the outer edge of the disk. The outflow
velocity shows layer-like structure, i.e., it decreases with radius where it is
launched. The outflow can be accelerated up to c from the inner
region of the disk, and the mass loss rate in the outflows is
of the mass accretion rate at the outer radius of the disk, which may account
for the fast outflows observed in some active galactic nuclei (AGNs).Comment: Accepted for publication in Ap
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