20,352 research outputs found
Scale-invariant radio jets and varying black hole spin
Compact radio cores associated with relativistic jets are often observed in
both active galactic nuclei and X-ray binaries. Their radiative properties
follow some general scaling laws which primarily depend on their masses and
accretion rates. However, it has been suggested that the black hole spin can
also strongly influence the power and radio flux of these. Here, we attempt to
estimate the dependency of the radio luminosity of steady jets launched by
accretion disks on black hole mass, accretion rate and spin using numerical
simulations. We make use of 3D GRMHD simulations of accretion disks around
low-luminosity black holes in which the jet radio emission is produced by the
jet sheath. We find that the radio flux increases roughly by a factor of 6 as
the back hole spin increases from a~0 to a=0.98. This is comparable to the
increase in accretion power with spin, meaning that the ratio between radio jet
and accretion power is hardly changing. Although our jet spine power scales as
expected for the Blandford-Znajek process, the dependency of jet radio
luminosity on the black hole spin is somewhat weaker. Also weakly rotating
black holes can produce visible radio jets. The overall scaling of the radio
emission with black hole mass and accretion rate is consistent with the
scale-invariant analytical models used to explain the fundamental plane of
black hole activity. Spin does not introduce a significant scatter in this
model. The jet-sheath model can describe well resolved accreting systems, such
as SgrA* and M87, as well as the general scaling behavior of low-luminosity
black holes. Hence the model should be applicable to a wide range of radio jets
in sub-Eddington black holes. The black hole spin has an effect on the
production of visible radio jet, but it may not be the main driver to produce
visible radio jets. An extension of our findings to powerful quasars remains
speculative.Comment: 10 pages, 6 figures, A&A accepte
The spin dependence of the Blandford-Znajek effect
The interaction of large scale magnetic fields with the event horizon of
rotating black holes (the Blandford-Znajek [1977] mechanism) forms the basis
for some models of the most relativistic jets. We explore a scenario in which
the central inward "plunging" region of the accretion flow enhances the
trapping of large scale poloidal field on the black hole. The study is carried
out using a fully relativistic treatment in Kerr spacetime, with the focus
being to determine the spin dependence of the Blandford-Znajek effect. We find
that large scale magnetic fields are enhanced on the black hole compared to the
inner accretion flow and that the ease with which this occurs for lower
prograde black hole spin, produces a spin dependence in the Blandford-Znajek
effect that has attractive applications to recent observations. Among these is
the correlation between inferred accretion rate and nuclear jet power observed
by Allen et al. (2006) in X-ray luminous elliptical galaxies. If the black hole
rotation in these elliptical galaxies is in the prograde sense compared with
that of the inner accretion disk, we show that both the absolute value and the
uniformity of the implied jet-production efficiency can be explained by the
flux-trapping model. The basic scenario that emerges from this study is that a
range of intermediate values of black hole spins could be powering these AGN.
We also suggest that the jets in the most energetic radio-galaxies may be
powered by accretion onto {\it retrograde} rapidly-rotating black holes.Comment: ApJ accepte
Magnetohydrodynamic Simulations of A Rotating Massive Star Collapsing to A Black Hole
We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of
the collapse of a rotating star of 40 Msun and in the light of the collapsar
model of gamma-ray burst. Considering two distributions of angular momentum, up
to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate
the formation of an accretion disk around a black hole and the jet production
near the hole. After material reaches to the black hole with the high angular
momentum, the disk is formed inside a surface of weak shock. The disk becomes
in a quasi-steady state for stars whose magnetic field is less than 10^{10} G
before the collapse. We find that the jet can be driven by the magnetic fields
even if the central core does not rotate as rapidly as previously assumed and
outer layers of the star has sufficiently high angular momentum. The magnetic
fields are chiefly amplified inside the disk due to the compression and the
wrapping of the field. The fields inside the disk propagate to the polar region
along the inner boundary near the black hole through the Alfv{\'e}n wave, and
eventually drive the jet. The quasi-steady disk is not an advection-dominated
disk but a neutrino cooling-dominated one. Mass accretion rates in the disks
are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent
for neutrinos. The dense part of the disk, which locates near the hole, emits
neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The
neutrino luminosity is much smaller than those from supernovae after the
neutrino burst.Comment: 42 pages, accepted for publication in the Astrophysical Journal. A
paper with higher-resolution figures available at
http://www.ec.knct.ac.jp/~fujimoto/collapsar/mhd-color.pd
A Magnetically-Switched, Rotating Black Hole Model For the Production of Extragalactic Radio Jets and the Fanaroff and Riley Class Division
A model is presented in which both Fanaroff and Riley class I and II
extragalactic jets are produced by magnetized accretion disk coronae in the
ergospheres of rotating black holes. While the jets are produced in the
accretion disk itself, the output power still is an increasing function of the
black hole angular momentum. For high enough spin, the black hole triggers the
magnetic switch, producing highly-relativistic, kinetic-energy-dominated jets
instead of Poynting-flux-dominated ones for lower spin. The coronal mass
densities needed to trigger the switch at the observed FR break power are quite
small (), implying that the source of the jet material
may be either a pair plasma or very tenuous electron-proton corona, not the
main accretion disk itself.
The model explains the differences in morphology and Mach number between FR I
and II sources and the observed trend for massive galaxies to undergo the FR
I/II transition at higher radio power. It also is consistent with the energy
content of extended radio lobes and explains why, because of black hole
spindown, the space density of FR II sources should evolve more rapidly than
that of FR I sources.
If the present model is correct, then the ensemble average speed of
parsec-scale jets in sources distinguished by their FR I morphology (not
luminosity) should be distinctly slower than that for sources with FR II
morphology. The model also suggests the existence of a population of
high-redshift, sub-mJy FR I and II radio sources associated with spiral or
pre-spiral galaxies that flared once when their black holes were formed but
were never again re-kindled by mergers.Comment: 14 pages, 2 figures, final version to appear in Sept Ap
Non-thermal Processes in Black-Hole-Jet Magnetospheres
The environs of supermassive black holes are among the universe's most
extreme phenomena. Understanding the physical processes occurring in the
vicinity of black holes may provide the key to answer a number of fundamental
astrophysical questions including the detectability of strong gravity effects,
the formation and propagation of relativistic jets, the origin of the highest
energy gamma-rays and cosmic-rays, and the nature and evolution of the central
engine in Active Galactic Nuclei (AGN). As a step towards this direction, this
paper reviews some of the progress achieved in the field based on observations
in the very high energy domain. It particularly focuses on non-thermal particle
acceleration and emission processes that may occur in the rotating
magnetospheres originating from accreting, supermassive black hole systems.
Topics covered include direct electric field acceleration in the black hole's
magnetosphere, ultra-high energy cosmic ray production, Blandford-Znajek
mechanism, centrifugal acceleration and magnetic reconnection, along with the
relevant efficiency constraints imposed by interactions with matter, radiation
and fields. By way of application, a detailed discussion of well-known sources
(Sgr A*; Cen A; M87; NGC1399) is presented.Comment: invited review for International Journal of Modern Physics D, 49
pages, 15 figures; minor typos corrected to match published versio
Maximum Spin of Black Holes Driving Jets
Unbounded outflows in the form of highly collimated jets and broad winds
appear to be a ubiquitous feature of accreting black hole systems. The most
powerful jets are thought to derive a significant fraction, if not the
majority, of their power from the rotational energy of the black hole. Whatever
the precise mechanism that causes them, these jets must therefore exert a
braking torque on the black hole. We calculate the spin-up function for an
accreting black hole, accounting for this braking torque. We find that the
predicted black hole spin-up function depends only on the black hole spin and
dimensionless parameters describing the accretion flow. Using recent
relativistic magnetohydrodynamical numerical simulation results to calibrate
the efficiency of angular momentum transfer in the flow, we find that an ADAF
flow will spin a black hole up (or down) to an equilibrium value of about 96%
of the maximal spin value in the absence of jets. Combining our ADAF system
with a simple model for jet power, we demonstrate that an equilibrium is
reached at approximately 93% of the maximal spin value, as found in the
numerical simulation studies of the spin-up of accreting black holes, at which
point the spin-up of the hole by accreted material is balanced by the braking
torque arising from jet production. Our model also yields a relationship
between jet efficiency and black hole spin that is in surprisingly good
agreement with that seen in the simulation studies, indicating that our simple
model is a useful and convenient description of ADAF inflow - jet outflow about
a spinning black hole for incorporation in models of the formation and
evolution of galaxies, groups and clusters of galaxies.Comment: 15 pages, 5 figures, accepted for publication in MNRAS. Corrected
errors in jet efficiency formula in text and some equations in Appendices.
Errors affected text only, results are unchange
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