3,480 research outputs found
Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole
We describe global, 3D, time-dependent, non-radiative, general-relativistic,
magnetohydrodynamic simulations of accreting black holes (BHs). The simulations
are designed to transport a large amount of magnetic flux to the center, more
than the accreting gas can force into the BH. The excess magnetic flux remains
outside the BH, impedes accretion, and leads to a magnetically arrested disc.
We find powerful outflows. For a BH with spin parameter a = 0.5, the efficiency
with which the accretion system generates outflowing energy in jets and winds
is eta ~ 30%. For a = 0.99, we find eta ~ 140%, which means that more energy
flows out of the BH than flows in. The only way this can happen is by
extracting spin energy from the BH. Thus the a = 0.99 simulation represents an
unambiguous demonstration, within an astrophysically plausible scenario, of the
extraction of net energy from a spinning BH via the Penrose-Blandford-Znajek
mechanism. We suggest that magnetically arrested accretion might explain
observations of active galactic nuclei with apparent eta ~ few x 100%.Comment: 5 pages, 2 figures, MNRAS, accepte
Equation of State in Relativistic Magnetohydrodynamics: variable versus constant adiabatic index
The role of the equation of state for a perfectly conducting, relativistic
magnetized fluid is the main subject of this work. The ideal constant
-law equation of state, commonly adopted in a wide range of
astrophysical applications, is compared with a more realistic equation of state
that better approximates the single-specie relativistic gas. The paper focus on
three different topics. First, the influence of a more realistic equation of
state on the propagation of fast magneto-sonic shocks is investigated. This
calls into question the validity of the constant -law equation of state
in problems where the temperature of the gas substantially changes across
hydromagnetic waves. Second, we present a new inversion scheme to recover
primitive variables (such as rest-mass density and pressure) from conservative
ones that allows for a general equation of state and avoids catastrophic
numerical cancellations in the non-relativistic and ultrarelativistic limits.
Finally, selected numerical tests of astrophysical relevance (including
magnetized accretion flows around Kerr black holes) are compared using
different equations of state. Our main conclusion is that the choice of a
realistic equation of state can considerably bear upon the solution when
transitions from cold to hot gas (or viceversa) are present. Under these
circumstances, a polytropic equation of state can significantly endanger the
solution.Comment: 14 pages, 14 figure
Prograde and Retrograde Black Holes: Whose Jet is More Powerful?
The outflow efficiency (eta) from black hole (BH) accretion disc systems is
known to depend upon both the BH spin (a) and the amount of large-scale
magnetic flux threading the BH and disc. Semi-analytical flux-trapping models
suggest retrograde BHs should trap much more large-scale magnetic flux near the
BH leading to much higher eta than for prograde BHs. We self-consistently
determine the amount of large-scale magnetic flux trapped by rapidly spinning
(a = -0.9 and 0.9) BHs using global 3D time-dependent non-radiative general
relativistic magnetohydrodynamic simulations of thick (h/r ~ 0.3-0.6) discs. We
find that BH-trapped flux builds up until it is strong enough to disrupt the
inner accretion disc. Contrary to prior flux-trapping models, which do not
include the back-reaction of magnetic flux on the disc, our simulations show
prograde BHs trap more magnetic flux, leading to about 3 times higher eta than
retrograde BHs for |a| = 0.9. Both spin orientations can produce highly
efficient jets, eta ~ 100%, with increasing eta for increasing disc thickness.
The similarity of eta for prograde and retrograde BHs makes it challenging to
infer the sign of BH spin based on jet energetics alone.Comment: 5 pages, 3 figures. Accepted to MNRAS. For associated movies see
http://youtu.be/yNZLjsrz0Wo and http://youtu.be/bQE69wti3a
On the efficiency of the Blandford-Znajek mechanism for low angular momentum relativistic accretion
Blandford-Znajek (BZ) mechanism has usually been studied in the literature
for accretion with considerably high angular momentum leading either to the
formation of a cold Keplerian disc, or a hot and geometrically thick
sub-Keplerian flow as described within the framework of ADAF/RIAF. However, in
nearby elliptical galaxies, as well as for our own Galactic centre, accretion
with very low angular momentum is prevalent. Such quasi-spherical strongly
sub-Keplerian accretion has complex dynamical features and can accommodate
stationary shocks. In this letter, we present our calculation for the maximum
efficiency obtainable through the BZ mechanism for complete general
relativistic weakly rotating axisymmetric flow in the Kerr metric. Both shocked
and shock free flow has been studied in detail for rotating and counter
rotating accretion. Such study has never been done in the literature before. We
find that the energy extraction efficiency is low, about 0.1%, and increases by
a factor 15 if the ram pressure is included. Such an efficiency is still much
higher than the radiative efficiency of such optically thin flows. For BZ
mechanism, shocked flow produces higher efficiency than the shock free
solutions and retrograde flow provides a slightly larger value of the
efficiency than that for the prograde flow.Comment: Substantially revised final version to appear in MNRAS Letters. Three
colour figure
Transport of Large Scale Poloidal Flux in Black Hole Accretion
We report on a global, three-dimensional GRMHD simulation of an accretion
torus embedded in a large scale vertical magnetic field orbiting a
Schwarzschild black hole. This simulation investigates how a large scale
vertical field evolves within a turbulent accretion disk and whether global
magnetic field configurations suitable for launching jets and winds can
develop. We find that a "coronal mechanism" of magnetic flux motion, which
operates largely outside the disk body, dominates global flux evolution. In
this mechanism, magnetic stresses driven by orbital shear create large-scale
half-loops of magnetic field that stretch radially inward and then reconnect,
leading to discontinuous jumps in the location of magnetic flux. In contrast,
little or no flux is brought in directly by accretion within the disk itself.
The coronal mechanism establishes a dipole magnetic field in the evacuated
funnel around the orbital axis with a field intensity regulated by a
combination of the magnetic and gas pressures in the inner disk. These results
prompt a reevaluation of previous descriptions of magnetic flux motion
associated with accretion. Local pictures are undercut by the intrinsically
global character of magnetic flux. Formulations in terms of an "effective
viscosity" competing with an "effective resistivity" are undermined by the
nonlinearity of of the magnetic dynamics and the fact that the same turbulence
driving mass motion (traditionally identified as "viscosity") can alter
magnetic topology.Comment: 45 pages, 17 figures, 1 movie; ApJ accepted; updated version contains
several new figures and a movie detailing the operation of the coronal
mechanism. The movie and a version of the paper with high resolution figures
can be found at http://www.astro.virginia.edu/~krb3u/0906.2784
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
Protecting privacy of users in brain-computer interface applications
Machine learning (ML) is revolutionizing research and industry. Many ML applications rely on the use of large amounts of personal data for training and inference. Among the most intimate exploited data sources is electroencephalogram (EEG) data, a kind of data that is so rich with information that application developers can easily gain knowledge beyond the professed scope from unprotected EEG signals, including passwords, ATM PINs, and other intimate data. The challenge we address is how to engage in meaningful ML with EEG data while protecting the privacy of users. Hence, we propose cryptographic protocols based on secure multiparty computation (SMC) to perform linear regression over EEG signals from many users in a fully privacy-preserving(PP) fashion, i.e., such that each individual's EEG signals are not revealed to anyone else. To illustrate the potential of our secure framework, we show how it allows estimating the drowsiness of drivers from their EEG signals as would be possible in the unencrypted case, and at a very reasonable computational cost. Our solution is the first application of commodity-based SMC to EEG data, as well as the largest documented experiment of secret sharing-based SMC in general, namely, with 15 players involved in all the computations
Three-Dimensional Simulations of Magnetized Thin Accretion Disks around Black Holes: Stress in the Plunging Region
We describe three-dimensional general relativistic magnetohydrodynamic
simulations of a geometrically thin accretion disk around a non-spinning black
hole. The disk has a thickness over the radial range
. In steady state, the specific angular momentum profile of the
inflowing magnetized gas deviates by less than 2% from that of the standard
thin disk model of
Novikov & Thorne (1973). Also, the magnetic torque at the radius of the
innermost stable circular orbit (ISCO) is only of the inward flux of
angular momentum at this radius. Both results indicate that magnetic coupling
across the ISCO is relatively unimportant for geometrically thin disks.Comment: 4 pages, 4 figures, ApJL accepte
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