2,793 research outputs found
The Dynamics of Truncated Black Hole Accretion Disks. I. Viscous Hydrodynamic Case
Truncated accretion disks are commonly invoked to explain the
spectro-temporal variability from accreting black holes in both small systems,
i.e. state transitions in galactic black hole binaries (GBHBs), and large
systems, i.e. low-luminosity active galactic nuclei (LLAGNs). In the canonical
truncated disk model of moderately low accretion rate systems, gas in the inner
region of the accretion disk occupies a hot, radiatively inefficient phase,
which leads to a geometrically thick disk, while the gas in the outer region
occupies a cooler, radiatively efficient phase that resides in the standard
geometrically thin disk. Observationally, there is strong empirical evidence to
support this phenomenological model, but a detailed understanding of the
dynamics of truncated disks is lacking. We present a well-resolved viscous,
hydrodynamic simulation that uses an ad hoc cooling prescription to drive a
thermal instability and, hence, produce the first sustained truncated accretion
disk. With this simulation, we perform a study of the dynamics, angular
momentum transport, and energetics of a truncated disk. We find that time
variability introduced by the quasi-periodic transition of gas from efficient
cooling to inefficient cooling impacts the evolution of the simulated disk. A
consequence of the thermal instability is that an outflow is launched from the
hot/cold gas interface which drives large, sub-Keplerian convective cells in
the disk atmosphere. The convective cells introduce a viscous
stress that is less than the generic viscous stress component, but
greatly influences the evolution of the disk. In the truncated disk, we find
that the bulk of the accreted gas is in the hot phase.Comment: 16 pgs, 14 figures, accepted for publication in Ap
The Influence of Accretion Disk Thickness on the Large-scale Magnetic Dynamo
The evolution of the magnetic field from the large-scale dynamo is considered
a central feature of the accretion disk around a black hole. The resulting
low-frequency oscillations introduced from the growth and decay of the field
strength, along with the change in field orientation, play an integral role in
the accretion disk behavior. Despite the importance of this process and how
commonly it is invoked to explain variable features, it still remains poorly
understood. We present a study of the dynamo using a suite of four global,
high-resolution, MHD accretion disk simulations. We systematically vary the
scale height ratio and find the large-scale dynamo fails to organize above a
scale height ratio of . Using spacetime diagrams of the
azimuthal magnetic field, we show the large-scale dynamo is well-ordered in the
thinner accretion disk models, but fails to develop the characteristic
"butterfly" pattern when the scale height ratio is increased, a feature which
is also reflected in the power spectra. Additionally, we calculate the dynamo
-parameter and generate synthetic light curves. Using an emission
proxy, we find the disks have markedly different characters as stochastic
photometric fluctuations have a larger amplitude when the dynamo is unordered
HOW AGN JETS HEAT the INTRACLUSTER MEDIUM - INSIGHTS from HYDRODYNAMIC SIMULATIONS
© 2016. The American Astronomical Society. All rights reserved. Feedback from active galactic nuclei (AGNs) is believed to prevent catastrophic cooling in galaxy clusters. However, how the feedback energy is transformed into heat, and how the AGN jets heat the intracluster medium (ICM) isotropically, still remain elusive. In this work, we gain insights into the relative importance of different heating mechanisms using three-dimensional hydrodynamic simulations including cold gas accretion and momentum-driven jet feedback, which are the most successful models to date in terms of reproducing the properties of cool cores. We find that there is net heating within two "jet cones" (within ∼30° from the axis of jet precession) where the ICM gains entropy by shock heating and mixing with the hot thermal gas within bubbles. Outside the jet cones, the ambient gas is heated by weak shocks, but not enough to overcome radiative cooling, therefore, forming a "reduced" cooling flow. Consequently, the cluster core is in a process of "gentle circulation" over billions of years. Within the jet cones, there is significant adiabatic cooling as the gas is uplifted by buoyantly rising bubbles; outside the cones, energy is supplied by the inflow of already-heated gas from the jet cones as well as adiabatic compression as the gas moves toward the center. In other words, the fluid dynamics self-adjusts such that it compensates and transports the heat provided by the AGN, and hence no fine-tuning of the heating profile of any process is necessary. Throughout the cluster evolution, turbulent energy is only at the percent level compared to gas thermal energy, and thus turbulent heating is not the main source of heating in our simulation
INEFFICIENT DRIVING of BULK TURBULENCE by ACTIVE GALACTIC NUCLEI in A HYDRODYNAMIC MODEL of the INTRACLUSTER MEDIUM
Central jetted active galactic nuclei (AGN) appear to heat the core regions
of the intracluster medium (ICM) in cooling-core galaxy clusters and groups,
thereby preventing a cooling catastrophe. However, the physical mechanism(s) by
which the directed flow of kinetic energy is thermalized throughout the ICM
core remains unclear. We examine one widely discussed mechanism whereby the AGN
induces subsonic turbulence in the ambient medium, the dissipation of which
provides the ICM heat source. Through controlled inviscid 3-d hydrodynamic
simulations, we verify that explosive AGN-like events can launch gravity waves
(g-modes) into the ambient ICM which in turn decay to volume-filling
turbulence. In our model, however, this process is found to be inefficient,
with less than 1% of the energy injected by the AGN activity actually ending up
in the turbulence of the ambient ICM. This efficiency is an order of magnitude
or more too small to explain the observations of AGN-feedback in galaxy
clusters and groups with short central cooling times. Atmospheres in which the
g-modes are strongly trapped/confined have an even lower efficiency since, in
these models, excitation of turbulence relies on the g-modes' ability to escape
from the center of the cluster into the bulk ICM. Our results suggest that, if
AGN-induced turbulence is indeed the mechanism by which the AGN heats the ICM
core, its driving may rely on physics beyond that captured in our ideal
hydrodynamic model
Relativistic X-ray Lines from the Inner Accretion Disks Around Black Holes
Relativistic X-ray emission lines from the inner accretion disk around black
holes are reviewed. Recent observations with the Chandra X-ray Observatory,
X-ray Multi-Mirror Mission-Newton, and Suzaku are revealing these lines to be
good probes of strong gravitational effects. A number of important
observational and theoretical developments are highlighted, including evidence
of black hole spin and effects such as gravitational light bending, the
detection of relativistic lines in stellar-mass black holes, and evidence of
orbital-timescale line flux variability. In addition, the robustness of the
relativistic disk lines against absorption, scattering, and continuum effects
is discussed. Finally, prospects for improved measures of black hole spin and
understanding the spin history of supermassive black holes in the context of
black hole-galaxy co-evolution are presented. The best data and most rigorous
results strongly suggest that relativistic X-ray disk lines can drive future
explorations of General Relativity and disk physics.Comment: 40 pages, includes color figures, to appear in ARAA, vol 45, in pres
An extension of the FRI framework for calcium transient detection
Two-photon calcium imaging of the brain allows the spatiotemporal activity of neuronal networks to be monitored at cellular resolution. In order to analyse this activity it must first be possible to detect, with high temporal resolution, spikes from the time series corresponding to single neurons. Previous work has shown that finite rate of innovation (FRI) theory can be used to reconstruct spike trains from noisy calcium imaging data. In this paper we extend the FRI framework for spike detection from calcium imaging data to encompass data generated by a larger class of calcium indicators, including the genetically encoded indicator GCaMP6s. Furthermore, we implement least squares model-order estimation and perform a noise reduction procedure ('pre-whitening') in order to increase the robustness of the algorithm. We demonstrate high spike detection performance on real data generated by GCaMP6s, detecting 90% of electrophysiologically-validated spikes
Towards modelling X-ray reverberation in AGN: Piecing together the extended corona
Models of X-ray reverberation from extended coronae are developed from
general relativistic ray tracing simulations. Reverberation lags between
correlated variability in the directly observed continuum emission and that
reflected from the accretion disc arise due to the additional light travel time
between the corona and reflecting disc. X-ray reverberation is detected from an
increasing sample of Seyfert galaxies and a number of common properties are
observed, including a transition from the characteristic reverberation
signature at high frequencies to a hard lag within the continuum component at
low frequencies, as well a pronounced dip in the reverberation lag at 3keV.
These features are not trivially explained by the reverberation of X-rays
originating from simple point sources. We therefore model reverberation from
coronae extended both over the surface of the disc and vertically. Causal
propagation through its extent for both the simple case of constant velocity
propagation and propagation linked to the viscous timescale in the underlying
accretion disc is included as well as stochastic variability arising due to
turbulence locally on the disc. We find that the observed features of X-ray
reverberation in Seyfert galaxies can be explained if the long timescale
variability is dominated by the viscous propagation of fluctuations through the
corona. The corona extends radially at low height over the surface of the disc
but with a bright central region in which fluctuations propagate up the black
hole rotation axis driven by more rapid variability arising from the innermost
regions of the accretion flow
Excess Galactic Molecular Absorption Toward the Radio Galaxy 3C 111
© 2017. The American Astronomical Society. All rights reserved. We show the combined spectral analysis of Chandra high-energy transmission grating and XMM-Newton reflection-grating spectrometer observations of the broad-line radio galaxy 3C 111. This source is known to show excess neutral absorption with respect to the one estimated from 21 cm radio surveys of atomic H i in the Galaxy. However, previous works were not able to constrain the origin of such an absorber as local to our Milky Way or intrinsic to the source (z = 0.0485). The high signal-to-noise grating spectra allow us to constrain the excess absorption as being due to intervening gas in the Milky Way, and we estimate a time-averaged total column density of NH = (7.4 ± 0.1) × 1021 cm-2, a factor of two higher than the tabulated H i value. We recommend using the total average Galactic column density estimated here when studying 3C 111. The origin of the extra Galactic absorption of NH = 4.4 × 1021 cm-2 is likely due to molecular gas associated with the Taurus molecular cloud complex toward 3C 111, which is our nearest star-forming region. We also detect a weak (EW = 16 ± 10 eV) and narrow (FWMH < 5500 km s-1, consistent with optical Hα) Fe Kα emission line at E = 6.4 keV, likely from the torus in the central regions of 3C 111, and we place an upper limit on the column density of a possible intrinsic warm absorber of N H < 2.5 ×1020 cm-2. These complexities make 3C 111 a very promising object for studying both the intrinsic properties of this active radio galaxy and the Galactic interstellar medium, if used as a background source
X-ray dips in the seyfert galaxy fairall 9: Compton-thick "cOMETS" or a failed radio galaxy?
We investigate the spectral variability of the Seyfert galaxy Fairall 9 using
almost 6 years of monitoring with the Rossi X-ray Timing Explorer (RXTE) with
an approximate time resolution of 4 days. We discover the existence of
pronounced and sharp dips in the X-ray flux, with a rapid decline of the 2--20
keV flux of a factor 2 or more followed by a recovery to pre-dip fluxes after
~10 days . These dips skew the flux distribution away from the commonly
observed log-normal distribution. Dips may result from the eclipse of the
central X-ray source by broad line region (BLR) clouds, as has recently been
found in NGC 1365 and Mrk 766. Unlike these other examples, however, the clouds
in Fairall 9 would need to be Compton-thick, and the non-dip state is
remarkably free of any absorption features. A particularly intriguing
alternative is that the accretion disk is undergoing the same cycle of
disruption/ejection as seen in the accretion disks of broad line radio galaxies
(BLRGs) such as 3C120 but, for some reason, fails to create a relativistic jet.
This suggests that a detailed comparison of Fairall 9 and 3C120 with future
high-quality data may hold the key to understanding the formation of
relativistic jets in AGN
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SUPPRESSION of ELECTRON THERMAL CONDUCTION in the HIGH β INTRACLUSTER MEDIUM of GALAXY CLUSTERS
Understanding the thermodynamic state of the hot intracluster medium (ICM) in a galaxy cluster requires knowledge of the plasma transport processes, especially thermal conduction. The basic physics of thermal conduction in plasmas with ICM-like conditions has yet to be elucidated, however. We use particle-in-cell simulations and analytic models to explore the dynamics of an ICM-like plasma (with small gyroradius, large mean free path, and strongly sub-dominant magnetic pressure) driven by the diffusive heat flux associated with thermal conduction. Linear theory reveals that whistler waves are driven unstable by electron heat flux, even when the heat flux is weak. The resonant interaction of electrons with these waves then plays a critical role in scattering electrons and suppressing the heat flux. In a 1D model where only whistler modes that are parallel to the magnetic field are captured, the only resonant electrons are moving in the opposite direction to the heat flux, and the electron heat flux suppression is small. In 2D or more, oblique whistler modes also resonate with electrons moving in the direction of the heat flux. The overlap of resonances leads to effective symmetrization of the electron distribution function and a strong suppression of heat flux. The results suggest that thermal conduction in the ICM might be strongly suppressed, possibly to negligible levels
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