110 research outputs found
Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region
We explore the conditions for the thermal instability to operate in the
mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold
media are known to coexist. The photoionisation Cloudy calculations are
performed for different physical states of plasma. We neglect the dynamics of
the material and concentrate on the study of the parameter ranges where the
thermal instability may operate, taking into account the past history of Sgr A*
bolometric luminosity. We show that the thermal instability does not operate at
the present very low level of the Sgr A* activity. However, Sgr A* was much
more luminous in the past. For the highest luminosity states the two-phase
medium can be created up to 1.4 pc from the centre. The presence of dust grains
tends to suppress the instability, but the dust is destroyed in the presence of
strong radiation field and hot plasma. The clumpiness is thus induced in the
high activity period, and the cooling/heating timescales are long enough to
preserve later the past multi-phase structure. The instability enhances the
clumpiness of the mini-spiral medium and creates a possibility of episodes of
enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the
range of masses and sizes of clouds; under the conditions of Sgr A*, the likely
values come out - for the cloud typical mass.Comment: Accepted for publication in MNRAS, 10 pages, 7 figure
Aberrational Effects for Shadows of Black Holes
In this paper, we discuss how the shadow of a Kerr black hole depends on the
motion of the observer. In particular, we derive an analytical formula for the
boundary curve of the shadow for an observer moving with given four-velocity at
given Boyer--Lindquist coordinates. We visualize the shadow for various values
of parameters.Comment: 12 pages, 3 figures; Proceedings of the 524. WE-Heraeus-Seminar held
at the Physikzentrum, Bad Honnef, Germany, 17.--23.2.201
Pair Production in Low Luminosity Galactic Nuclei
Electron-positron pairs may be produced near accreting black holes by a
variety of physical processes, and the resulting pair plasma may be accelerated
and collimated into a relativistic jet. Here we use a self-consistent dynamical
and radiative model to investigate pair production by \gamma\gamma collisions
in weakly radiative accretion flows around a black hole of mass M and accretion
rate \dot{M}. Our flow model is drawn from general relativistic
magnetohydrodynamic simulations, and our radiation field is computed by a Monte
Carlo transport scheme assuming the electron distribution function is thermal.
We argue that the pair production rate scales as r^{-6} M^{-1} \dot{M}^{6}. We
confirm this numerically and calibrate the scaling relation. This relation is
self-consistent in a wedge in M, \dot{M} parameter space. If \dot{M} is too low
the implied pair density over the poles of the black hole is below the
Goldreich-Julian density and \gamma\gamma pair production is relatively
unimportant; if \dot{M} is too high the models are radiatively efficient. We
also argue that for a power-law spectrum the pair production rate should scale
with the observables L_X \equiv X-ray luminosity and M as L_X^2 M^{-4}. We
confirm this numerically and argue that this relation likely holds even for
radiatively efficient flows. The pair production rates are sensitive to black
hole spin and to the ion-electron temperature ratio which are fixed in this
exploratory calculation. We finish with a brief discussion of the implications
for Sgr A* and M87.Comment: 21 pages, 10 figures, 1 table. Accepted for publication in Ap
Visibility of black hole shadows in low-luminosity AGN
Accreting black holes tend to display a characteristic dark central region called the black hole shadow, which depends only on space–time/observer geometry and which conveys information about the black hole’s mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the black hole shadow: evacuation of the innermost region of the accretion flow, which can render the image shadow larger than the black hole shadow, and obscuration of the black hole shadow by optically thick regions of the accretion flow, which can render the image shadow smaller than the black hole shadow, or eliminate it altogether. We investigate in which models the image shadows of our models match their corresponding black hole shadows, and in which models the two deviate from each other. We find that, given a compact and optically thin emission region, our models allow for measurement of the black hole shadow size to an accuracy of 5 per cent. We show that these conditions are generally met for all MAD simulations we considered, as well as some of the SANE simulations
Magnetized Accretion Flows: Effects of Gas Pressure
We study how axisymmetric magnetohydrodynamical (MHD) accretion flows depend
on gamma adiabatic index in the polytropic equation of state. This work is an
extension of Moscibrodzka & Proga (2008), where we investigated the gamma
dependence of 2-D Bondi-like accretion flows in the hydrodynamical (HD) limit.
Our main goal is to study if simulations for various gamma can give us insights
into to the problem of various modes of accretion observed in several types of
accretion systems such as black hole binaries (BHB), active galactic nuclei
(AGN), and gamma-ray bursts (GRBs). We find that for gamma >~ 4/3, the fast
rotating flow forms a thick torus that is supported by rotation and gas
pressure. As shown before for gamma=5/3, such a torus produces a strong,
persistent bipolar outflow that can significantly reduce the polar funnel
accretion of a slowly rotating flow. For low gamma, close to 1, the torus is
thin and is supported by rotation. The thin torus produces an unsteady outflow
which is too weak to propagate throughout the polar funnel inflow. Compared to
their HD counterparts, the MHD simulations show that the magnetized torus can
produce an outflow and does not exhibit regular oscillations. Generally, our
simulations demonstrate how the torus thickness affects the outflow production.
They also support the notion that the geometrical thickness of the torus
correlates with the power of the torus outflow. Our results, applied to
observations, suggest that the torus ability to radiatively cool and become
thin can correspond to a suppression of a jet as observed in the BHB during a
transition from a hard/low to soft/high spectral state and a transition from a
quiescent to hard/low state in AGN.Comment: 17 pages, 7 figures, accepted for publication in MNRA
Pair production in low luminosity galactic nuclei
ABSTRACT We compute the distribution of pair production by γγ collisions in weakly radiative accretion flows around a black hole of mass M and accretion rateṀ . We use a flow model drawn from general relativistic magnetohydrodynamic simulations and a Monte Carlo radiation field that assumes the electron distribution function is thermal. We find that
Observing supermassive black holes in virtual reality
We present a 360∘ (i.e., 4π steradian) general-relativistic ray-tracing and radiative transfer calculations of accreting supermassive black holes. We perform state-of-the-art three-dimensional general-relativistic magnetohydrodynamical simulations using the BHAC code, subsequently post-processing this data with the radiative transfer code RAPTOR. All relativistic and general-relativistic effects, such as Doppler boosting and gravitational redshift, as well as geometrical effects due to the local gravitational field and the observer’s changing position and state of motion, are therefore calculated self-consistently. Synthetic images at four astronomically-relevant observing frequencies are generated from the perspective of an observer with a full 360∘ view inside the accretion flow, who is advected with the flow as it evolves. As an example we calculated images based on recent best-fit models of observations of Sagittarius A*. These images are combined to generate a complete 360∘ Virtual Reality movie of the surrounding environment of the black hole and its event horizon. Our approach also enables the calculation of the local luminosity received at a given fluid element in the accretion flow, providing important applications in, e.g., radiation feedback calculations onto black hole accretion flows. In addition to scientific applications, the 360∘ Virtual Reality movies we present also represent a new medium through which to interactively communicate black hole physics to a wider audience, serving as a powerful educational tool
Radiative Models of Sagittarius A* and M87 from Relativistic MHD Simulations
Ongoing millimeter VLBI observations with the Event Horizon Telescope allow
unprecedented study of the innermost portion of black hole accretion flows.
Interpreting the observations requires relativistic, time-dependent physical
modeling. We discuss the comparison of radiative transfer calculations from
general relativistic MHD simulations of Sagittarius A* and M87 with current and
future mm-VLBI observations. This comparison allows estimates of the viewing
geometry and physical conditions of the Sgr A* accretion flow. The viewing
geometry for M87 is already constrained from observations of its large-scale
jet, but, unlike Sgr A*, there is no consensus for its millimeter emission
geometry or electron population. Despite this uncertainty, as long as the
emission region is compact, robust predictions for the size of its jet
launching region can be made. For both sources, the black hole shadow may be
detected with future observations including ALMA and/or the LMT, which would
constitute the first direct evidence for a black hole event horizon.Comment: 8 pages, 2 figures, submitted to the proceedings of AHAR 2011: The
Central Kiloparse
Low angular momentum flow model of Sgr A* activity
Sgr A* is the closest massive black hole and can be observed with the highest
angular resolution. Nevertheless, our current understanding of the accretion
process in this source is very poor. The inflow is almost certainly of low
radiative efficiency and it is accompanied by a strong outflow and the flow is
strongly variable but the details of the dynamics are unknown. Even the amount
of angular momentum in the flow is an open question. Here we argue that low
angular momentum scenario is better suited to explain the flow variability. We
present a new hybrid model which describes such a flow and consists of an outer
spherically symmetric Bondi flow and an inner axially symmetric flow described
through MHD simulations. The assumed angular momentum of the matter is low,
i.e. the corresponding circularization radius in the equatorial plane of the
flow is just above the innermost stable circular orbit in pseudo-Newtonian
potential. We compare the radiation spectrum from such a flow to the broad band
observational data for Sgr A*.Comment: Proceedings of the AHAR 2008 Conference: The Universe under the
Microscope; Astrophysics at High Angular Resolution, Bad Honef
An Inverse Compton Scattering Origin of X-ray Flares from Sgr A*
The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a
quiescent component dominates the emission at radio and sub-mm wavelengths. The
spectral energy distribution of the quiescent emission from Sgr A* peaks at
sub-mm wavelengths and is modeled as synchrotron radiation from a thermal
population of electrons in the accretion flow, with electron temperatures
ranging up to \,MeV. Here we investigate the mechanism by which
X-ray flare emission is produced through the interaction of the quiescent and
flaring components of Sgr A*. The X-ray flare emission has been interpreted as
inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present
results of simultaneous X-ray and near-IR observations and show evidence that
X-ray peak flare emission lags behind near-IR flare emission with a time delay
ranging from a few to tens of minutes. Our Inverse Compton scattering modeling
places constraints on the electron density and temperature distributions of the
accretion flow and on the locations where flares are produced. In the context
of this model, the strong X-ray counterparts to near-IR flares arising from the
inner disk should show no significant time delay, whereas near-IR flares in the
outer disk should show a broadened and delayed X-ray flare.Comment: 22 pages, 6 figures, 2 tables, AJ (in press
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