232 research outputs found
Heating and Cooling of Hot Accretion Flows by Non Local Radiation
We consider non-local effects which arise when radiation emitted at one
radius of an accretion disk either heats or cools gas at other radii through
Compton scattering. We discuss three situations:
1. Radiation from the inner regions of an advection-dominated flow Compton
cooling gas at intermediate radii and Compton heating gas at large radii.
2. Soft radiation from an outer thin accretion disk Compton cooling a hot
one- or two-temperature flow on the inside.
3. Soft radiation from an inner thin accretion disk Compton cooling hot gas
in a surrounding one-temperature flow.
We describe how previous results are modified by these non-local
interactions. We find that Compton heating or cooling of the gas by the
radiation emitted in the inner regions of a hot flow is not important.
Likewise, Compton cooling by the soft photons from an outer thin disk is
negligible when the transition from a cold to a hot flow occurs at a radius
greater than some minimum . However, if the hot flow terminates at
, non-local cooling is so strong that the hot gas is cooled to
a thin disk configuration in a runaway process. In the case of a thin disk
surrounded by a hot one-temperature flow, we find that Compton cooling by soft
radiation dominates over local cooling in the hot gas for \dot{M} \gsim
10^{-3} \alpha \dot{M}_{Edd}, and R \lsim 10^4 R_{Schw}. As a result, the
maximum accretion rate for which an advection-dominated one-temperature
solution exists, decreases by a factor of , compared to the value
computed under an assumption of local energy balance.Comment: LaTeX aaspp.sty, 25 pages, and 6 figures; to appear in Ap
Energy and Momentum Transfer via Coulomb Frictions in Relativistic Two Fluids
We numerically calculate the energy and momentum transfer rates due to
Coulomb scattering between two fluids moving with a relative velocity. The
results are fitted by simple functions. The fitting formulae are useful to
simulate outflows from active galactic nuclei and compact high energy sources.Comment: 29 pages, 11 figures. accepted for publication in ApJ
Formal Specification and Testing of a Management Architecture
The importance of network and distributed systems management to supply and maintain services required by users has led to a demand for management facilities. Open network management is assisted by representing the system resources to be managed as objects, and providing standard services and protocols for interrogating and manipulating these objects. This paper examines the application of formal description techniques to the specification of managed objects by presenting a case study in the specification and testing of a management architecture. We describe a formal specification of a management architecture suitable for scheduling and distributing services across nodes in a distributed system. In addition, we show how formal specifications can be used to generate conformance tests for the management architecture
X-ray Images of Hot Accretion Flows
We consider the X-ray emission due to bremsstrahlung processes from hot, low
radiative-efficiency accretion flows around supermassive and galactic black
holes. We calculate surface brightness profiles and Michelson visibility
functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to
allow for the presence of outflows. We find that although the 1 keV emitting
region in these flows can always extend up to 10^6 Schwarzschild radii (R_S),
their surface brightness profiles and visibility functions are strongly
affected by the specific density profile. The advection-dominated solutions
with no outflows (p=0) lead to centrally peaked profiles with characteristic
sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to
flat intensity profiles with significantly larger characteristic sizes of up to
10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may
appear as extended X-ray sources when observed with current X-ray imaging
instruments. We show that X-ray brightness profiles and their associated
visibility functions may be powerful probes for determining the relevant mode
of accretion and, in turn, the properties of hot accretion flows. We discuss
the implications of our results for observations with the Chandra X-ray
Observatory and the planned X-ray interferometer MAXIM.Comment: 14 pages, 4 figures, accepted by The Astrophysical Journal, minor
change
Spectrum of Optically Thin Advection Dominated Accretion Flow around a Black Hole: Application to Sgr A*
The global structure of optically thin advection dominated accretion flows
which are composed of two-temperature plasma around black holes is calculated.
We adopt the full set of basic equations including the advective energy
transport in the energy equation for the electrons. The spectra emitted by the
optically thin accretion flows are also investigated. The radiation mechanisms
which are taken into accout are bremsstrahlung, synchrotron emission, and
Comptonization. The calculation of the spectra and that of the structure of the
accretion flows are made to be completely consistent by calculating the
radiative cooling rate at each radius. As a result of the advection domination
for the ions, the heat transport from the ions to the electrons becomes
practically zero and the radiative cooling balances with the advective heating
in the energy equation of the electrons. Following up on the successful work of
Narayan et al. (1995), we applied our model to the spectrum of Sgr A*. We find
that the spectrum of Sgr A* is explained by the optically thin advection
dominated accretion flow around a black hole of the mass M_bh=10^6 M_sun. The
parameter dependence of the spectrum and the structure of the accretion flows
is also discussed.Comment: AAS LaTeX file; 26 pages; 12 ps figures; to be published in ApJ. PDF
files are obtainable via following anonymous ftp.
ftp://ftp.kusastro.kyoto-u.ac.jp/pub/manmoto/preprint/spec_sgrA.tar.g
Emergence in genetic programming:let's exploit it!
Banzhaf explores the concept of emergence and how and where it happens in genetic programming [1]. Here we consider the question: what shall we do with it? We argue that given our ultimate goal to produce genetic programming systems that solve new and difficult problems, we should take advantage of emergence to get closer to this goal
Are Particles in Advection-Dominated Accretion Flows Thermal?
We investigate the form of the momentum distribution function for protons and
electrons in an advection-dominated accretion flow (ADAF). We show that for all
accretion rates, Coulomb collisions are too inefficient to thermalize the
protons. The proton distribution function is therefore determined by the
viscous heating mechanism, which is unknown. The electrons, however, can
exchange energy quite efficiently through Coulomb collisions and the emission
and absorption of synchrotron photons. We find that for accretion rates greater
than \sim 10^{-3} of the Eddington accretion rate, the electrons have a thermal
distribution throughout the accretion flow. For lower accretion rates, the
electron distribution function is determined by the electron's source of
heating, which is primarily adiabatic compression. Using the principle of
adiabatic invariance, we show that an adiabatically compressed collisionless
gas maintains a thermal distribution until the particle energies become
relativistic. We derive a new, non-thermal, distribution function which arises
for relativistic energies and provide analytic formulae for the synchrotron
radiation from this distribution. Finally, we discuss its implications for the
emission spectra from ADAFs.Comment: 29 pages (Latex), 3 Figures. Submitted to Ap
Going Around Again: Modelling Standing Ovations with a Flexible Agent-based Simulation Framework
We describe how we have used the CoSMoS process to trans- form a computer simulation originally developed for the simulation of plant development for use in modelling aspects of audience behaviour. An existing agent-based simulator is re-factored to simulate a completely dierent type of agent in 2D space. This is possible and desirable because the original simulator was designed with the intention that it could eas- ily be use to model a variety of dierent agents interacting in 2D and 3D space. The resulting simulation will be used to simulate the phe- nomena of standing ovations in audiences as a model system of tipping point behaviour. Continued development of this simulator, assisted by the CoSMoS process, has resulted in a general purpose lightweight sim- ulation framework
Reaction–diffusion chemistry implementation of associative memory neural network
Unconventional computing paradigms are typically very difficult to program. By implementing efficient parallel control architectures such as artificial neural networks, we show that it is possible to program unconventional paradigms with relative ease. The work presented implements correlation matrix memories (a form of artificial neural network based on associative memory) in reaction–diffusion chemistry, and shows that implementations of such artificial neural networks can be trained and act in a similar way to conventional implementations
Pair plasma relaxation time scales
By numerically solving the relativistic Boltzmann equations, we compute the
time scale for relaxation to thermal equilibrium for an optically thick
electron-positron plasma with baryon loading. We focus on the time scales of
electromagnetic interactions. The collisional integrals are obtained directly
from the corresponding QED matrix elements. Thermalization time scales are
computed for a wide range of values of both the total energy density (over 10
orders of magnitude) and of the baryonic loading parameter (over 6 orders of
magnitude). This also allows us to study such interesting limiting cases as the
almost purely electron-positron plasma or electron-proton plasma as well as
intermediate cases. These results appear to be important both for laboratory
experiments aimed at generating optically thick pair plasmas as well as for
astrophysical models in which electron-positron pair plasmas play a relevant
role.Comment: Phys. Rev. E, in pres
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