66 research outputs found
Model Checkers Are Cool: How to Model Check Voting Protocols in Uppaal
The design and implementation of an e-voting system is a challenging task.
Formal analysis can be of great help here. In particular, it can lead to a
better understanding of how the voting system works, and what requirements on
the system are relevant. In this paper, we propose that the state-of-art model
checker Uppaal provides a good environment for modelling and preliminary
verification of voting protocols. To illustrate this, we present an Uppaal
model of Pr\^et \`a Voter, together with some natural extensions. We also show
how to verify a variant of receipt-freeness, despite the severe limitations of
the property specification language in the model checker
Sgr A* Polarization: No ADAF, Low Accretion Rate, and Non-Thermal Synchrotron Emission
The recent detection of polarized radiation from Sgr A* requires a
non-thermal electron distribution for the emitting plasma. The Faraday rotation
measure must be small, placing strong limits on the density and magnetic field
strength. We show that these constraints rule out advection-dominated accretion
flow models. We construct a simple two-component model which can reproduce both
the radio to mm spectrum and the polarization. This model predicts that the
polarization should rise to nearly 100% at shorter wavelengths. The first
component, possibly a black-hole powered jet, is compact, low density, and
self-absorbed near 1 mm with ordered magnetic field, relativistic Alfven speed,
and a non-thermal electron distribution. The second component is poorly
constrained, but may be a convection-dominated accretion flow with dM/dt~10^-9
M_Sun/yr, in which feedback from accretion onto the black hole suppresses the
accretion rate at large radii. The black hole shadow should be detectable with
sub-mm VLBI.Comment: 4 pages, 1 figure, accepted by ApJL, several changes from submitted
versio
The origin of emission and absorption features in Ton S180 Chandra observations
We present new interpretation of Ton S180 spectrum obtained by {\it Chandra}
Spectrometer (Low Energy Transmission Grating). Several narrow absorption lines
and a few emission disk lines have been successfully fitted to the data. We
have not found any significant edges accompanying line emission. We propose the
interpretation of narrow lines consistent with the paper recently written by
Krolik (2002), where warm absorber is strongly inhomogeneous. Such situation is
possible in so called multi-phase medium, where regions with different
ionization states, densities and temperatures may coexist in thermal
equilibrium under constant pressure. We illustrate this scenario with
theoretical spectra of radiation transfered through a stratified cloud with
constant pressure (instead of constant density) computed by code {\sc titan} in
plane parallel approximation. Detected spectral features are faint and their
presence do not alter the broad band continuum. We model the broad band
continuum of Ton S180 assuming an irradiated accretion disk with a dissipative
warm skin. The set of parameters appropriate for the data cannot be determined
uniquely but models with low values of the black hole mass have too hot and
radially extended warm skin to explain the formation of soft X-ray disk lines
seen in the data.Comment: accepted to Ap
General-relativistic model of hot accretion flows with global Compton cooling
We present a model of optically thin, two-temperature, accretion flows using
an exact Monte Carlo treatment of global Comptonization, with seed photons from
synchrotron and bremsstrahlung emission, as well as with a fully general
relativistic description of both the radiative and hydrodynamic processes. We
consider accretion rates for which the luminosities of the flows are between
~0.001 and 0.01 of the Eddington luminosity. The black hole spin parameter
strongly affects the flow structure within the innermost 10 gravitational
radii. The resulting large difference between the Coulomb heating in models
with a non-rotating and a rapidly rotating black hole is, however, outweighed
by a strong contribution of compression work, much less dependent on spin. The
consequent reduction of effects related to the value of the black spin is more
significant at smaller accretion rates. For a non-rotating black hole, the
compressive heating of electrons dominates over their Coulomb heating, and
results in an approximately constant radiative efficiency of approximately 0.4
per cent in the considered range of luminosities. For a rapidly rotating black
hole, the Coulomb heating dominates, the radiative efficiency is ~1 per cent
and it slightly increases (but less significantly than estimated in some
previous works) with increasing accretion rate. We find an agreement between
our model, in which the synchrotron emission is the main source of seed
photons, and observations of black-hole binaries in their hard states and AGNs
at low luminosities. In particular, our model predicts a hardening of the X-ray
spectrum with increasing luminosity, as indeed observed below ~0.01 of the
Eddington luminosity in both black-hole binaries and AGNs. Also, our model
approximately reproduces the luminosity and the slope of the X-ray emission in
Cen A.Comment: 13 pages, MNRAS, accepte
A Structure for Quasars
This paper proposes a simple, empirically derived, unifying structure for the
inner regions of quasars. This structure is constructed to explain the broad
absorption line (BAL) regions, the narrow `associated' ultraviolet and X-ray
warm absorbers (NALs); and is also found to explain the broad emission line
regions (BELR), and several scattering features, including a substantial
fraction of the broad X-ray Iron-K emission line, and the bi-conical extended
narrow emission line region (ENLR) structures seen on large kiloparsec scales
in Seyfert images. Small extensions of the model to allow luminosity dependent
changes in the structure may explain the UV and X-ray Baldwin effects and the
greater prevalence of obscuration in low luminosity AGN.Comment: 35 pages, including 8 color figures (figures 4abc are big).
Astrophysical Journal, in press. Expanded version of conference paper
astro-ph/000516
Monte Carlo simulations of global Compton cooling in inner regions of hot accretion flows
Hot accretion flows such as advection-dominated accretion flows are generally
optically thin in the radial direction. Thus photons generated at some radii
can cool or heat electrons at other radii via Compton scattering. Such global
Compton scattering has previously been shown to be important for the dynamics
of accretion flows. Here, we extend previous treatments of this problem by
using accurate global general relativistic Monte Carlo simulations. We focus on
an inner region of the accretion flow (R < 600R_g), for which we obtain a
global self-consistent solution. As compared to the initial, not
self-consistent solution, the final solution has both the cooling rate and the
electron temperature significantly reduced at radii >=10 gravitational radii.
On the other hand, the radiation spectrum of the self-consistent solution has
the shape similar to that of the initial iteration, except for the high-energy
cut-off being at an energy lower by a factor of ~2 and the bolometric
luminosity decreased by a factor of ~2. We also compare the global Compton
scattering model with local models in spherical and slab geometry. We find that
the slab model approximates the global model significantly better than the
spherical one. Still, neither local model gives a good approximation to the
radial profile of the cooling rate, and the differences can be up to two orders
of magnitude. The local slab model underestimates the cooling rate at outer
regions whereas it overestimates that rate at inner regions. We compare our
modelling results to observed hard-state spectra of black-hole binaries and
find an overall good agreement provided any disc outflow is weak. We find that
general-relativistic effects in flows which dynamics is modified by global
Comptonization is crucial in approaching this agreement.Comment: 9 pages, 4 figures. Accepted to MNRAS. Add a new section to discuss
on the impact of outflow and viscous electron heatin
Peering through the dust: Evidence for a supermassive Black Hole at the nucleus of Centaurus A from VLT IR spectroscopy
We used the near infrared spectrometer ISAAC at the ESO 'Very Large
Telescope' to map the velocity field of Centaurus A (NGC 5128) at several
position angles and locations in the central 20" of the galaxy. The high
spatial resolution (~0.5") velocity fields from both ionized and molecular gas
(PaBeta, [FeII], BrGamma, and H2) are not compromised by either excitation
effects or obscuration. We identify three distinct kinematical systems: (i) a
rotating 'nuclear disk' of ionized gas, confined to the inner 2", the
counterpart of the PaAlpha feature previously revealed by HST/NICMOS imaging;
(ii) a ring-like system with a ~6" inner radius detected only in H2, likely the
counterpart of the 100pc-scale structure detected in CO by other authors; (iii)
a normal extended component of gas rotating in the galactic potential. The
nuclear disk is in keplerian rotation around a central mass concentration, dark
(M/L>20 Msun/LsunK) and point-like at the spatial resolution of the data
(R<0.25" ~4pc). We interpret this mass concentration as a supermassive black
hole. Its dynamical mass based on the line velocities and disk inclination
(i>15deg) is M=2(+3.0;-1.4) 10^8 Msun. The ring-like system is probably
characterized by non-circular motions; a 'figure-of-8' pattern observed in the
H2 position-velocity diagram might provide kinematical evidence for the
presence of a nuclear bar.Comment: 43 pages, 19 figures, Astrophysical Journal in press, higher quality
figures available at http://www.arcetri.astro.it/~marconi/pubs.htm
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