2,071 research outputs found
A Rewriting Based Model for Probabilistic Distributed Object Systems
Concurrent and distributed systems have traditionally been modelled using nondeterministic transitions over configurations. The nondeterminism provides an abstraction over scheduling, network delays, failures and randomization. However a probabilistic model can capture these sources of nondeterminism more precisely and enable statistical analysis, simulations and reasoning. We have developed a general semantic framework for probabilistic systems using probabilistic rewriting. Our framework also allows nondeterminism in the system. In this paper, we briefly describe the framework and its application to concurrent object based systems such as actors. We also identify a su#ciently expressive fragment of the general framework and describe its implementation. The concepts are illustrated by a simple client-server example
Absorption lines from magnetically-driven winds in X-ray binaries
High resolution X-ray spectra of black hole X-ray binaries (BHBs) show
blueshifted absorption lines from disk winds which seem to be equatorial. Winds
occur in the Softer (disk-dominated) states of the outburst and are less
prominent or absent in the Harder (power-law dominated) states. We use
self-similar magneto-hydrodynamic (MHD) accretion-ejection models to explain
the disk winds in BHBs. In our models, the density at the base of the outflow
from the accretion disk is not a free parameter, but is determined by solving
the full set of dynamical MHD equations. Thus the physical properties of the
outflow are controlled by the global structure of the disk. We studied
different MHD solutions characterized by different values of (a) the disk
aspect ratio () and (b) the ejection efficiency (). We use two
kinds of MHD solutions depending on the absence (cold solution) or presence
(warm solution) of heating at the disk surface. Such heating could be from e.g.
dissipation of energy due to MHD turbulence in the disk or from illumination.
We use each of these MHD solutions to predict the physical parameters of an
outflow; put limits on the ionization parameter (), column density and
timescales, motivated by observational results; and thus select regions within
the outflow which are consistent with the observed winds. The cold MHD
solutions cannot account for winds due to their low ejection efficiency. But
warm solutions can explain the observed physical quantities in the wind because
they can have sufficiently high values of (, implying larger
mass loading at the base of the outflow). Further from our thermodynamic
equilibrium curve analysis for the outflowing gas, we found that in the Hard
state a range of is thermodynamically unstable, and had to be excluded.
This constrain made it impossible to have any wind at all, in the Hard state.Comment: 16 Pages, 10 figures in the main body and 4 figures in the appendix.
Accepted for publication in A&
Dust Emission from Active Galactic Nuclei
Unified schemes of active galactic nuclei (AGN) require an obscuring dusty
torus around the central source, giving rise to Seyfert 1 line spectrum for
pole-on viewing and Seyfert 2 characteristics in edge-on sources. Although the
observed IR is in broad agreement with this scheme, the behavior of the 10
micron silicate feature and the width of the far-IR emission peak remained
serious problems in all previous modeling efforts. We show that these problems
find a natural explanation if the dust is contained in about 5-10 clouds along
radial rays through the torus. The spectral energy distributions (SED) of both
type 1 and type 2 sources are properly reproduced from different viewpoints of
the same object if the visual optical depth of each cloud is larger than about
60 and the clouds' mean free path increases roughly in proportion to radial
distance.Comment: 11 pages, submitted to ApJ Letter
Modelling and analyzing adaptive self-assembling strategies with Maude
Building adaptive systems with predictable emergent behavior is a challenging task and it is becoming a critical need. The research community has accepted the challenge by introducing approaches of various nature: from software architectures, to programming paradigms, to analysis techniques. We recently proposed a conceptual framework for adaptation centered around the role of control data. In this paper we show that it can be naturally realized in a reflective logical language like Maude by using the Reflective Russian Dolls model. Moreover, we exploit this model to specify and analyse a prominent example of adaptive system: robot swarms equipped with obstacle-avoidance self-assembly strategies. The analysis exploits the statistical model checker PVesta
The international AGN watch: A multiwavelength monitoring consortium
The International AGN Watch, an informal consortium of over 100 astronomers, was established to coordinate multiwavelength monitoring of a limited number of active galactic nuclei and thus obtain comprehensive continuum and emission-line variability data with unprecedented temporal and wavelength coverage. We summarize the principal scientific results from two completed space-based and ground-based campaigns on the Seyfert galaxies NGC 5548 and NGC 3783. We describe a project in progress and outline our future plans
The Detection of Crystalline Silicates in Ultra-Luminous Infrared Galaxies
Silicates are an important component of interstellar dust and the structure
of these grains -- amorphous versus crystalline -- is sensitive to the local
physical conditions. We have studied the infrared spectra of a sample of
ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow
absorption features at 11, 16, 19, 23, and 28 microns, characteristic of
crystalline silicates, superimposed on the broad absorption bands at 10 and 18
microns due to amorphous silicates in a subset of this sample. These features
betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of
the olivines. Previously, crystalline silicates have only been observed in
circumstellar environments. The derived fraction of forsterite to amorphous
silicates is typically 0.1 in these ULIRGs. This is much larger than the upper
limit for this ratio in the interstellar medium of the Milky Way, 0.01. These
results suggest that the timescale for injection of crystalline silicates into
the ISM is short in a merger-driven starburst environment (e.g., as compared to
the total time to dissipate the gas), pointing towards massive stars as a
prominent source of crystalline silicates. Furthermore, amorphization due to
cosmic rays, which is thought to be of prime importance for the local ISM, lags
in vigorous starburst environments.Comment: 7 pages, 5 figures, accepted for publication in Ap
Reddening, Emission-Line, and Intrinsic Absorption Properties in the Narrow-Line Seyfert 1 Galaxy Akn 564
We use Hubble Space Telescope UV and optical spectra of the narrow-line
Seyfert 1 (NLS1) galaxy Akn 564 to investigate its internal reddening and
properties of its emission-line and intrinsic UV absorption gas. We find that
the extinction curve of Akn 564, derived from a comparison of its UV/optical
continuum to that of an unreddened NLS1, lacks a 2200 A bump and turns up
towards the UV at a longer wavelength (4000 A) than the standard Galactic, LMC,
and SMC curves. However, it does not show the extremely steep rise to 1200 A
that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The
emission-lines and continuum experience the same amount of reddening,
indicating the presence of a dust screen that is external to the narrow-line
region (NLR). Echelle spectra from the Space Telescope Imaging Spectrograph
show intrinsic UV absorption lines due to Ly-alpha, N V, C IV, Si IV, and Si
III, centered at a radial velocity of -190 km/s (relative to the host galaxy).
Photoionization models of the UV absorber indicate that it has a sufficient
columnand is at a sufficient distance from the nucleus (D > 95 pc) to be the
source of the dust screen. Thus, Akn 564 contains a dusty ``lukewarm absorber''
similar to that seen in NGC 3227.Comment: 25 pages, 4 figures. LaTeX with encapsulated postscript figures.
Accepted for publication in the Astrophysical Journa
STIS Echelle Observations of the Seyfert Galaxy NGC 4151: Physical Conditions in the Ultraviolet Absorbers
We have examined the physical conditions in intrinsic UV-absorbing gas in the
Seyfert galaxy NGC 4151, using echelle spectra obtained with the Space
Telescope Imaging Spectrograph (STIS). We confirm the presence of the kinematic
components detected in earlier GHRS observations as well as a new broad
absorption feature at a radial velocity of -1680 km/s. The UV continuum of NGC
4151 decreased by a factor of 4 over the previous two years, and we argue the
changes in the column density of the low ionization absorption lines associated
with the broad component at -490 km/s reflect the decrease in the ionizing
flux. Most of the strong absorption lines (e.g., N V, C IV, Si IV) from this
component are saturated, but show substantial residual flux in their cores,
indicating that the absorber does not fully cover the source of emission. Our
interpretation is that the unocculted light is due to scattering by free
electrons from an extended region, which reflects continuum, emission lines,
and absorption lines. We have been able to constrain the densities for the
kinematic components based on absorption lines from metastable states of C III
and Fe II, and/or the ratios of ground and fine structure lines of O I,C II,
and Si II. We have generated a set of photoionization models which match the
ionic column densities for each component during the present low flux state and
those seen in previous high flux states with the GHRS and STIS, confirming that
the absorbers are photoionized and respond to the changes in the continuum
flux. We have been able to map the relative radial positions of the absorbers,
and find that the gas decreases in density with distance. None of the UV
absorbers is of sufficiently large column density or high enough ionization
state to account for the X-ray absorption.Comment: 46 pages (Latex), 14 figures (postscript), plus a landscape table
(Latex), to appear in the Astrophysical Journa
The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infra-Red to X-Ray Spectral Energy Distribution
We present extinction curves derived from the broad emission lines and
continua of large samples of both radio-loud and radio-quiet AGNs. The curves
are significantly flatter in the UV than are curves for the local ISM. The
reddening curves for the radio-quiet LBQS quasars are slightly steeper than
those of the radio-loud quasars in the UV, probably because of additional
reddening by dust further out in the host galaxies of the former. The UV
extinction curves for the radio-loud AGNs are very flat. This is explicable
with slight modifications to standard MRN dust models: there is a relative lack
of small grains in the nuclear dust. Our continuum and broad-emission line
reddening curves agree in both shape and amplitude, confirming that the
continuum shape is indeed profoundly affected by reddening for all but the
bluest AGNs. With correction by our generic extinction curve, all of the
radio-loud AGNs have continuous optical-UV spectra consistent with a single
shape. We show that radio-quiet AGNs have very similar intrinsic UV to optical
shape over orders of magnitude in luminosity. We also argue that radio-loud and
radio-quiet AGNs probably share the same underlying continuum shape and that
most of the systematic differences between their observed continuum shapes are
due to higher nuclear reddening in radio-selected AGNs, and additional
reddening from dust further out in the host galaxies in radio-quiet AGNs. Our
conclusions have important implications for the modelling of quasar continua
and the analysis of quasar demographics.Comment: 41 pages, including 6 figures and 3 tables. To appear in ApJ vol.
614, October 20 issue. Some slight wording changes. Some additional
references added. Small changes in the model fit in section 6.2, to the
analytical fit in the Appendix, and to the tabulated reddening curve in the
Appendi
Automated Certification of Authorisation Policy Resistance
Attribute-based Access Control (ABAC) extends traditional Access Control by
considering an access request as a set of pairs attribute name-value, making it
particularly useful in the context of open and distributed systems, where
security relevant information can be collected from different sources. However,
ABAC enables attribute hiding attacks, allowing an attacker to gain some access
by withholding information. In this paper, we first introduce the notion of
policy resistance to attribute hiding attacks. We then propose the tool ATRAP
(Automatic Term Rewriting for Authorisation Policies), based on the recent
formal ABAC language PTaCL, which first automatically searches for resistance
counter-examples using Maude, and then automatically searches for an Isabelle
proof of resistance. We illustrate our approach with two simple examples of
policies and propose an evaluation of ATRAP performances.Comment: 20 pages, 4 figures, version including proofs of the paper that will
be presented at ESORICS 201
- …