732 research outputs found
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&
AC conductivity analysis for a metal core-silver orthosilicate shell nanostructure
Nanocomposites containing silver particles of diameter of 20 nm with silver orthosilicate crystals forming the shell with thickness around 21 nm closely packed in a silicate glass were prepared. The ac conductivity of samples subjected to different heat treatments were measured over the frequency range of 100 Hz to 6 MHz in the temperature range of 500-570 K. The data were analyzed by the Macdonald model based on Kohlrausch-related frequency response formalism designated as CK0. The reference system was taken as the glass-crystal composite containing a lithium orthosilicate crystalline phase. The shape parameter βo for the reference system was found to be 0.33, whereas that for the nanocomposites was extracted to be 0.46. The former implied a one-dimensional lithium ion motion along the grain boundaries of the orthosilicate crystals, whereas the latter indicated that there was a one-dimensional silver ion motion in an effective two-dimensional structure in the shell surface because of a high stress condition along the radial direction of the core-shell composite
A solvable model of the genesis of amino-acid sequences via coupled dynamics of folding and slow genetic variation
We study the coupled dynamics of primary and secondary structure formation
(i.e. slow genetic sequence selection and fast folding) in the context of a
solvable microscopic model that includes both short-range steric forces and and
long-range polarity-driven forces. Our solution is based on the diagonalization
of replicated transfer matrices, and leads in the thermodynamic limit to
explicit predictions regarding phase transitions and phase diagrams at genetic
equilibrium. The predicted phenomenology allows for natural physical
interpretations, and finds satisfactory support in numerical simulations.Comment: 51 pages, 13 figures, submitted to J. Phys.
A unified accretion-ejection paradigm for black hole X-ray binaries. III. Spectral signatures of hybrid disk configurations
It has been suggested that the cycles of activity of X-ray Binaries (XrB) are
triggered by a switch in the dominant disk torque responsible for accretion
(paper I). As the disk accretion rate increases, the disk innermost regions
would change from a jet-emitting disk (JED) to a standard accretion disk (SAD).
While JEDs have been proven to successfully reproduce hard states (paper II),
the existence of an outer cold SAD introduces an extra non local cooling term.
We investigate the thermal structure and associated spectra of such a hybrid
disk configuration. We use the 2T plasma code elaborated in paper II, allowing
to compute outside-in the disk local thermal equilibrium with self-consistent
advection and optically thin-to-thick transitions, in both radiation and gas
supported regimes. The non-local inverse Compton cooling introduced by the
external soft photons is computed by the BELM code. This additional term has a
profound influence on JED solutions, allowing a smooth temperature transition
from the outer SAD to the inner JED. We explore the full parameter space in
disk accretion rate and transition radius, and show that the whole domain in
X-ray fluxes and hardness ratios covered by standard XrB cycles is well
reproduced by such hybrid configurations. Precisely, a reasonable combination
of these parameters allows to reproduce the 3-200 keV spectra of five canonical
XrB states. Along with X-ray signatures, JED-SAD configurations also naturally
account for the radio emission whenever it is observed. By varying only the
transition radius and the accretion rate, hybrid disk configurations combining
an inner JED and an outer SAD are able to reproduce simultaneously the X-ray
spectral states and radio emission of X-ray binaries during their outburst.
Adjusting these two parameters, it is then possible to reproduce a full cycle.
This will be shown in a forthcoming paper (paper IV).Comment: Accepted for publication in A&
A natural orbital functional for the many-electron problem
The exchange-correlation energy in Kohn-Sham density functional theory is
expressed as a functional of the electronic density and the Kohn-Sham orbitals.
An alternative to Kohn-Sham theory is to express the energy as a functional of
the reduced first-order density matrix or equivalently the natural orbitals. In
the former approach the unknown part of the functional contains both a kinetic
and a potential contribution whereas in the latter approach it contains only a
potential energy and consequently has simpler scaling properties. We present an
approximate, simple and parameter-free functional of the natural orbitals,
based solely on scaling arguments and the near satisfaction of a sum rule. Our
tests on atoms show that it yields on average more accurate energies and charge
densities than the Hartree Fock method, the local density approximation and the
generalized gradient approximations
The outflow in Mrk 509: A method to calibrate XMM-Newton EPIC-pn and RGS
We have analyzed three XMM-Newton observations of the Seyfert 1 galaxy Mrk
509, with the goal to detect small variations in the ionized outflow
properties. Such measurements are limited by the quality of the
cross-calibration between RGS, the best instrument to characterize the
spectrum, and EPIC-pn, the best instrument to characterize the variability. For
all three observations we are able to improve the relative calibration of RGS
and pn consistently to 4 %. In all observations we detect three different
outflow components and, thanks to our accurate cross-calibration we are able to
detect small differences in the ionization parameter and column density in the
highest ionized component of the outflow. This constrains the location of this
component of the outflow to within 0.5 pc of the central source. Our method for
modeling the relative effective area is not restricted to just this source and
can in principle be extended to other types of sources as well.Comment: 11 pages, 9 figure
Multiwavelength Campaign on Mrk 509 X. Lower limit on the distance of the absorber from HST COS and STIS spectroscopy
Active Galactic Nuclei often show evidence of photoionized outflows. A major
uncertainty in models for these outflows is the distance () to the gas from
the central black hole. In this paper we use the HST/COS data from a massive
multi-wavelength monitoring campaign on the bright Seyfert I galaxy Mrk 509, in
combination with archival HST/STIS data, to constrain the location of the
various kinematic components of the outflow. We compare the expected response
of the photoionized gas to changes in ionizing flux with the changes measured
in the data using the following steps: 1) We compare the column densities of
each kinematic component measured in the 2001 STIS data with those measured in
the 2009 COS data; 2) We use time-dependent photionization calculations with a
set of simulated lightcurves to put statistical upper limits on the hydrogen
number density that are consistent with the observed small changes in the ionic
column densities; 3) From the upper limit on the number density, we calculate a
lower limit on the distance to the absorber from the central source via the
prior determination of the ionization parameter. Our method offers two
improvements on traditional timescale analysis. First, we account for the
physical behavior of AGN lightcurves. Second, our analysis accounts for the
quality of measurement in cases where no changes are observed in the absorption
troughs. The very small variations in trough ionic column densities (mostly
consistent with no change) between the 2001 and 2009 epochs allow us to put
statistical lower limits on the distance between 100--200 pc for all the major
UV absorption components at a confidence level of 99%. These results are mainly
consistent with the independent distance estimates derived for the warm
absorbers from the simultaneous X-ray spectra.Comment: Accepted to A&A (06 APR 2012
A Theory of Sampling for Continuous-time Metric Temporal Logic
This paper revisits the classical notion of sampling in the setting of
real-time temporal logics for the modeling and analysis of systems. The
relationship between the satisfiability of Metric Temporal Logic (MTL) formulas
over continuous-time models and over discrete-time models is studied. It is
shown to what extent discrete-time sequences obtained by sampling
continuous-time signals capture the semantics of MTL formulas over the two time
domains. The main results apply to "flat" formulas that do not nest temporal
operators and can be applied to the problem of reducing the verification
problem for MTL over continuous-time models to the same problem over
discrete-time, resulting in an automated partial practically-efficient
discretization technique.Comment: Revised version, 43 pages
The warm absorber and X-ray variability of the Seyfert 1 galaxy NGC 3516 as seen by the XMM-Newton RGS
We present a new analysis of the soft and medium energy X-ray spectrum of the
Seyfert 1 galaxy NGC 3516 taken with the Reflection Grating Spectrometer (RGS)
and European Photon Imaging Camera (EPIC) on board the XMM-Newton observatory.
We examine four observations made in October 2006. We investigate whether the
observed variability is due to absorption by the warm absorber and/or is
intrinsic to the source emission. We analyse in detail the EPIC-pn and RGS
spectra of each observation separately. The warm absorber in NGC 3516 is found
to consist of three phases of ionisation, two of which have outflow velocities
of more than 1000 km/s. The third phase (the least ionised one) is much slower
at 100 km/s. One of the high ionisation phases, with log xi of 2.4, is found to
have a partial covering fraction of about 60%. It has previously been suggested
that the passage of a cloud, part of a disk wind, in front of the source
(producing a change in the covering fraction) was the cause of a significant
dip in the lightcurve during one of the observations. From our modelling of the
EPIC-pn and RGS spectra, we find that variation in the covering fraction cannot
be solely responsible for this. We show that intrinsic change in the source
continuum plays a much more significant role in explaining the observed flux
and spectral variability than originally thought.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
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