18,408 research outputs found
The origin of blue-shifted absorption features in the X-ray spectrum of PG 1211+143: Outflow or disc?
In some radio-quiet active galaxies (AGN), high-energy absorption features in
the x-ray spectra have been interpreted as Ultrafast Outflows (UFOs) -- highly
ionised material (e.g. Fe XXV and Fe XXVI) ejected at mildly relativistic
velocities. In some cases, these outflows can carry energy in excess of the
binding energy of the host galaxy. Needless to say, these features demand our
attention as they are strong signatures of AGN feedback and will influence
galaxy evolution. For the same reason, alternative models need to be discussed
and refuted or confirmed. Gallo & Fabian proposed that some of these features
could arise from resonance absorption of the reflected spectrum in a layer of
ionised material located above and corotating with the accretion disc.
Therefore, the absorbing medium would be subjected to similar blurring effects
as seen in the disc. A priori, the existence of such plasma above the disc is
as plausible as a fast wind. In this work, we highlight the ambiguity by
demonstrating that the absorption model can describe the ~7.6 keV absorption
feature (and possibly other features) in the quasar PG 1211+143, an AGN that is
often described as a classic example of an UFO. In this model, the 2-10 keV
spectrum would be largely reflection dominated (as opposed to power law
dominated in the wind models) and the resonance absorption would be originating
in a layer between about 6 and 60 gravitational radii. The studies of such
features constitutes a cornerstone for future X-ray observatories like Astro-H
and Athena+. Should our model prove correct, or at least important in some
cases, then absorption will provide another diagnostic tool with which to probe
the inner accretion flow with future missions.Comment: 4 pages. Accepted for publication in MNRAS Letter
An alternative method of analysis for base accelerated dynamic response in NASTRAN
An alternative method of analysis to determine the dynamic response of structures subjected to base accelerations is presented. The method is exact as opposed to the approximate technique of using unusually large masses and loads to enforce desired base accelerations. This paper presents the relevant equations to motion, ALTERs for direct and modal frequency-, random- and transient-response rigid formats, and illustrative examples
Acoustical modes of arbitrary volumes using NASTRAN transient heat transfer RF9
An equivalence between temperature and pressure, heat conduction and stiffness and heat capacity and mass is defined, enabling acoustical modal analysis of arbitrary three dimensional volumes. The transient heat transfer analysis rigid format in NASTRAN, RF9, has been altered providing the acoustical analysis capability. Examples and ALTERs are included
Random sequential adsorption and diffusion of dimers and k-mers on a square lattice
We have performed extensive simulations of random sequential adsorption and
diffusion of -mers, up to in two dimensions with particular attention
to the case . We focus on the behavior of the coverage and of vacancy
dynamics as a function of time. We observe that for a complete coverage
of the lattice is never reached, because of the existence of frozen
configurations that prevent isolated vacancies in the lattice to join. From
this result we argue that complete coverage is never attained for any value of
. The long time behavior of the coverage is not mean field and nonanalytic,
with as leading term. Long time coverage regimes are independent of
the initial conditions while strongly depend on the diffusion probability and
deposition rate and, in particular, different values of these parameters lead
to different final values of the coverage. The geometrical complexity of these
systems is also highlighted through an investigation of the vacancy population
dynamics.Comment: 9 pages, 9 figures, to be published in the Journal of Chemical
Physic
A longer XMM-Newton look at I Zwicky 1: Distinct modes of X-ray spectral variability
The short-term spectral variability of the narrow-line Seyfert 1 galaxy I
Zwicky 1 (I Zw 1) as observed in an 85 ks XMM-Newton observation is discussed
in detail. I Zw 1 shows distinct modes of variability prior to and after a flux
dip in the broad-band light curve. Before the dip the variability can be
described as arising from changes in shape and normalisation of the spectral
components. Only changes in normalisation are manifested after the dip. The
change in the mode of behaviour occurs on dynamically short timescales in I Zw
1. The data suggest that the accretion-disc corona in I Zw 1 could have two
components that are co-existing. The first, a uniform, physically diffuse
plasma responsible for the "typical'' long-term (e.g. years) behaviour; and a
second compact, centrally located component causing the rapid flux and spectral
changes. This compact component could be the base of a short or aborted jet as
sometimes proposed for radio-quiet active galaxies. Modelling of the average
and time-resolved rms spectra demonstrate that a blurred Compton-reflection
model can describe the spectral variability if we allow for pivoting of the
continuum component prior to the dip.Comment: 8 pages, 9 figures. Accepted by MNRA
Probing the geometry and motion of AGN coronae through accretion disc emissivity profiles
To gain a better understanding of the inner disc region that comprises active
galactic nuclei it is necessary to understand the pattern in which the disc is
illuminated (the emissivity profile) by X-rays emitted from the continuum
source above the black hole (corona). The differences in the emissivity
profiles produced by various corona geometries are explored via general
relativistic ray tracing simulations. Through the analysis of various
parameters of the geometries simulated it is found that emissivity profiles
produced by point source and extended geometries such as cylindrical slabs and
spheroidal coronae placed on the accretion disc are distinguishable. Profiles
produced by point source and conical geometries are not significantly
different, requiring an analysis of reflection fraction to differentiate the
two geometries. Beamed point and beamed conical sources are also simulated in
an effort to model jet-like coronae, though the differences here are most
evident in the reflection fraction. For a point source we determine an
approximation for the measured reflection fraction with the source height and
velocity. Simulating spectra from the emissivity profiles produced by the
various geometries produce distinguishable differences. Overall spectral
differences between the geometries do not exceed 15 per cent in the most
extreme cases. It is found that emissivity profiles can be useful in
distinguishing point source and extended geometries given high quality spectral
data of extreme, bright sources over long exposure times. In combination with
reflection fraction, timing, and spectral analysis we may use emissivity
profiles to discern the geometry of the X-ray source.Comment: 15 pages, 12 figures. Accepted for publication in MNRA
NASTRAN forced vibration analysis of rotating cyclic structures
Theoretical aspects of a new capability developed and implemented in NASTRAN level 17.7 to analyze forced vibration of a cyclic structure rotating about its axis of symmetry are presented. Fans, propellers, and bladed shrouded discs of turbomachines are some examples of such structures. The capability includes the effects of Coriolis and centripetal accelerations on the rotating structure which can be loaded with: (1) directly applied loads moving with the structure and (2) inertial loas due to the translational acceleration of the axis of rotation (''base' acceleration). Steady-state sinusoidal or general periodic loads are specified to represent: (1) the physical loads on various segments of the complete structure, or (2) the circumferential harmonic components of the loads in (1). The cyclic symmetry feature of the rotating structure is used in deriving and solving the equations of forced motion. Consequently, only one of the cyclic sectors is modelled and analyzed using finite elements, yielding substantial savings in the analysis cost. Results, however, are obtained for the entire structure. A tuned twelve bladed disc example is used to demonstrate the various features of the capability
Caught in the act: Measuring the changes in the corona that cause the extreme variability of 1H 0707-495
The X-ray spectra of the narrow line Seyfert 1 galaxy, 1H 0707-495, obtained
with XMM-Newton, from time periods of varying X-ray luminosity are analysed in
the context of understanding the changes to the X-ray emitting corona that lead
to the extreme variability seen in the X-ray emission from active galactic
nuclei (AGN). The emissivity profile of the accretion disc, illuminated by the
X-ray emitting corona, along with previous measurements of reverberation time
lags are used to infer the spatial extent of the X-ray source. By fitting a
twice-broken power law emissivity profile to the relativistically-broadened
iron K fluorescence line, it is inferred that the X-ray emitting corona expands
radially, over the plane of the accretion disc, by 25 to 30 per cent as the
luminosity increases, contracting again as the luminosity decreases, while
increases in the measured reverberation lag as the luminosity increases would
require also variation in the vertical extent of the source above the disc. The
spectrum of the X-ray continuum is found to soften as the total X-ray
luminosity increases and we explore the variation in reflected flux as a
function of directly-observed continuum flux. These three observations combined
with simple, first-principles models constructed from ray tracing simulations
of extended coron self-consistently portray an expanding corona whose average
energy density decreases, but with a greater number of scattering particles as
the luminosity of this extreme object increases.Comment: 12 pages, 4 figures. Accepted for publication in MNRA
Forced vibration analysis of rotating cyclic structures in NASTRAN
A new capability was added to the general purpose finite element program NASTRAN Level 17.7 to conduct forced vibration analysis of tuned cyclic structures rotating about their axis of symmetry. The effects of Coriolis and centripetal accelerations together with those due to linear acceleration of the axis of rotation were included. The theoretical, user's, programmer's and demonstration manuals for this new capability are presented
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