40,968 research outputs found
Comptonization of the cosmic microwave background by high energy particles residing in AGN cocoons
X-ray cavities and extended radio sources (`cocoons') surrounding active
galactic nuclei (AGN) have been detected by the Chandra X-ray mission and radio
interferometers. A joint analysis of X-ray and radio maps suggests that
pressure values of non-thermal radio-emitting particles derived from the radio
maps are not sufficient to inflate the X-ray cavities. We propose using the
Sunyaev-Zel'dovich (SZ) effect, whose intensity strongly depends on the
pressure, to find the hitherto undetected, dynamically-dominant component in
the radio cocoons.
We demonstrate that the spectral function at a frequency of 217 GHz has an
absolute maximum at a temperature higher than K, therefore the
measurement of the SZ effect at this frequency is a powerful tool for
potentially revealing the dynamically-dominant component inside AGN jet-driven
radio cocoons. A new method is proposed for excluding the contribution from the
low energy, non-relativistic electrons to the SZ effect by means of
observations at two frequencies. We show how one may correct for a possible
contribution from the kinematic SZ effect. The intensity maps of the SZ effect
are calculated for the self-similar Sedov solution, and application of a
predicted ring-like structure on the SZ map at a frequency of 217 GHz is
proposed to determine the energy released during the active jet stage. The SZ
intensity map for an AGN cocoon in a distant elliptical is calculated using a
2-D numerical simulation and including relativistic corrections to the SZ
effect. We show the intensity spectrum of the SZ effect is flat at high
frequencies if gas temperature is as high as
keV.Comment: 12 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
External Mass Accumulation onto Core Potentials: Implications for Star Clusters, Galaxies and Galaxy Clusters
Accretion studies have been focused on the flow around bodies with point mass
gravitational potentials, but few general results are available for non-point
mass distributions. Here, we study the accretion flow onto non-divergent, core
potentials moving through a background medium. We use Plummer and Hernquist
potentials as examples to study gas accretion onto star clusters, dwarf and
large galaxy halos and galaxy clusters in a variety of astrophysical
environments. The general conditions required for a core potential to
collectively accrete large quantities of gas from the external medium are
derived using both simulations and analytic results. The consequences of large
mass accumulation in galaxy nuclei, dwarf galaxies and star clusters are
twofold. First, if the gas cools effectively star formation can be triggered,
generating new stellar members in the system. Second, if the collective
potential of the system is able to alter the ambient gas properties before the
gas is accreted onto the individual core members, the augmented mass supply
rates could significantly alter the state of the various accreting stellar
populations and result in an enhanced central black hole accretion luminosity.Comment: 24 pages, 15 figures, accepted to Ap
The Evolution of X-ray Clusters and the Entropy of the Intra Cluster Medium
The thermodynamics of the diffuse, X-ray emitting gas in clusters of galaxies
is determined by gravitational processes associated with shock heating,
adiabatic compression, and non-gravitational processes such as heating by SNe,
stellar winds, activity in the central galactic nucleus, and radiative cooling.
The effect of gravitational processes on the thermodynamics of the Intra
Cluster Medium (ICM) can be expressed in terms of the ICM entropy S ~
ln(T/\rho^{2/3}). We use a generalized spherical model to compute the X-ray
properties of groups and clusters for a range of initial entropy levels in the
ICM and for a range of mass scales, cosmic epochs and background cosmologies.
We find that the statistical properties of the X-ray clusters strongly depend
on the value of the initial excess entropy. Assuming a constant, uniform value
for the excess entropy, the present-day X-ray data are well fitted for the
following range of values K_* = kT/\mu m_p \rho^{2/3} = (0.4\pm 0.1) \times
10^{34} erg cm^2 g^{-5/3} for clusters with average temperatures kT>2 keV; K_*
= (0.2\pm 0.1) \times 10^{34} erg cm^2 g^{-5/3} for groups and clusters with
average temperatures kT<2 keV. These values correspond to different excess
energy per particle of kT \geq 0.1 (K_*/0.4\times 10^{34}) keV. The dependence
of K_* on the mass scale can be well reproduced by an epoch dependent external
entropy: the relation K_* = 0.8(1+z)^{-1}\times 10^{34} erg cm^2 g^{-5/3} fits
the data over the whole temperature range. Observations of both local and
distant clusters can be used to trace the distribution and the evolution of the
entropy in the cosmic baryons, and ultimately to unveil the typical epoch and
the source of the heating processes.Comment: 53 pages, LateX, 19 figures, ApJ in press, relevant comments and
references adde
Screening and metamodeling of computer experiments with functional outputs. Application to thermal-hydraulic computations
To perform uncertainty, sensitivity or optimization analysis on scalar
variables calculated by a cpu time expensive computer code, a widely accepted
methodology consists in first identifying the most influential uncertain inputs
(by screening techniques), and then in replacing the cpu time expensive model
by a cpu inexpensive mathematical function, called a metamodel. This paper
extends this methodology to the functional output case, for instance when the
model output variables are curves. The screening approach is based on the
analysis of variance and principal component analysis of output curves. The
functional metamodeling consists in a curve classification step, a dimension
reduction step, then a classical metamodeling step. An industrial nuclear
reactor application (dealing with uncertainties in the pressurized thermal
shock analysis) illustrates all these steps
Conformal mapping methods for interfacial dynamics
The article provides a pedagogical review aimed at graduate students in
materials science, physics, and applied mathematics, focusing on recent
developments in the subject. Following a brief summary of concepts from complex
analysis, the article begins with an overview of continuous conformal-map
dynamics. This includes problems of interfacial motion driven by harmonic
fields (such as viscous fingering and void electromigration), bi-harmonic
fields (such as viscous sintering and elastic pore evolution), and
non-harmonic, conformally invariant fields (such as growth by
advection-diffusion and electro-deposition). The second part of the article is
devoted to iterated conformal maps for analogous problems in stochastic
interfacial dynamics (such as diffusion-limited aggregation, dielectric
breakdown, brittle fracture, and advection-diffusion-limited aggregation). The
third part notes that all of these models can be extended to curved surfaces by
an auxilliary conformal mapping from the complex plane, such as stereographic
projection to a sphere. The article concludes with an outlook for further
research.Comment: 37 pages, 12 (mostly color) figure
Statistics of seismic cluster durations
Using the standard ETAS model of triggered seismicity, we present a rigorous
theoretical analysis of the main statistical properties of temporal clusters,
defined as the group of events triggered by a given main shock of fixed
magnitude m that occurred at the origin of time, at times larger than some
present time t. Using the technology of generating probability function (GPF),
we derive the explicit expressions for the GPF of the number of future
offsprings in a given temporal seismic cluster, defining, in particular, the
statistics of the cluster's duration and the cluster's offsprings maximal
magnitudes. We find the remarkable result that the magnitude difference between
the largest and second largest event in the future temporal cluster is
distributed according to the regular Gutenberg-Richer law that controls the
unconditional distribution of earthquake magnitudes. For earthquakes obeying
the Omori-Utsu law for the distribution of waiting times between triggering and
triggered events, we show that the distribution of the durations of temporal
clusters of events of magnitudes above some detection threshold \nu has a power
law tail that is fatter in the non-critical regime than in the critical
case n=1. This paradoxical behavior can be rationalised from the fact that
generations of all orders cascade very fast in the critical regime and
accelerate the temporal decay of the cluster dynamics.Comment: 45 pages, 15 figure
Inference on the tail process with application to financial time series modelling
To draw inference on serial extremal dependence within heavy-tailed Markov
chains, Drees, Segers and Warcho{\l} [Extremes (2015) 18, 369--402] proposed
nonparametric estimators of the spectral tail process. The methodology can be
extended to the more general setting of a stationary, regularly varying time
series. The large-sample distribution of the estimators is derived via
empirical process theory for cluster functionals. The finite-sample performance
of these estimators is evaluated via Monte Carlo simulations. Moreover, two
different bootstrap schemes are employed which yield confidence intervals for
the pre-asymptotic spectral tail process: the stationary bootstrap and the
multiplier block bootstrap. The estimators are applied to stock price data to
study the persistence of positive and negative shocks.Comment: 22 page
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