982 research outputs found
Measurability of kinetic temperature from metal absorption-line spectra formed in chaotic media
We present a new method for recovering the kinetic temperature of the
intervening diffuse gas to an accuracy of 10%. The method is based on the
comparison of unsaturated absorption-line profiles of two species with
different atomic weights. The species are assumed to have the same temperature
and bulk motion within the absorbing region. The computational technique
involves the Fourier transform of the absorption profiles and the consequent
Entropy-Regularized chi^2-Minimization [ERM] to estimate the model parameters.
The procedure is tested using synthetic spectra of CII, SiII and FeII ions. The
comparison with the standard Voigt fitting analysis is performed and it is
shown that the Voigt deconvolution of the complex absorption-line profiles may
result in estimated temperatures which are not physical. We also successfully
analyze Keck telescope spectra of CII1334 and SiII1260 lines observed at the
redshift z = 3.572 toward the quasar Q1937--1009 by Tytler {\it et al.}.Comment: 25 pages, 6 Postscript figures, aaspp4.sty file, submit. Ap
Electron Acceleration and Time Variability of High Energy Emission from Blazars
Blazars are known to emit a broad band emission from radio to gamma-rays with
rapid time variations, particularly, in X- and gamma-rays. Synchrotron
radiation and inverse Compton scattering are thought to play an important role
in emission and the time variations are likely related to the acceleration of
nonthermal electrons. As simultaneous multiwavelength observations with
continuous time spans are recently available, some characteristics of electron
acceleration are possibly inferred from the spectral changes of high energy
emission. In order to make such inferences, we solve the time-dependent kinetic
equations of electrons and photons simultaneously using a simple model for
electron acceleration. We then show how the time variations of emission are
dependent on electron acceleration. We also present a simple model for a flare
in X-rays and TeV gamma-rays by temporarily changing the acceleration
timescale. Our model will be used, in future, to analyze observed data in
detail to obtain information on electron acceleration in blazars.Comment: 24 pages, 12 figures, accepted by the Astrophysical Journa
The X-ray Jet in Centaurus A: Clues on the Jet Structure and Particle Acceleration
We report detailed studies of the X-ray emission from the kpc scale jet in
the nearest active galaxy, Cen A. 41 compact sources were found within the jet,
13 of which were newly identified. We construct the luminosity function for the
detected jet-knots and argue that the remaining emission is most likely to be
truly diffuse, rather than resulting from the pile-up of unresolved faint
knots. The transverse jet profile reveals that the extended emission has the
intensity peak at the jet boundaries. We note that limb-brightened jet
morphologies have been observed previously at radio frequencies in some jet
sources, but never so clearly at higher photon energies. Our result therefore
supports a stratified jet model, consisting of a relativistic outflow including
a boundary layer with a velocity shear. In addition, we found that the X-ray
spectrum of the diffuse component is almost uniform across and along the jet.
We discuss this spectral behavior within a framework of shock and stochastic
particle acceleration processes. We note some evidence for a possible spectral
hardening at the outer sheath of the jet. Due to the limited photon statistics
of the present data, further deep observations of Cen A are required to
determine the reality of this finding, however we note that the existence of
the hard X-ray features at outer jet boundaries would provide an important
challenge to theories for the evolution of ultra-relativistic particles within
the jets.Comment: 27page, 8 figures, ver2, accepted for publication in the Ap
Compton scattering in the Klein-Nishina Regime Revisited
In blazars such as 3C 279, GeV gamma-rays are thought to be produced by
inverse Compton scattering of soft photons injected from external sources into
the jet. Because of the large bulk Lorentz factor of the jet, the energy of
soft photons is Doppler shifted in the comoving frame of the jet, and the
scattering is likely to occur in the Klein-Nishina regime. Although the
Klein-Nishina effects are well known, the properties of the electron and
emission spectra have not been studied in detail in the environment of blazars.
We solve the kinetic equation of electrons with the spatial escape term of the
electrons to obtain the electron energy spectrum in the jet and calculated the
observed emission spectrum. In calculations of the Compton losses in the
Klein-Nishina regime, we use the discrete loss formalism to take into account
the significant energy loss in a single scattering. Although the scattering
cross section decreases because of the Klein-Nishina effects, ample gamma rays
are emitted by inverse Compton scattering. When the injection spectrum of
electrons obeys a power law, the electron spectrum does not follow a broken
power law, as a result of the Klein-Nishina effects, and a large number of
high-energy electrons remain in the emitting region.Comment: 10 pages, 9 figure
Energetics of Tev Blazars and Physical Constraints on their Emission Regions
Using multi-frequency spectra from TeV blazars in quiescent states, we obtain
the physical parameters of the emission region of blazars within the framework
of the one-zone synchrotron self-Compton (SSC) model. We numerically calculate
the steady-state energy spectra of electrons by self-consistently taking into
account the effects of radiative cooling with a proper account of the
Klein-Nishina effects. Here electrons are assumed to be injected with a
power-law spectrum and to escape on a finite time scale, which naturally leads
to the existence of a break energy scale. Although we do not use time
variabilities but utilize a model of electron escape to constrain the size of
the emission region, the resultant size turns out to be similar to that
obtained based on time variabilities. Through detailed comparison of the
predicted emission spectra with observations, we find that for Mrk 421, Mrk
501, and PKS 2155--304, the energy density of relativistic electrons is about
an order of magnitude larger than that of magnetic fields with an uncertainty
within a factor of a few.Comment: Accepted for publication in Ap
Towards Semi-Markov Model-based Dependability Evaluation of VM-based Multi-Domain Service Function Chain
In NFV networks, service functions (SFs) can be deployed on virtual machines
(VMs) across multiple domains and then form a service function chain (MSFC) for
end-to-end network service provision. However, any software component in a
VM-based MSFC must experience software aging issue after a long period of
operation. This paper quantitatively investigates the capability of proactive
rejuvenation techniques in reducing the damage of software aging on a VM-based
MSFC. We develop a semi-Markov model to capture the behaviors of SFs, VMs and
virtual machine monitors (VMMs) from software aging to recovery under the
condition that failure times and recovery times follow general distributions.
We derive the formulas for calculating the steady-state availability and
reliability of the VM-based MSFC composed of multiple SFs running on VMs hosted
by VMMs. Sensitivity analysis is also conducted to identify potential
dependability bottlenecks
Lieb-Thirring Bound for Schr\"odinger Operators with Bernstein Functions of the Laplacian
A Lieb-Thirring bound for Schr\"odinger operators with Bernstein functions of
the Laplacian is shown by functional integration techniques. Several specific
cases are discussed in detail.Comment: We revised the first versio
The D/H ratio at z = 3.57 toward Q 1937-1009
Deuterium abundance re-measurements by Burles and Tytler (1998; hereafter BT)
yielded D/H = (3.3 +/- 0.3) 10^{-5} and the robust upper limit D/H < 3.9
10^{-5} from the z_a = 3.572 system toward Q1937-1009. In this new analysis BT
adopted multicomponent microturbulent models together with the possibility to
vary freely the local continuum level around each HI line to improve the fit.
The procedure failed, however, to fit adequately D Ly-beta without recourse to
an additional H Ly-alpha contamination at the position of D Ly-beta. We show
that this obstacle may be successfully overcome within the framework of the
mesoturbulent model accounting (in contrast to the microturbulent
approximation) for a correlated structure of the large scale velocity field.
Using the same observational data and the original continuum as determined by
Tytler et al. (1996), we obtained good fits. The one-component mesoturbulent
models provide D/H in the range (3.2 - 4.8) 10^{-5} and the total hydrogen
column density N(HI) = (5.6 - 7.0) 10^{17} cm^{-2}. This result is consistent
with that found by us from the z_a = 2.504 and z_a = 0.701 systems toward
Q1009+2956 and Q1718+4807, respectively. The range for D/H common to all three
analyses is D/H = (4.1 - 4.6) 10^{-5}. This value is consistent with standard
big bang nucleosynthesis [SBBN] if the baryon-to-photon ratio, \eta, is in the
range 4.2 10^{-10} <= \eta <= 4.6 10^{-10}, implying 0.0155 <= \Omega_b
h^2_{100} <= 0.0167.Comment: 8 pages, 2 Postscript figures, aaspp4.sty file, submit. ApJ Let
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