785 research outputs found
A simple model for electron plasma heating in supernova remnants
Context: Multiwavelength observations of supernova remnants can be explained
within the framework of diffusive shock acceleration theory, which allows
effective conversion of the explosion energy into cosmic rays. Although the
models of nonlinear shocks describe reasonably well the nonthermal component of
emission, certain issues, including the heating of the thermal electron plasma
and the related X-ray emission, still remain open.
Methods: Numerical solution of the equations of the Chevalier model for
supernova remnant evolution, coupled with Coulomb scattering heating of the
electrons.
Results: The electron temperature and the X-ray thermal Bremsstrahlung
emission from supernova remnants have been calculated as functions of the
relevant parameters. Since only the Coulomb mechanism was considered for
electron heating, the values obtained for the electron temperatures should be
treated as lower limits. Results from this work can be useful to constrain
model parameters for observed SNRs.Comment: Accepted to A&A as a research not
Fibre DFB lasers in a 4x10 Gbit/s WDM link with a single sinc-sampled fibre grating dispersion compensator
WDM transmission and dispersion compensation at 40 Gbit/s over 200 km standard fibre is demonstrated on a 100 GHz grid using four high power single-polarisation single-sided output DFB fibre laser based transmitters and a single 4 channel WDM chirped fibre Bragg grating dispersion compensator
Ion Charge States in the Fast Solar Wind: New Data Analysis and Theoretical Refinements
We present a further investigation into the increased ionization observed in
element charge states in the fast solar wind compared to its coronal hole
source regions. Once ions begin to be perpendicularly heated by ion cyclotron
waves and execute large gyro-orbits, density gradients in the flow can excite
lower hybrid waves that then damp by heating electrons in the parallel
direction. We give further analysis of charge state data from polar coronal
holes at solar minimum and maximum, and also from equatorial coronal holes. We
also consider further the damping of lower hybrid waves by ions and the effect
of non-Maxwellian electron distribution functions on the degree of increased
ionization, both of which appear to be negligible for the solar wind case
considered here. We also suggest that the density gradients required to heat
electrons sufficiently to further ionize the solar wind can plausibly result
from the turbulent cascade of MHD waves.Comment: 27 pages, accepted by Ap
Fast Variability of Nonthermal X-Ray Emission in Cassiopeia A: Probing Electron Acceleration in Reverse-Shocked Ejecta
Recent discovery of the year-scale variability in the synchrotron X-ray
emission of the supernova remnant (SNR) RX J1713.7-3946 has initiated our study
of multi-epoch X-ray images and spectra of the young SNR Cassiopeia A based on
the Chandra archive data taken in 2000, 2002, and 2004. We have found
year-scale time variations in the X-ray intensity for a number of X-ray
filaments or knots associated with the reverse-shocked regions. The X-ray
spectra of the variable filaments are characterized by a featureless continuum,
and described by a power law with a photon index within 1.9-2.3. The upper
limits on the iron K-line equivalent width are 110 eV, which favors a
synchrotron origin of the X-ray emission. The characteristic variability
timescale of 4 yr can be explained by the effects of fast synchrotron cooling
and diffusive shock acceleration with a plausible magnetic field of 1 mG. The
X-ray variability provides a new effective way of studying particle
acceleration at supernova shocks.Comment: 4 pages, 4 figures, accepted for publication in ApJ Lette
The Polar Regions of Cassiopeia A: The Aftermath of a Gamma Ray Burst?
Probably not, but it is interesting nevertheless to investigate just how
close Cas A might have come to generating such an event. Focusing on the
northeast jet filaments, we analyze the polar regions of the recently acquired
very deep 1 Ms Chandra X-ray observation. We infer that the so-called "jet"
regions are indeed due to jets emanating from the explosion center, and not due
to polar cavities in the circumstellar medium at the time of explosion. We
place limits on the equivalent isotropic explosion energy in the polar regions
(around 2.3 x 10^52 ergs), and the opening angle of the x-ray emitting ejecta
(around 7 degrees), which give a total energy in the NE jet of order 10^50
ergs; an order of magnitude or more lower than inferred for "typical" GRBs.
While the Cas A progenitor and explosion exhibit many of the features
associated with GRB hosts, e.g. extensive presupernova mass loss and rotation,
and jets associated with the explosion, we speculate that the recoil of the
compact central object, with velocity 330 km/s, may have rendered the jet
unstable. In such cases the jet rapidly becomes baryon loaded, if not truncated
altogether. Although unlikely to have produced a gamma ray burst, the jets in
Cas A suggest that such outflows may be common features of core-collapse SNe.Comment: 35 pages, 7 figures, accepted by Ap
Coronal abundances of X-ray bright pre-main sequence stars in the Taurus Molecular Cloud
We studied the thermal properties and chemical composition of the X-ray
emitting plasma of a sample of bright members of the Taurus Molecular Cloud to
investigate possible differences among classical and weak-lined T Tauri stars
and possible dependences of the abundances on the stellar activity level and/or
on the presence of accretion/circumstellar material. We used medium-resolution
X-ray spectra obtained with the sensitive EPIC/PN camera in order to analyse
the possible sample. The PN spectra of 20 bright (L_X ~ 10^30 - 10^31 erg/s)
Taurus members, with at least ~ 4500 counts, were fitted using thermal models
of optically thin plasma with two components and variable abundances of O, Ne,
Mg, Si, S, Ar, Ca, and Fe. Extensive preliminary investigations were employed
to study the performances of the PN detectors regarding abundance
determinations, and finally to check the results of the fittings. We found that
the observed X-ray emission of the studied stars can be attributed to coronal
plasma having similar thermal properties and chemical composition both in the
classical and in the weak-lined T Tauri stars. The results of the fittings did
not show evidence for correlations of the abundance patterns with activity or
accretion/disk presence. The iron abundance of these active stars is
significantly lower than (~ 0.2 of) the solar photospheric value. An indication
of slightly different coronal properties in stars with different spectral type
is found from this study. G-type and early K-type stars have, on average,
slightly higher Fe abundances (Fe ~ 0.24 solar) with respect to stars with
later spectral type (Fe ~ 0.15 solar), confirming previous findings from
high-resolution X-ray spectroscopy; stars of the former group are also found to
have, on average, hotter coronae.Comment: 14 pages, 11 figures, to be published in Astronomy & Astrophysic
Sequence effects in the categorization of tones varying in frequency
In contrast to exemplar and decision-bound categorization models, the memory and contrast models described here do not assume that long-term representations of stimulus magnitudes are available. Instead, stimuli are assumed to be categorized using only their differences from a few recent stimuli. To test this alternative, the authors examined sequential effects in a binary categorization of 10 tones varying in frequency. Stimuli up to 2 trials back in the sequence had a significant effect on the response to the current stimulus. The effects of previous stimuli interacted with one another. A memory and contrast model, according to which only ordinal information about the differences between the current stimulus and recent preceding stimuli is used, best accounted for these dat
Ion Charge States in Halo CMEs: What can we Learn about the Explosion?
We describe a new modeling approach to develop a more quantitative
understanding of the charge state distributions of the ions of various elements
detected in situ during halo Coronal Mass Ejection (CME) events by the Advanced
Composition Explorer (ACE) satellite. Using a model CME hydrodynamic evolution
based on observations of CMEs propagating in the plane of the sky and on
theoretical models, we integrate time dependent equations for the ionization
balance of various elements to compare with ACE data. We find that plasma in
the CME ``core'' typically requires further heating following filament
eruption, with thermal energy input similar to the kinetic energy input. This
extra heating is presumably the result of post eruptive reconnection. Plasma
corresponding to the CME ``cavity'' is usually not further ionized, since
whether heated or not, the low density gives freeze-in close the the Sun. The
current analysis is limited by ambiguities in the underlying model CME
evolution. Such methods are likely to reach their full potential when applied
to data to be acquired by STEREO when at optimum separation. CME evolution
observed with one spacecraft may be used to interpret CME charge states
detected by the other.Comment: 20 pages, accepted by Ap
Comparison and contrast in perceptual categorization
People categorized pairs of perceptual stimuli that varied in both category membership and pairwise similarity. Experiments 1 and 2 showed categorization of 1 color of a pair to be reliably contrasted from that of the other. This similarity-based contrast effect occurred only when the context stimulus was relevant for the categorization of the target (Experiment 3). The effect was not simply owing to perceptual color contrast (Experiment 4), and it extended to pictures from common semantic categories (Experiment 5). Results were consistent with a sign-and-magnitude version of N. Stewart and G. D. A. Brown's (2005) similarity-dissimilarity generalized context model, in which categorization is affected by both similarity to and difference from target categories. The data are also modeled with criterion setting theory (M. Treisman & T. C. Williams, 1984), in which the decision criterion is systematically shifted toward the mean of the current stimuli
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