1,617 research outputs found
Non-stationary heat conduction in one-dimensional chains with conserved momentum
The Letter addresses the relationship between hyperbolic equations of heat
conduction and microscopic models of dielectrics. Effects of the non-stationary
heat conduction are investigated in two one-dimensional models with conserved
momentum: Fermi-Pasta-Ulam (FPU) chain and chain of rotators (CR). These models
belong to different universality classes with respect to stationary heat
conduction. Direct numeric simulations reveal in both models a crossover from
oscillatory decay of short-wave perturbations of the temperature field to
smooth diffusive decay of the long-wave perturbations. Such behavior is
inconsistent with parabolic Fourier equation of the heat conduction. The
crossover wavelength decreases with increase of average temperature in both
models. For the FPU model the lowest order hyperbolic Cattaneo-Vernotte
equation for the non-stationary heat conduction is not applicable, since no
unique relaxation time can be determined.Comment: 4 pages, 5 figure
Detection of Circular Polarization in the Galactic Center Black Hole Candidate Sagittarius A*
We report here the detection of circular polarization in the Galactic Center
black hole candidate, Sagittarius A*. The detection was made at 4.8 GHz and 8.4
GHz with the Very Large Array. We find that the fractional circular
polarization at 4.8 GHz is and that the spectral index of
the circular polarization is (). The systematic error in is less than 0.04% at both
frequencies. In light of our recent lower limits on the linear polarization in
Sgr A*, this detection is difficult to interpret with standard models. We
consider briefly whether scattering mechanisms could produce the observed
polarization. Detailed modeling of the source and the scattering medium is
necessary. We propose a simple model in which low energy electrons reduce
linear polarization through Faraday depolarization and convert linear
polarization into circular polarization. Circular polarization may represent a
significant new parameter for studying the obscured centimeter wavelength radio
source in Sgr A*.Comment: ApJL accepted, 11 pages including 1 figur
Habitat filtering determines spatial variation of macroinvertebrate community traits in northern headwater streams
Although our knowledge of the spatial distribution of stream organisms has been increasing rapidly in the last decades, there is still little consensus about trait-based variability of macroinvertebrate communities within and between catchments in near-pristine systems. Our aim was to examine the taxonomic and trait based stability vs. variability of stream macroinvertebrates in three high-latitude catchments in Finland. The collected taxa were assigned to unique trait combinations (UTCs) using biological traits. We found that only a single or a highly limited number of taxa formed a single UTC, suggesting a low degree of redundancy. Our analyses revealed significant differences in the environmental conditions of the streams among the three catchments. Linear models, rarefaction curves and beta-diversity measures showed that the catchments differed in both alpha and beta diversity. Taxon- and trait-based multivariate analyses also indicated that the three catchments were significantly different in terms of macroinvertebrate communities. All these findings suggest that habitat filtering, i.e., environmental differences among catchments, determines the variability of macroinvertebrate communities, thereby contributing to the significant biological differences among the catchments. The main implications of our study is that the sensitivity of trait-based analyses to natural environmental variation should be carefully incorporated in the assessment of environmental degradation, and that further studies are needed for a deeper understanding of trait-based community patterns across near-pristine streams
The Linear Polarization of Sagittarius A* II. VLA and BIMA Polarimetry at 22, 43 and 86 GHz
We present a search for linear polarization at 22 GHz, 43 GHz and 86 GHz from
the nearest super massive black hole candidate, Sagittarius A*. We find upper
limits to the linear polarization of 0.2%, 0.4% and 1%, respectively. These
results strongly support the conclusion of our centimeter wavelength
spectro-polarimetry that Sgr A* is not depolarized by the interstellar medium
but is in fact intrinsically depolarized.Comment: Accepted for publication in ApJ, 13 pages, 2 figure
The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz
We report here multi-epoch, multi-frequency observations of the circular
polarization in Sagittarius A*, the compact radio source in the Galactic
Center. Data taken from the VLA archive indicate that the fractional circular
polarization at 4.8 GHz was -0.31% with an rms scatter of 0.13% from 1981 to
1998, in spite of a factor of 2 change in the total intensity. The sign
remained negative over the entire time range, indicating a stable magnetic
field polarity. In the Summer of 1999 we obtained 13 epochs of VLA A-array
observations at 1.4, 4.8, 8.4 and 15 GHz. In May, September and October of 1999
we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8
and 8.5 GHz. In all three of the data sets, we find no evidence for linear
polarization greater than 0.1% in spite of strong circular polarization
detections. Both VLA and ATCA data sets support three conclusions regarding the
fractional circular polarization: the average spectrum is inverted with a
spectral index ~0.5 +/- 0.2; the degree of variability is roughly constant on
timescales of days to years; and, the degree of variability increases with
frequency. We also observed that the largest increase in fractional circular
polarization was coincident with the brightest flare in total intensity.
Significant variability in the total intensity and fractional circular
polarization on a timescale of 1 hour was observed during this flare,
indicating an upper limit to the size of 70 AU at 15 GHz. The fractional
circular polarization at 15 GHz reached -1.1% and the spectral index is
strongly inverted during this flare. We conclude that the spectrum has two
components that match the high and low frequency total intensity components.
(abridged)Comment: Accepted for publication in ApJ, 40 pages, 18 figure
Radio Variability of Radio Quiet and Radio Loud Quasars
The majority of quasars are weak in their radio emission, with flux densities
comparable to those in the optical, and energies far lower. A small fraction,
about 10%, are hundreds to thousands of times stronger in the radio.
Conventional wisdom holds that there are two classes of quasars, the radio
quiets and radio louds, with a deficit of sources having intermediate power.
Are there really two separate populations, and if so, is the physics of the
radio emission fundamentally different between them? This paper addresses the
second question, through a study of radio variability across the full range of
radio power, from quiet to loud. The basic findings are that the root mean
square amplitude of variability is independent of radio luminosity or
radio-to-optical flux density ratio, and that fractionally large variations can
occur on timescales of months or less in both radio quiet and radio loud
quasars. Combining this with similarities in other indicators, such as radio
spectral index and the presence of VLBI-scale components, leads to the
suggestion that the physics of radio emission in the inner regions of all
quasars is essentially the same, involving a compact, partially opaque core
together with a beamed jet.Comment: 32 pages, 9 figures. Astrophysical Journal, in pres
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