3,337 research outputs found
Flux density measurements of a complete sample of faint blazars
We performed observations with the Effelsberg 100-m radio telescope to
measure flux densities and polarised emission of sources selected from the
"Deep X-ray Radio Blazar Survey" (DXRBS) to better define their spectral index
behaviour in the radio band, with the aim to construct a homogeneous sample of
blazars. Sources were observed at four different frequencies with the
Effelsberg 100-m telescope. We complemented these measurements with flux
density data at 1.4GHz derived from the NRAO VLA Sky Survey.The spectral
indices of a sample of faint blazars were computed making use of almost
simultaneous measurements. Sixty-six percent of the sources can be classified
as "bona fide" blazars. Seven objects show a clearly inverted spectral index.
Seventeen sources previously classified as flat spectrum radio quasars (FSRQs)
are actually steep spectrum radio quasars (SSRQs). The flux densities obtained
with the Effelsberg 100-m telescope at 5GHz are compared with the flux
densities listed in the Green Bank GB6 survey and in the Parkes-MIT-NRAO PMN
catalogue. About 43% of the sources in our sample exhibit flux density
variations on temporal scales of 19 or 22 years. We confirm that 75 out of 103
sources of the DXRBS are indeed FSRQs. Twenty-seven sources show a spectral
index steeper than -0.5 and should be classified as SSRQs. Polarised emission
was detected for 36 sources at 4.85GHz. The median value of the percentage of
polarised emission is (5.8+-0.9$)%. Five sources show rotation measure (RM)
values >200 rad m^-2.Comment: Accepted for publication in Astronomy and Astrophysic
The X-ray Emissions from the M87 Jet: Diagnostics and Physical Interpretation
We reanalyze the deep Chandra observations of the M87 jet, first examined by
Wilson & Yang (2002). By employing an analysis chain that includes image
deconvolution, knots HST-1 and I are fully separated from adjacent emission. We
find slight but significant variations in the spectral shape, with values of
ranging from . We use VLA radio observations, as well
as HST imaging and polarimetry data, to examine the jet's broad-band spectrum
and inquire as to the nature of particle acceleration in the jet. As shown in
previous papers, a simple continuous injection model for synchrotron-emitting
knots, in which both the filling factor, , of regions within which
particles are accelerated and the energy spectrum of the injected particles are
constant, cannot account for the X-ray flux or spectrum. Instead, we propose
that is a function of position and energy and find that in the inner
jet, , and
in knots A and B, , where is the emitted photon energy and and is the
emitting electron energy. In this model, the index of the injected electron
energy spectrum () is at all locations in
the jet, as predicted by models of cosmic ray acceleration by ultrarelativistic
shocks. There is a strong correlation between the peaks of X-ray emission and
minima of optical percentage polarization, i.e., regions where the jet magnetic
field is not ordered. We suggest that the X-ray peaks coincide with shock waves
which accelerate the X-ray emitting electrons and cause changes in the
direction of the magnetic field; the polarization is thus small because of beam
averaging.Comment: Accepted for publication in ApJ; 21 pages, 9 figures, 2 tables;
abstract shortened for astro-ph; Figures 1, 7 and 8 at reduced resolutio
Critical view of WKB decay widths
A detailed comparison of the expressions for the decay widths obtained within
the semiclassical WKB approximation using different approaches to the tunneling
problem is performed. The differences between the available improved formulae
for tunneling near the top and the bottom of the barrier are investigated.
Though the simple WKB method gives the right order of magnitude of the decay
widths, a small number of parameters are often fitted. The need to perform the
fitting procedure remaining consistently within the WKB framework is emphasized
in the context of the fission model based calculations. Calculations for the
decay widths of some recently found super heavy nuclei using microscopic
alpha-nucleus potentials are presented to demonstrate the importance of a
consistent WKB calculation. The half-lives are found to be sensitive to the
density dependence of the nucleon-nucleon interaction and the implementation of
the Bohr-Sommerfeld quantization condition inherent in the WKB approach.Comment: 18 pages, Late
Spectrum of two component flows around a super-massive black hole: an application to M87
We calculate the spectra of two-component accretion flows around black holes
of various masses, from quasars to nano-quasars. Specifically, we fit the
observational data of M87 very satisfactorily using our model and find that the
spectrum may be well fitted by a sub-Keplerian component alone, and there is
little need of any Keplerian component. The non-thermal distribution of
electrons produced by their acceleration across the standing shock in the
sub-Keplerian component is enough to produce the observed flat spectrum through
the synchrotron radiation.Comment: 12 pages, 2 figures and 1 table. Accepted in Astrophysical Journal
Lette
Iconicity in English and Spanish and its relation to lexical category and age of acquisition
Signed languages exhibit iconicity (resemblance between form and meaning) across their vocabulary, and many non-Indo-European spoken languages feature sizable classes of iconic words known as ideophones. In comparison, Indo-European languages like English and Spanish are believed to be arbitrary outside of a small number of onomatopoeic words. In three experiments with English and two with Spanish, we asked native speakers to rate the iconicity of ~600 words from the English and Spanish MacArthur-Bates Communicative Developmental Inventories. We found that iconicity in the words of both languages varied in a theoretically meaningful way with lexical category. In both languages, adjectives were rated as more iconic than nouns and function words, and corresponding to typological differences between English and Spanish in verb semantics, English verbs were rated as relatively iconic compared to Spanish verbs. We also found that both languages exhibited a negative relationship between iconicity ratings and age of acquisition. Words learned earlier tended to be more iconic, suggesting that iconicity in early vocabulary may aid word learning. Altogether these findings show that iconicity is a graded quality that pervades vocabularies of even the most “arbitrary” spoken languages. The findings provide compelling evidence that iconicity is an important property of all languages, signed and spoken, including Indo-European languages
Variability of the extreme z=4.72 blazar, GB 1428+4217
We report X-ray and radio variability of GB 1428+4217 which confirm its
blazar nature. IR observations reveal a powerful optical-UV component, not
obscured by dust, which is suggestive of the presence of a billion solar mass
black hole, already formed by z ~ 5. A detailed comparison of the broad band
spectral properties of GB 1428+4217 with those of nearby blazars shows it to be
extreme, but nevertheless consistent with the trend found for nearby sources.Comment: MNRAS, in press - 5 pages, 5 figure
Polarization of synchrotron emission from relativistic reconfinement shocks with ordered magnetic fields
We calculate the polarization of synchrotron radiation produced at the
relativistic reconfinement shocks, taking into account globally ordered
magnetic field components, in particular toroidal and helical fields. In these
shocks, toroidal fields produce high parallel polarization (electric vectors
parallel to the projected jet axis), while chaotic fields generate moderate
perpendicular polarization. Helical fields result in a non-axisymmetric
distribution of the total and polarized brightness. For a diverging downstream
velocity field, the Stokes parameter U does not vanish and the average
polarization is neither strictly parallel nor perpendicular. A distance at
which the downstream flow is changing from diverging to converging can be
easily identified on polarization maps as the turning point, at which
polarization vectors switch, e.g., from clockwise to counterclockwise.Comment: 10 pages, 6 figures, accepted for publication in A&
The mitochondrial ribosome from Locusta migratoria
Mitochondrial ribosomes from locust thorax muscles have been identified recently in our labo-ratory by specific labelling of the adhering nascent peptide chains and have been isolated from mito
The radio-ultraviolet spectral energy distribution of the jet in 3C273
We present deep VLA and HST observations of the large-scale jet in 3C 273
matched to 0.3" resolution. The observed spectra show a significant flattening
in the infrared-ultraviolet wavelength range. The jet's emission cannot
therefore be assumed to arise from a single electron population and requires
the presence of an additional emission component. The observed smooth
variations of the spectral indices along the jet imply that the physical
conditions vary correspondingly smoothly. We determine the maximum particle
energy for the optical jet using synchrotron spectral fits. The slow decline of
the maximum energy along the jet implies particle reacceleration acting along
the entire jet. In addition to the already established global anti-correlation
between maximum particle energy and surface brightness, we find a weak positive
correlation between small-scale variations in maximum particle energy and
surface brightness. The origin of these conflicting global and local
correlations is unclear, but they provide tight constraints for reacceleration
models.Comment: 28 pages, lots of figures, accepted for publication in A&
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