16,229 research outputs found
Frequency dependence of pulsar radiation patterns
We report on new results from simultaneous, dual frequency, single pulse
observation of PSR B0329+54 using the Giant Metrewave Radio Telescope. We find
that the longitude separation of subpulses at two different frequencies (238
and 612 MHz) is less than that for the corresponding components in the average
profile. A similar behaviour has been noticed before in a number of pulsars. We
argue that subpulses are emitted within narrow flux tubes of the dipolar field
lines and that the mean pulsar beam has a conal structure. In such a model the
longitudes of profile components are determined by the intersection of the line
of sight trajectory with subpulse-associated emission beams. Thus, we show that
the difference in the frequency dependence of subpulse and profile component
longitudes is a natural property of the conal model of pulsar emission beam. We
support our conclusions by numerical modelling of pulsar emission, using the
known parameters for this pulsar, which produce results that agree very well
with our dual frequency observations.Comment: 24 pages, 8 figures. Accepted for publication in Ap
Persistence on airline accidents
This paper analyses airline accidents data from 1927-2006. The fractional integration methodology is adopted. It is shown that airline accidents are persistent and (fractionally) cointegrated with airline traffic. Thus, there exists an equilibrium relation between air accidents and airline traffic, with the effect of the shocks to that relationship disappearing in the long run. Policy implications are derived for countering accidents events.
How to generate pentagonal symmetry using Turing systems
We explore numerically the formation of Turing patterns in a confined circular domain with small aspect ratio. Our results show that stable fivefold patterns are formed over a well defined range of disk sizes, offering a possible mechanism for inducing the fivefold symmetry observed in early development of regular echinoids. Using this pattern as a seed, more complex biological structures can be mimicked, such as the pigmentation pattern of sea urchins and the plate arrangements of the calyxes of primitive camerate crinoids
The redshift and broad band spectral energy distribution of NRAO 150
Context. NRAO 150 is one of the brightest radio and mm AGN sources on the
northern sky. It has been revealed as an interesting source where to study
extreme relativistic jet phenomena. However, its cosmological distance has not
been reported so far, because of its optical faintness produced by strong
Galactic extinction.
Aims. Aiming at measuring the redshift of NRAO 150, and hence to start making
possible quantitative studies from the source.
Methods. We have conducted spectroscopic and photometric observations of the
source in the near-IR, as well as in the optical.
Results. All such observations have been successful in detecting the source.
The near-IR spectroscopic observations reveal strong H and H
emission lines from which the cosmological redshift of NRAO 150
() has been determined for the first time. We classify the
source as a flat-spectrum radio-loud quasar, for which we estimate a large
super-massive black-hole mass . After
extinction correction, the new near-IR and optical data have revealed a
high-luminosity continuum-emission excess in the optical (peaking at
\,\AA, rest frame) that we attribute to thermal emission from the
accretion disk for which we estimate a high accretion rate, \,% of the
Eddington limit.
Conclusions. Comparison of these source properties, and its broad-band
spectral-energy distribution, with those of Fermi blazars allow us to predict
that NRAO 150 is among the most powerful blazars, and hence a high luminosity
-although not detected yet- -ray emitter.Comment: 8 pages, 4 figure
On the calibration of the relation between geometric albedo and polarimetric properties for the asteroids
We present a new extensive analysis of the old problem of finding a
satisfactory calibration of the relation between the geometric albedo and some
measurable polarization properties of the asteroids. To achieve our goals, we
use all polarimetric data at our disposal. For the purposes of calibration, we
use a limited sample of objects for which we can be confident to know the
albedo with good accuracy, according to previous investigations of other
authors. We find a new set of updated calibration coefficients for the
classical slope - albedo relation, but we generalize our analysis and we
consider also alternative possibilities, including the use of other
polarimetric parameters, one being proposed here for the first time, and the
possibility to exclude from best-fit analyzes the asteroids having low albedos.
We also consider a possible parabolic fit of the whole set of data.Comment: Accepted by MNRA
Radial Age and Metal Abundance Gradients in the Stellar Content of M32
We present long-slit spectroscopy of the elliptical galaxy M32, obtained with
the 8-m Subaru telescope at Mauna Kea, the 1.5-m Tillinghast telescope at the
F. L. Whipple Observatory, and the 4-m Mayall telescope at the Kitt Peak
National Observatory. The spectra cover the Lick index red spectral region as
well as higher order Balmer lines in the blue. Spectra have been taken with the
slit off-set from the nucleus to avoid scattered light contamination from the
bright nucleus of M32. An analysis of numerous absorption features,
particularly involving the H and H Balmer lines, reveals that
systematic radial trends are evident in the integrated spectrum of M32.
Population synthesis models indicate a radial change in both the age and
chemical composition of the light-weighted mean stellar population in M32, from
the nucleus out to 33", i.e., approximately 1.0 effective radius, R_e.
Specifically, the light-weighted mean stellar population at 1 R_e is older, by
\~3 Gyr, and more metal-poor, by ~-0.25 dex in [Fe/H], t han the central value
of ~4 Gyr and [Fe/H]~0.0. We show that this apparent population trend cannot be
attributed to a varying contribution from either hot stars or emission line
contamination. The increase in age and decrease in metal-abundance with radius
are sufficiently well-matched to explain the flat radial color profiles
previously observed in M32. In addition, the ratio of Mg to Fe abundance,
[Mg/Fe], increases from ~-0.25 in the nucleus to ~-0.08 at 1 R_e. Finally, we
find spuriously pronounced line strength gradients in the Mayall data that are
an artifact of scattered light from the bright nucleus. Scattered light issues
may explain the lack of consistency among previously published studies of
radial line strength gradients in M32.Comment: 25 pages, 14 figures, 3 tables, accepted for publication in the
Astronomical Journa
On the Nature of Precursors in the Radio Pulsar Profiles
In the average profiles of several radio pulsars, the main pulse is
accompanied by the preceding component. This so called precursor is known for
its distinctive polarization, spectral, and fluctuation properties. Recent
single-pulse observations hint that the sporadic activity at the extreme
leading edge of the pulse may be prevalent in pulsars. We for the first time
propose a physical mechanism of this phenomenon. It is based on the induced
scattering of the main pulse radiation into the background. We show that the
scattered component is directed approximately along the ambient magnetic field
and, because of rotational aberration in the scattering region, appears in the
pulse profile as a precursor to the main pulse. Our model naturally explains
high linear polarization of the precursor emission, its spectral and
fluctuation peculiarities as well as suggests a specific connection between the
precursor and the main pulse at widely spaced frequencies. This is believed to
stimulate multifrequency single-pulse studies of intensity modulation in
different pulsars.Comment: 5 pages, no figures. Accepted for publication in MNRAS Letter
Universal point contact resistance between thin-film superconductors
A system comprising two superconducting thin films connected by a point
contact is considered. The contact resistance is calculated as a function of
temperature and film geometry, and is found to vanish rapidly with temperature,
according to a universal, nearly activated form, becoming strictly zero only at
zero temperature. At the lowest temperatures, the activation barrier is set
primarily by the superfluid stiffness in the films, and displays only a weak
(i.e., logarithmic) temperature dependence. The Josephson effect is thus
destroyed, albeit only weakly, as a consequence of the power-law-correlated
superconducting fluctuations present in the films below the
Berezinskii-Kosterlitz-Thouless transition temperature. The behavior of the
resistance is discussed, both in various limiting regimes and as it crosses
over between these regimes. Details are presented of a minimal model of the
films and the contact, and of the calculation of the resistance. A formulation
in terms of quantum phase-slip events is employed, which is natural and
effective in the limit of a good contact. However, it is also shown to be
effective even when the contact is poor and is, indeed, indispensable, as the
system always behaves as if it were in the good-contact limit at low enough
temperature. A simple mechanical analogy is introduced to provide some
heuristic understanding of the nearly-activated temperature dependence of the
resistance. Prospects for experimental tests of the predicted behavior are
discussed, and numerical estimates relevant to anticipated experimental
settings are provided.Comment: 29 pages (single column format), 7 figure
Relaciones históricas entre olmos y vides
In Spanish, the expression «You cannot ask an elm for pears» denotes something that is impossible. Although its origin is unknown, it can be dated back to the 1st century BC, when appeared the Latin maxim Pirum, non ulmum accedas, si cupias pira (You should go to a pear-tree for pears, not to an elm), a sentence from which we believe the Spanish saying comes. The objective of this paper is to show how the historical relationship of elms and vines can be related to these expressions, because elms did not give pears but, figuratively, did give grapes. The cultivation of vines was soon included among the domestic plants at the beginning of the Neolithic Age. During the Assyrian Empire (7th century BC), vines are represented growing up around pine-like trees. The first documentary evidence of the marriage between elms and vines is found in the Ancient Greece: a wine called Pteleaikós oinos is mentioned, which refers to the region where it was produced, Ptelea (Elm). During the Roman Ages, the cultivation of vines married to elms became more important as it is reflected in the treatises in agriculture. This technique was so common that it appears recurrently as a topic in Poetry and Drama. The classical books were copied during the Middle Ages, and only the Arabian agronomists in the Iberian Peninsula gave new evidence of the relationship between vines and elms in the 12th century. Some four hundred years later the use of elms as props for vines was rare in Spain and, although not to elms, the marriage of vines to trees lasted in the South of Spain until the 19th century. In Italy, elms and vines were even planted together in the 20th century, before the Dutch Elm Disease began to kill the plantations of trees and farmers were forced to replace them with poles.En castellano, la expresión «No se le puede pedir peras al olmo» indica algo imposible. A pesar de que su origen es desconocido, puede situarse en el siglo I A.C., cuando aparece la máxima latina Pirum, non ulmum accedas, si cupias pira (Al peral acude, no al olmo, si quieres peras), de la cual creemos que procede la expresión española. En este trabajo se pretende mostrar que la relación histórica de los olmos y las vides puede estar relacionada con estas expresiones en la medida en que los olmos no daban peras mas, en sentido figurado, daban uvas. La vid fue una de las primeras plantas cultivadas ya desde el comienzo del Neolítico. En el Imperio Asirio (s. VII A.C.), las vides aparecen representadas creciendo alrededor de árboles que se asemejan a pinos. La primera evidencia documental del maridaje de olmos y vides la encontramos en la Grecia Clásica, donde se menciona un vino llamado Pteleaikós oinos que hace referencia a la región dónde se producía, Ptelea (Olmo). Durante la época Romana el cultivo de las vides maridadas a los olmos adquiere mayor importancia, como se refleja en los tratados de agricultura. La práctica fue tan común que aparece de modo recurrente como tópico en la literatura. Los textos clásicos son copiados durante la Edad Media, y sólo los agrónomos árabes de la península Ibérica proporcionan nuevas evidencias de la relación entre vides y olmos ya en el siglo XII. Cuatro siglos después la utilización de olmos como tutores de las vides era rara en España, pero el maridaje de las vides con árboles, aunque no olmos, duró en el Sur de España hasta el siglo XIX. Sin embargo en Italia olmos y vides se plantaban juntos incluso en el siglo XX, antes de que la grafiosis comenzara a matar las plantaciones de árboles y los agricultores se vieran obligados a sustituirlos por postes
- …