5,034 research outputs found
Time-dependent modeling of TeV-detected, young pulsar wind nebulae
The increasing sensitivity of instruments at X-ray and TeV energies have
revealed a large number of nebulae associated with bright pulsars. Despite this
large data set, the observed pulsar wind nebulae (PWNe) do not show a uniform
behavior and the main parameters driving features like luminosity,
magnetization, and others are still not fully understood. To evaluate the
possible existence of common evolutive trends and to link the characteristics
of the nebula emission with those of the powering pulsar, we selected a sub-set
of 10 TeV detections which are likely ascribed to young PWNe and model the
spectral energy distribution with a time-dependent description of the nebulae's
electron population. In 9 of these cases, a detailed PWNe model, using
up-to-date multiwavelength information, is presented. The best-fit parameters
of these nebula are discussed, together with the pulsar characteristics. We
conclude that TeV PWNe are particle-dominated objects with large
multiplicities, in general far from magnetic equipartition, and that relatively
large photon field enhancements are required to explain the high level of
Comptonized photons observed. We do not find significant correlations between
the efficiencies of emission at different frequencies and the magnetization.
The injection parameters do not appear to be particularly correlated with the
pulsar properties either. We find that a normalized comparison of the SEDs
(e.g., with the corresponding spin-down flux) at the same age significantly
reduces the spectral distributions dispersion.Comment: 38 pages, 20 figures, 8 tables. Accepted for publication in the
Journal of High Energy Astrophysics (JHEAp
Gamma-ray upper limits on magnetars with 6 years of Fermi-LAT observations
We report on the search for gamma-ray emission from 20 magnetars using 6
years of Fermi, Large Area Telescope (LAT) observations. No significant
evidence for gamma-ray emission from any of the currently-known magnetars is
found. We derived the most stringent upper limits to date on the 0.1--10 GeV
emission of Galactic magnetars, which are estimated between
erg s cm. Gamma-ray pulsations were
searched for the four magnetars having reliable ephemerides over the observing
period, but none were detected. On the other hand, we also studied the
gamma-ray morphology and spectra of seven Supernova Remnants associated or
adjacent to the magnetars.Comment: 22 pages, 4 figures, 2 tables, submitted to Ap
Gamma-ray emission from PSR J0007+7303 using 7 years of Fermi Large Area Telescope observations
Based on more than seven years of Fermi Large Area Telescope (LAT) Pass 8
data, we report on a detailed analysis of the bright gamma-ray pulsar (PSR)
J0007+7303. We confirm that PSR J0007+7303 is significantly detected as a point
source also during the off-peak phases with a TS value of 262 ( 16
). In the description of PSR J0007+7303 off-peak spectrum, a power law
with an exponential cutoff at 2.71.21.3 GeV (the first/second
uncertainties correspond to statistical/systematic errors) is preferred over a
single power law at a level of 3.5 . The possible existence of a cutoff
hints at a magnetospheric origin of the emission. In addition, no extended
gamma-ray emission is detected compatible with either the supernova remnant
(CTA 1) or the very high energy (> 100 GeV) pulsar wind nebula. A flux upper
limit of 6.510 erg cm s in the 10-300 GeV energy
range is reported, for an extended source assuming the morphology of the
VERITAS detection. During on-peak phases, a sub-exponential cutoff is
significantly preferred (11 ) for representing the spectral
energy distribution, both in the phase-averaged and in the phase-resolved
spectra. Three glitches are detected during the observation period and we found
no flux variability at the time of the glitches or in the long-term behavior.
We also report the discovery of a previously unknown gamma-ray source in the
vicinity of PSR J0007+7303, Fermi J0020+7328, which we associate with the z =
1.781 quasar S5 0016+73. A concurrent analysis of this source is needed to
correctly characterize the behavior of CTA 1 and it is also presented in the
paper.Comment: 26 pages, 10 figures, 4 tables; Accepted for publication in Ap
Entierro del juicio final y vivificacion de la astrologia herida con tres llagas en lo natural, moral y politico curada con tres parches...
Copia digital. Valladolid : Junta de Castilla y León. ConsejerÃa de Cultura y Turismo, 2009-2010Sign.: A-E4, F2Port. con orla tip
Machine learning applied to active fixed-income portfolio management: a Lasso logit approach
El uso de métodos cuantitativos es fundamental en la gestión de carteras de inversores institucionales. En la última década, se han realizado diversos estudios empÃricos que emplean modelos probabilÃsticos o de clasificación para predecir los rendimientos del mercado de valores, modelar calificaciones de riesgo y probabilidades de incumplimiento de bonos, asà como pronosticar la curva de rendimientos. Sin embargo, existe una escasa investigación sobre la aplicación de estos modelos en la gestión activa de renta fija. Este documento busca abordar esta brecha al comparar un algoritmo de aprendizaje automático, la regresión logÃstica Lasso, con una estrategia de inversión pasiva (comprar y mantener) en la construcción de un modelo de gestión de duración para carteras de bonos gubernamentales, con enfoque especÃfico en los bonos del Tesoro de Estados Unidos. Además, se propone un procedimiento de dos pasos, junto con un promedio simple entre variables de caracterÃsticas estadÃsticas similares, con el objetivo de minimizar el posible sobreajuste de los algoritmos tradicionales de aprendizaje automático. Asimismo, se introduce un método para seleccionar umbrales que conviertan probabilidades en señales basadas en distribuciones de probabilidad condicional. Se utiliza un amplio conjunto de variables financieras y económicas para obtener una señal de duración y se comparan otras estrategias de inversión. Como resultado, la mayorÃa de las variables seleccionadas por el modelo están relacionadas con flujos financieros y fundamentos económicos, aunque los parámetros no parecen ser estables a lo largo del tiempo, lo que sugiere que la relevancia de las variables es dinámica y se requiere una evaluación continua del modelo. Además, el modelo logra un exceso de retorno estadÃsticamente significativo en comparación con la estrategia pasiva. Estos resultados respaldan la inclusión de herramientas cuantitativas en el proceso de gestión activa de carteras para inversores institucionales, con especial atención en el posible sobreajuste y en los parámetros inestables. Las herramientas cuantitativas deben considerarse como un complemento del análisis cualitativo y fundamental, junto con la experiencia del gestor de carteras, para tomar decisiones de inversión fundamentadas de manera más sólida.The use of quantitative methods constitutes a standard component of the institutional investors’ portfolio management toolkit. In the last decade, several empirical studies have employed probabilistic or classification models to predict stock market excess returns, model bond ratings and default probabilities, as well as to forecast yield curves. To the authors’ knowledge, little research exists into their application to active fixed-income management. This paper contributes to filling this gap by comparing a machine learning algorithm, the Lasso logit regression, with a passive (buy-and-hold) investment strategy in the construction of a duration management model for high-grade bond portfolios, specifically focusing on US treasury bonds. Additionally, a two-step procedure is proposed, together with a simple ensemble averaging aimed at minimising the potential overfitting of traditional machine learning algorithms. A method to select thresholds that translate probabilities into signals based on conditional probability distributions is also introduced
Querella que don Quixote de la Mancha da en el Tribunal de la Muerte contra Don francisco de Quevedo, sobre la primera y segunda parte de las visiones, y visitas de D. Diego de Torres
Copia digital. Valladolid : Junta de Castilla y León. ConsejerÃa de Cultura y Turismo, 2009-2010Nicolás de Molani Nogui Interiano es pseudónimo de Diego Torres y VillarroelPort. con orla tip
Cathedra de morir : puntos que se han de tomar en la vida para la leccion del ultimo instante
DedÃcalo al IlustrÃsimo Señor DonSilvestre GarcÃa de Escalona, Obispo de SalamancaCopia digital. Valladolid : Junta de Castilla y León. ConsejerÃa de Cultura y Turismo, 2009-2010Aprobación y censura fechadas en 1726Sign.: A-D4, E2Port. con orla tip
Segunda parte de las visiones y visitas de Torres con D. Francisco de Quevedo, por la corte
Copia digital. Valladolid : Junta de Castilla y León. ConsejerÃa de Cultura y Turismo, 2009-2010Port. con orla tip
Tercera parte de las visiones y visitas de Torres con D. Francisco de Quevedo, por la corte
Copia digital. Valladolid : Junta de Castilla y León. ConsejerÃa de Cultura y Turismo, 2009-2010Port. con orla tip
Testamento del reverendo Don Diego de Torres y Villarroel, cathedratico de astrologia en la Universidad de Salamanca
Copia digital. Valladolid : Junta de Castilla y León. ConsejerÃa de Cultura y Turismo, 2009-2010El pie de imp. consta en colofónSign.: A
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