Search results for "astronomía"
showing 10 items of 247 documents
Gravitational radiation from the magnetic field of a strongly magnetized star
2003
We consider the electromagnetic (e.m.) field of a compact strongly magnetized star. The star is idealized as a perfect conducting sphere, rigidly rotating in a vacuum, with a magnetic moment not aligned with its rotation axis. Then we use the exterior e.m. solution, obtained by Deutsch (1955) in his classic paper, to calculate the gravitational waves emitted by the e.m. field when its wavelength is much longer than the radius of the star. In some astrophysical situations, this gravitational radiation can overcome the quadrupole one emitted by the matter of the star, and, for some magnetars, would be detectable in the near future, once the present detectors, planned or under construction, be…
JEM–X inflight performance
2003
We summarize the inflight performance of JEM-X, the X-ray monitor on the INTEGRAL mission during the initial ten months of operations. The JEM-X instruments have now been tuned to stable operational conditions. The performance is found to be close to the pre-launch expectations. The ground calibrations and the inflight calibration data permit to determine the instruments characteristics to fully support the scientific data analysis. Reglero Velasco, Victor, Victor.Reglero@uv.es ; Martinez Nuñez, Silvia, Silvia.Martinez@uv.es
Absolute kinematics of radio source components in the complete S5 polar cap sample I. First and second epoch maps at 8.4 GHz
2001
We observed the thirteen extragalactic radio sources of the S5 polar cap sample at 8.4 GHz with the Very Long Baseline Array, on 1997.93 and 1999.41. We present the maps from those two epochs and briefly discuss the morphological changes experimented by some of the radio sources in the 1.4 yr elapsed. These results correspond to the first two epochs at 8.4 GHz of a program directed to study the absolute kinematics of the radio source components of the members of the sample by means of phase delay astrometry at 8.4, 15 and 43 GHz.
Nonlinear stability of relativistic sheared planar jets
2005
The linear and non-linear stability of sheared, relativistic planar jets is studied by means of linear stability analysis and numerical hydrodynamical simulations. Our results extend the previous Kelvin-Hemlholtz stability studies for relativistic, planar jets in the vortex sheet approximation performed by Perucho et al. (2004a,b) by including a shear layer between the jet and the external medium and more general perturbations. The models considered span a wide range of Lorentz factors ($2.5-20$) and internal energies ($0.08 c^2-60 c^2$) and are classified into three classes according to the main characteristics of their long-term, non-linear evolution. We observe a clear separation of thes…
Energy and time-lag spectra of galactic black-hole X-ray sources in the low/hard state
2003
Most, probably all, accreting binaries that are believed to contain a black-hole emit radio waves when they are in the low/hard state. Whenever this radio emission has been resolved, a jet-like structure has become apparent. We propose that Compton upscattering of low-energy photons in the jet can explain both the energy spectra and the time lags versus Fourier frequency observed in the low/hard state of black-hole systems. The soft photons originate in the inner part of the accretion disk. We have performed Monte Carlo simulations of Compton upscattering in a jet and have found that for a rather wide range of values of the parameters we can obtain power-law high-energy X-ray spectra with p…
On the existence of a reverse shock in magnetized gamma-ray burst ejecta
2007
The role of magnetic fields in gamma-ray burst (GRB) flows remains controversial. The study of the early afterglow phases and, in particular, of the reverse shock dynamics and associated emission offers a promising probe of the magnetization of the ejecta. In this paper, we derive the conditions for the existence of a reverse shock in arbitrarily magnetized ejecta that decelerate and interact with the circumburst medium. Both constant and wind-like density profiles are considered. We show, in contrast to previous estimates, that ejecta with magnetization σ0 >∼ 1 are not crossed by a reverse shock for a large fraction of the parameter space relevant to GRB flows. Allowing for shell spreading…
Two-dimensional spectroscopy for the study of ion Coulomb crystals
2015
Ion Coulomb crystals are currently establishing themselves as a highly controllable test-bed for mesoscopic systems of statistical mechanics. The detailed experimental interrogation of the dynamics of these crystals however remains an experimental challenge. In this work, we show how to extend the concepts of multi-dimensional nonlinear spectroscopy to the study of the dynamics of ion Coulomb crystals. The scheme we present can be realized with state-of-the-art technology and gives direct access to the dynamics, revealing nonlinear couplings even in the presence of thermal excitations. We illustrate the advantages of our proposal showing how two-dimensional spectroscopy can be used to detec…
CCD uvby-beta photometry of young open clusters. I. The double cluster h and chi Persei
2002
We present CCD uvby-beta photometry for stars in the nuclei of the young double cluster h and chi Persei. We find that the reddening is highly variable through the h Persei nucleus, increasing from west to east, with values ranging from E(b-y)=0.328+-0.022 in the western part to E(b-y)=0.465+-0.025 in the south-east. Towards chi Persei the reddening is fairly constant, with E(b-y)=0.398+-0.025. Both clusters share a common distance modulus of 11.7+-0.1 mag., and an age of log t=7.10+-0.05 years.
Schematic and realistic model calculations of the isovector spin monopole excitations in 116In
2012
The excitation of Gamow-Teller (GT) and isovector spin monopole (IVSM) Jπ=1+ modes in 116In by (p,n) and (n,p) charge-exchange reactions on 116Cd and on 116Sn, respectively, is studied within the framework of the quasiparticle random-phase approximation. The calculations have been performed both for schematic and realistic model situations. It appears that the calculated admixture of the IVSM and Gamow-Teller (GT) Jπ=1+ excitations is negligible and that the contribution to the strength above 20 MeV of excitation energy, in 116In, is due to the IVSM (σr2t ±) mode. This result is compared with the recent experimental work that reported a large amount of both (p,n) and (n,p) strength beyond 1…
A decade of SN 1993J : discovery of radio wavelength effects in the expansion rate
2009
We studied the growth of the shell-like radio structure of supernova SN 1993J in M 81 from September 1993 to October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18 cm. We developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure such as SN 1993J. The source structure of SN 1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J until approximately day 1500 after explosion with an expansion parameter m = 0.845 ± 0.005 (R ∝ tm). However, from that day onwards the expansion differs whe…