Search results for "UNESCO::ASTRONOMÍA Y ASTROFÍSICA::Cosmología y cosmogonia::Estrellas"
showing 10 items of 12 documents
The pre-outburst flare of the A 0535+26 August/September 2005 outburst
2008
We study the spectral and temporal behavior of the High Mass X-ray Binary A 0535+26 during a `pre-outburst flare' which took place ~5 d before the peak of a normal (type I) outburst in August/September 2005. We compare the studied behavior with that observed during the outburst. We analyse RXTE observations that monitored A 0535+26 during the outburst. We complete spectral and timing analyses of the data. We study the evolution of the pulse period, present energy-dependent pulse profiles both at the initial pre-outburst flare and close to outburst maximum, and measure how the cyclotron resonance-scattering feature (hereafter CRSF) evolves. We present three main results: a constant period P=…
Unveiling the nature of six HMXBs through IR spectroscopy
2008
The International Gamma-Ray Astrophyiscs Laboratory (INTEGRAL) is discovering a large number of new hard X-ray sources, many of them being HMXBs. The identification and spectral characterization of their optical/infrared counterparts is a necessary step to undertake detailed study of these systems. In particular, the determination of the spectral type is crucial in the case of the new class of Supergiant Fast X-ray Transients (SFXTs), which show X-ray properties common to other objects. We used the ESO/NTT SofI spectrograph to observe proposed IR counterparts to HMXBs, obtaining Ks medium resolution spectra (R = 1320) with a S/N >= 100. We classified them through comparison with publishe…
A new general relativistic magnetohydrodynamics code for dynamical spacetimes
2008
We present a new numerical code which solves the general relativistic magneto-hydrodynamics (GRMHD) equations coupled to the Einstein equations for the evolution of a dynamical spacetime within the conformally-flat approximation. This code has been developed with the main objective of studying astrophysical scenarios in which both, high magnetic fields and strong gravitational fields appear, such as the magneto-rotational collapse of stellar cores, the collapsar model of GRBs, and the evolution of neutron stars. The code is based on an existing and thoroughly tested purely hydrodynamics code and on its extension to accommodate weakly magnetized fluids (passive magnetic field approximation).…
Local simulations of the magnetized Kelvin-Helmholtz instability in neutron-star mergers
2010
Context. Global MHD simulations show Kelvin-Helmholtz (KH) instabilities at the contact surface of two merging neutron stars. That region has been identified as the site of efficient amplification of magnetic fields. However, these global simulations, due to numerical limitations, were unable to determine the saturation level of the field strength, and thus the possible back-reaction of the magnetic field onto the flow. Aims. We investigate the amplification of initially weak fields in KH unstable shear flows, and the back-reaction of the field onto the flow. Methods. We use a high-resolution ideal MHD code to perform 2D and 3D local simulations of shear flows. Results. In 2D, the magnetic …
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…
Semi-global simulations of the magneto-rotational instability in core collapse supernovae
2009
Possible effects of magnetic fields in core collapse supernovae rely on an efficient amplification of the weak pre-collapse fields. It has been suggested that the magneto-rotational instability (MRI) leads to rapid field growth. Although MRI studies exist for accretion discs, the application of their results to core collapse supernovae is inhibited as the physics of supernova cores is substantially different from that of accretion discs. We address the problem of growth and saturation of the MRI by means of semi-global simulations, which combine elements of global and local simulations by taking the presence of global background gradients into account and using a local computational grid. W…
Relativistic Hydrodynamics and Spectral Evolution of GRB Jets
2017
En la presente tesis hemos estudiado los sistemas progenitores de las erupciones de rayos gamma (ERG, o GRBs por sus siglas en inglés --Gamma-Ray Bursts--) de larga duración mediante modelos numéricos de su evolución dinámica y emisión electromagnética. De todas las posibles clases de eventos, en particular nos centramos en aquellas que muestran una componente prominente de emisión térmica, la cual podría generarse por medio de la interacción de un chorro constituido por plasma relativista con el medio en el que se propaga. La parte central de esta tesis se ha dedicado al modelado de ERG a partir de dos clases diferentes de progenitores: ERG ultra largas dominadas en su espectro de emisión …
3D simulations of wind-jet interaction in massive X-ray binaries
2010
High-mass microquasars may produce jets that will strongly interact with surrounding stellar winds on binary system spatial scales. We study the dynamics of the collision between a mildly relativistic hydrodynamical jet of supersonic nature and the wind of an OB star. We performed numerical 3D simulations of jets that cross the stellar wind with the code Ratpenat. The jet head generates a strong shock in the wind, and strong recollimation shocks occur due to the initial overpressure of the jet with its environment. These shocks can accelerate particles up to TeV energies and produce gamma-rays. The recollimation shock also strengthens jet asymmetric Kelvin-Helmholtz instabilities produced i…
23 GHz VLBI observations of SN 2008ax (Research Note)
2009
We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI) observations of the type IIb supernova SN 2008ax, made with the Very Long Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These observations resulted in a marginal detection of the supernova. The total flux density recovered from our VLBI image is 0.8 ± 0.3 mJy (one standard deviation). As it appears, the structure may be interpreted as either a core-jet or a double source. However, the supernova structure could be somewhat confused with a possible close by noise peak. In such a case, the recovered flux density would decrease to 0.48 ± 0.12 mJy, compatible with the flux densities measured with the…
Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54
2008
Context. The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pul- sations have not been detected in the time range 10−3–103 s. A cyclotron line at ∼30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Aims. The main objective of our RXTE monitoring of 4U 2206+54 is to study the X-ray orbital variability of the spectral and timing parameters. The new long and uninterrupted RXTE observations allow us to search for long (∼1 h) puls…