Search results for "Compact star"
showing 10 items of 39 documents
Spin down of an Accreting Millisecond Pulsar, the case of XTE J1814‐338
2007
We report about a timing analysis performed on the data gathered by RXTE of the accreting millisecond pulsar XTE J1814-338 during its 2003 outburst. The first full orbital solution of this binary system is given. Moreover the evolution of the phase of the pulsed emission reveals that the rotating compact object is spinning down at a rate ν˙ = (-6.7 +/- 0.7) × 10-14 Hz/s, while accreting. This behavior is considered as a result of the braking effect due to the interaction between the magnetosphere and the inner parts of the accretion disc, in the case of an accretion rate low enough to allow the expansion of the magnetospheric radius to the corotation limit. In this context we derive an esti…
Spectral Evolution of Circinus X-1 along Its Orbit
2001
We report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in March 1998 along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from $2.5 \times 10^{38}$ erg s$^{-1}$ at the periastron (orbital phase 0.01) to $1.5 \times 10^{38}$ erg s$^{-1}$ at orbital phase 0.3. From the spectral analysis and the lightcurve we argue that Cir X-1 shows three states along the orbital evolution. The first state is at the orbital phase interval 0.97-0.3: the luminosity becames super-Eddington and a strong flaring activity is present. In this state a shock could form in the inner region of the system due to the super-Eddington accretion rate, producing an…
On the soft excess in the x-ray spectrum of circinus X-1: Revisitation of the distance to circinus X-1
2005
We report on a 300 ks BeppoSAX (0.12-200 keV) observation of Circinus X-1 (Cir X-1) at phases between 0.62 and 0.84 and on a 90 ks BeppoSAX observation of Cir X-1 at phases 0.11-0.16. Using the canonical model adopted until now to fit the energy spectrum of this source, large residuals appear below 1 keV. These are well fitted using an equivalent hydrogen column of 0.66¿1022 cm-2, adding absorption edges of O VII, O VIII, and Ne IX in the spectra extracted from the observation at phases 0.62-0.84 and adding absorption edges of O VII, O VIII, Mg XI, and Mg XII and absorption lines of O VIII and Mg XII in the spectra extracted from the observation at phases 0.11-0.16. During the observation a…
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…
High Resolution and Broad Band Spectra of Low Mass X-ray Binaries: A Comparison between Black Holes and Neutron Stars
2005
A common question about compact objects in high energy astrophysics is whether it is possible to distinguish black hole from neutron star systems with some other property that is not the mass of the compact object. Up to now a few characteristics have been found which are typical of neutron stars (like quasi periodic oscillations at kHz frequencies or type-I X-ray bursts), but in many respects black hole and neutron star systems show very similar behaviors. We present here a spectral study of low mass X-ray binaries containing neutron stars and show that these systems have spectral characteristics that are very similar to what is found for black hole systems. This implies that it is unlikel…
3-D collapse of rotating stars to Kerr black holes
2005
We study gravitational collapse of uniformly rotating neutron stars to Kerr black holes, using a new three-dimensional, fully general relativistic hydrodynamics code, which uses high-resolution shock-capturing techniques and a conformal traceless formulation of the Einstein equations. We investigate the gravitational collapse by carefully studying not only the dynamics of the matter, but also that of the trapped surfaces, i.e. of both the apparent and event horizons formed during the collapse. The use of these surfaces, together with the dynamical horizon framework, allows for a precise measurement of the black-hole mass and spin. The ability to successfully perform these simulations for su…
Accreting magnetars: a new type of high-mass X-ray binaries?
2012
The discovery of very slow pulsations (Pspin=5560s) has solved the long-standing question of the nature of the compact object in the high-mass X-ray binary 4U 2206+54 but has posed new ones. According to spin evolutionary models in close binary systems, such slow pulsations require a neutron star magnetic field strength larger that the quantum critical value, suggesting the presence of a magnetar. We present the first XMM-Newton observations of 4U 2206+54 and investigate its spin evolution. We find that the observed spin-down rate agrees with the magnetar scenario. We analyse ISGRI/INTEGRAL observations of 4U 2206+54 to search for the previously suggested cyclotron resonance scattering feat…
Neutron star formation with presence of hyperons
2001
We study the influence of hyperons during the early stages of the birth of a neutron star (Kelvin-Helmholtz phase), employing neutrino opacities calculated consistently with the equation of state by considering all possible neutrino-hyperon reactions. Our results from numerical simulations of newly born neutron stars, or proto-neutron stars, show an increasingly important influence of hyperons at later times. It is remarkable the existence of metastable stars, which are stable at birth but become unstable during the evolution as the deleptonization proceeds and the hyperon concentration increases. We also present results from hydrodynamical simulations of the collapse to a black hole of met…
Accretion-driven gravitational radiation from nonrotating compact objects. Infalling quadrupolar shells
2004
This paper reports results from numerical simulations of the gravitational radiation emitted from non--rotating compact objects(both neutron stars and Schwarzschild black holes) as a result of the accretion of matter. A hybrid procedure is adopted: we evolve, in axisymmetry, the linearized equations describing metric and fluid perturbations, coupled with a nonlinear hydrodynamics code that calculates the motion of the accreting matter. The initial matter distribution is shaped in the form of extended quadrupolar shells of dust or perfect fluid. Self--gravity and radiation reaction effects of the accreting fluid are neglected. This idealized setup is used to understand the qualitative featur…
Orbital X‐Ray Variability of the Microquasar LS 5039
2005
The properties of the orbit and the donor star in the high mass X-ray binary microquasar LS 5039 indicate that accretion processes should mainly occur via a radiatively driven wind. In such a scenario, significant X-ray variability would be expected due to the eccentricity of the orbit. The source has been observed at X-rays by several missions, although with a poor coverage that prevents to reach any conclusion about orbital variability. Therefore, we conducted RossiXTE observations of the microquasar system LS 5039 covering a full orbital period of 4 days. Individual observations are well fitted with an absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line emission that …