Navegando por Autor "Castro, Manuel"

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    Breaking the Degeneracy in Magnetic Cataclysmic Variable X-Ray Spectral Modeling Using X-Ray Light Curves
    (IOP science) Belloni, Diogo; Rodrigues, Claudia Vilega; Schreiber, Matthias R.; Castro, Manuel; Costa, Joaquim Eduardo Rezende; Hayashi, Takayuki; Lima, Isabel de Jesus; Luna, Gerardo Juan Manuel; Martins, Murilo; Oliveira, Alexandre Soares de; Parsons, Steven G.; Silva, Karleyne Medeiros Gomes da; Stecchini, Paulo Eduardo; Stuchi, Teresa de Jesus; Zorotovic, Monica
    We present an analysis of mock X-ray spectra and light curves of magnetic cataclysmic variables using an upgraded version of the 3D cyclops code. This 3D representation of the accretion flow allows us to properly model total and partial occultation of the postshock region by the white dwarf as well as the modulation of the X-ray light curves due to the phase-dependent extinction of the preshock region. We carried out detailed postshock region modeling in a four-dimensional parameter space by varying the white dwarf mass and magnetic field strength as well as the magnetosphere radius and the specific accretion rate. To calculate the postshock region temperature and density profiles, we assumed equipartition between ions and electrons; took into account the white dwarf gravitational potential, the finite size of the magnetosphere, and a dipole-like magnetic field geometry; and considered cooling by both bremsstrahlung and cyclotron radiative processes. By investigating the impact of the parameters on the resulting X-ray continuum spectra, we show that there is an inevitable degeneracy in the four-dimensional parameter space investigated here, which compromises X-ray continuum spectral fitting strategies and can lead to incorrect parameter estimates. However, the inclusion of X-ray light curves in different energy ranges can break this degeneracy, and it therefore remains, in principle, possible to use X-ray data to derive fundamental parameters of magnetic cataclysmic variables, which represents an essential step toward understanding their formation and evolution.
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    On the behavior of the black hole candidate 1E 1740.7-2942’scorona based on long-term INTEGRAL database
    (Wiley) Stecchini, Paulo Eduardo; Leão, Jurandi; Castro, Manuel; D'Amico, Flavio
    One of the most straightforward ways to explain the hard X-ray spectra observed in X-ray binaries is to assume that comptonization of soft photons from the accretion disk is occurring. The region where this process takes place, called the corona, is characterized by only two parameters: its thermal energy kT and its optical depth τ. Hard X-ray spectra analysis is, thus, an imperative tool in diagnosing the behavior of these parameters. The lack of consistency in obtain-ing/analyzing long-term databases, however, may have been hindering this kind of characterization from being attained. With the aim of better understanding the corona behavior in the black hole candidate 1E 1740.7-2942, we performed a homogeneous analysis for a large hard X-ray data set from the ISGRI telescope on-board the INTEGRAL satellite. Results from modeling the spectra show that, for most of our sample, unsaturated thermal comptonization is the main mechanism responsible for the hard X-ray spectra observed in 1E 1740.7-2942.Moreover, such extensive database allowed us to produce what is probably the longest hard X-ray light curve of 1E 1740.7-2942 and whose units—due to recent findings regarding dynamical quantities of the system—could be expressed in %of Eddington’s luminosity.