Examinando por Autor "Parsons, Steven G."

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    Breaking the Degeneracy in Magnetic Cataclysmic Variable X-Ray Spectral Modeling Using X-Ray Light Curves
    (American Astronomical Society (Aas), 2021) Belloni, Diogo; Rodrigues, Claudia; Schreiber, Matthias R.; Castro, Manuel; Costa, Joaquim E. R.; Hayashi, Takayuki; Lima, Isabel J.; Luna, Gerardo J. M.; Martins, Murilo; Oliveira, Alexandre S.; Parsons, Steven G.; Silva, Karleyne M. G.; Stecchini, Paulo E.; Stuchi, Teresa J.; 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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    Ítem
    The origin and evolution of magnetic white dwarfs in close binary stars
    (Springer, 2021) Schreiber, Matthias R.; Belloni, Diogo; Gänsicke, Boris T.; Parsons, Steven G.; Zorotovic, Monica
    The origin of magnetic fields in white dwarfs remains a fundamental unresolved problem in stellar astrophysics. In particular, the very different fractions of strongly (more than about a megagauss) magnetic white dwarfs in evolutionarily linked populations of close white dwarf binary stars cannot be reproduced by any scenario suggested so far. Strongly magnetic white dwarfs are absent among detached white dwarf binary stars that are younger than approximately a billion years. In contrast, of cataclysmic variables (semi-detached binary star systems that contain a white dwarf) in which the white dwarf accretes from a low-mass star companion, more than a third host a strongly magnetic white dwarf1. Here we present binary star evolutionary models that include the spin evolution of accreting white dwarfs and crystallization of their cores, as well as magnetic field interactions between the stars. We show that a crystallization- and rotation-driven dynamo similar to those working in planets and low-mass stars2 can generate strong magnetic fields in the white dwarfs in cataclysmic variables, which explains their large fraction among the observed population. When the magnetic field generated in the white dwarf connects with that of the secondary star in the binary system, synchronization torques and reduced angular momentum loss cause the binary to detach for a relatively short period of time. The few known strongly magnetic white dwarfs in detached binaries, such as AR Scorpii3, are in this detached phase.