Accretion disk coronae of Intermediate Polar Cataclysmic Variables: 3D MagnetoHydroDynamic modeling and thermal X-ray emission

Intermediate Polar Cataclysmic Variables (IPCVs), also known as DQ Her stars after their prototype, are cataclysmic variable stars characterized by the presence of an accreting, magnetic, rapidly rotating white dwarf surrounded by an accretion disk magnetically truncated by the inner magnetosphere of the star. These objects exhibit a magnetic field of the order of few MG, able to disrupt the inner part of the disk but not strong enough to inhibit the formation of a disk. These stars show a strong X-ray emission with a stochastic pulsation that in some cases arise in the inner part of the disk. However, the observations of the X-ray luminosity for these objects do not match the expected value: IPCVs appear in general to be underluminous in X-ray than expected for an accreting white dwarf. Nevertheless, accretion processes have often been successfully taken into account in order to explain the observed X-ray emission from IPCVs, although the observed features suggest that accretion phenomena occurring in these objects could be not the only process behind the X-ray emission. The main purposes of the present thesis are the following: the primary aim is to investigate the formation of an extended corona above the accretion disk, due to an intense flaring activity occurring on the disk surface; another objective is investigate its effects on the disk, on the accretion and on the stellar magnetosphere; the last, but not less important, intent is to assess the contribution of this coronal activity to the observed thermal X-ray fluxes. To this end we have developed a 3D magnetohydrodynamic (MHD) model describing an IPCV system. The model takes into account gravity, viscosity, thermal conduction, radiative losses and coronal heating. In order to carry out a parameter space exploration, several system conditions have been considered, with different magnetic field intensity and disk density. From the results of the evolution of the model, we have synthesized the thermal X-ray emission. The simulations actually show the formation of an extended corona, linking disk and star. The coronal activity is capable of strongly influencing the disk configuration and possibly its stability, effectively deforming the magnetic field lines. Hot plasma evaporation phenomena occur in the layer immediately above the disk. Furthermore, the flaring activity give rise to a conspicuous thermal X-ray emission in both the [0.1-2.0] keV and the [2.0-10] keV X-ray bands. In the light of our findings, it is possible to conclude that an intense coronal activity occurring on the disk surface of an IPCV can affect significantly the structure of the disk and the magnitude of this effect appears noticeably dependent on the density of the disk and the magnetic field of the central object. Moreover, the synthesis of the thermal X-ray fluxes shows that this flaring activity may contribute to the observed flickering thermal X-ray emission.