PHD THESIS (in Italian)



In this work we have tried to solve some open problems related to a new class of galactic X-ray transient sources: the superluminal galactic sources GRS 1915+ 105 and GRO J1655-40. In particular we have dealt with the energetics of the jets, the jet formation and the polarimetric properties of GRO J1655-40.
A method to derive the minimum power of jets is discussed and used to find a lower limit to the jet kinetic power of GRS 1915+105 both during the major ejections and the so called ``baby jets``. The large value obtained severly limits the possible carriers of the kinetic luminosity at the base of the jet: neither a pure pair plasma, nor a relativistic normal (electron-proton) plasma can carry the kinetic power; the only two possible carriers are either the ``cold" plasma or the Poynting vector. Although there were no strong observational arguments to decide between the two possibilities, the magnetic transport seems to be more suitable, because it also provide an effective mechanism accelerating and collimating the jets.
Starting from the results obtained for the minimum kinetic power, we have studied the jet formation from a magnetized accretion disk, using a finite difference MHD code (ZEUS-2D) in two dimensions. By comparing the numerical results (in particular mass, energy and magnetic fluxes in the jet) with the observational ones, we found first a good agreement in the case of baby jets, suggesting that the MHD acceleration could work for the superluminal jets, and secondly that the energy transport at the base of the jet is mainly magnetic.
In the last part of the thesis we present the results of optical polarimetric observations of the superluminal source GRO J1655-40, carried out in July 1996 and 1997, with the multichannel photopolarimeter of the Torino Observatory, using the 2.15-m telescope of Complejo Astronomico El Leoncito (Argentina). During the first observations a significant amount of intrinsic linear polarization (>3%) has been detected in the VRI bands, with the polarization direction parallel to the accretion disk plane, however we were not able to say whether the origin of the polarized optical continuum was due to a nonthermal emission process (synchrotron emission from relativistic plasma inside the jet) or to electron scattering. During the last observations we found that the amount of polarization showed oscillations which are consistent with the orbital period of the system. This gives information about physical and geometrical properties of the binary system and confirms that the optical continuum is intrinsically polarized. Although the origin of the polarized flux was likely to be related to electron scattering of photons from the accretion disk, the polarimetric observations also allow us to put some constraints on the energy transport inside the jets and confirm that at the base of the jet the transport occurs via Poynting vector.


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