Taverna, Roberto (2016) Polarized emission from highly-magnetized neutron stars. [Ph.D. thesis]
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The study of magnetars, the anomalous X-ray pulsars (AXPs) and the soft gamma repeaters (SGRs), and of X-ray Dim Isolated Neutron Stars (XDINSs) is of particular relevance, since these objects exhibit the strongest magnetic fields ever observed in the universe (10^13-10^15 G) and represent the only laboratories where physics in the presence of such strong magnetic fields can be tested. Until now, these peculiar neutron stars have been investigated through spectroscopic and timing measurements, which led to validate the theoretical models developed to explain their phenomenology, as in the case of the "twisted magnetosphere'' model for magnetars or the different surface emission models for XDINSs. Nevertheless, this kind of analysis alone is far from providing complete information. In this respect, X-ray polarimetry may disclose an entirely new approach in the study of highly magnetized neutron stars. Radiation emitted in the presence of strong magnetic fields, in fact, is expected to be highly polarized; polarization measurements provide two additional observables, the linear polarization fraction and the polarization angle, that can unambiguously determine the model parameters also when spectral analysis alone fails. The polarization signal that an observer at infinity would collect, however, do not necessary coincide with model predictions for the polarization at the surface, due to the effects of quantum electrodynamics in the highly magnetized vacuum around the star, coupled with the rotation of the Stokes parameters in the plane perpendicular to the line of sight, induced by the non-uniform magnetic field. In this thesis I present the results of the numerical codes I developed to simulate the polarization pattern, both at the surface and as observed at infinity, of the radiation emitted from highly magnetized, isolated neutron stars, using as templates the bright AXP 1RXS J170849.0-400910 and the XDINS RX J1856.5-3754. I demonstrate that polarization measurements can indeed provide key information about the physical and geometrical properties of these sources, allowing to directly test theoretical models. This work is also relevant in view of the launch of new-generation X-ray polarimeters, currently under development, like the X-ray Imaging Polarimeter Explorer (XIPE). For this reason, I also compare theoretical models with XIPE simulated observations, in order to show how polarization measurements can be used to extract the values of magnetospheric parameters and viewing angles.
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Lo studio delle magnetars, anomalous X-ray pulsars (AXPs) e soft gamma repeaters (SGRs), e delle X-ray Dim Isolated Neutron Stars (XDINSs) è di particolare rilevanza, dal momento che questi oggetti mostrano i più forti campi magnetici mai osservati nell'universo (10^13-10^15 G) e rappresentano i soli laboratori dove la fisica in presenza di campi magnetici così forti può essere testata. Fino ad ora, queste particolari stelle di neutroni sono state studiate attraverso misure spettroscopiche e di timing, che hanno portato a corroborare i modelli teorici formulati per spiegare la loro fenomenologia, come nel caso del "twisted magnetosphere'' model per le magnetars o dei diversi modelli di emissione superficiale per le XDINSs. Cionondimeno, questa analisi da sola non riesce a fornire informazioni complete. A questo riguardo, la polarimetria X può svelare un approccio
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