Ceccobello, Chiara (2012) Radiative Transfer Problem in the Presence of Strong Magnetic Fields. Analytical and Numerical Treatment. Tesi di Dottorato , Università degli studi di Ferrara.

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Abstract
In this thesis we investigate analytical and numerical methods to find a solution of the radiative transfer equation in the presence of strong magnetic fields. My Ph.D research theme is focused on those astrophysical objects which presumably show an evidence of a strong magnetic field (B & 1012 G), with a particular emphasis on the physics of Xray spectral formation in these objects. The radiative transfer equation which describes spectral formation is, in general, rather complicated because of its integrodifferential nature. If we are interested in finding a solution, even numerically, we need to simplify the problem. For instance, we assume that stellar atmospheres can be represented, in first approximation, by a planeparallel slab of fully ionized plasma of nonrelativistic thermal electrons with an external uniform magnetic field. Since we are interested in modelling the high energy photon emission coming from the interaction with such medium, we assume also that the dominant radiative process which modifies the Xray photon spectrum is multiple inverse Compton scattering. We propose two approaches to the study of this problem and we discuss the related solutions. In the first part of this thesis, we present an analytical and numerical study of the radiative transfer problem in the presence of a strong uniform magnetic field (B & 4:4 � 1013G) taking place in a medium filled by nonrelativistic thermal electrons in planeparallel geometry. Even after making some initial assumptions, the equation governing such system is still an integrodifferential equation. Additional conditions are required to handle the radiative transfer equation the with separation of variable method. Then the radiative transfer problem can be reduced to the solution of the equation which has a diffusion operator for the energy variable and an integral operator for the space variable. Such an integrodifferential equation was firstly derived and its solution was estimated in 1988 by Lyubarskii in [1],[2]. We have solved numerically the equation proposed by Lyubarskii and we have confirmed this solution using the analytical methods. The second part of the thesis is devoted to the description of a numerical algorithm that we implemented for the resolution of radiative transfer equation, when it reduces to a pure differential form. This is usually the case when the FokkerPlanck (diffusion) approximation is applicable. The algorithm is essentially based on relaxation methods and, generally, it solves all inhomogeneous second order elliptic partial differential equations with vanishing mixed derivatives. The numerical code gives a stable solution of the equation when the system has reached its steadystate equilibrium. We test the code solving the radiative transfer problem in the case of cylindrical accretion onto a magnetised neutron star, when a combined effect of bulk and thermal Comptonization takes place. Finally, we implemented the algorithm in the Xray spectral fitting package XSPEC and we successfully fitted the Xray spectra of the two Supergiant Fast Xray Transients (SFXTs) XTE J1739302 and IGR J175442619, observed with the Swift Gammaray Burst Telescope. Our model is then compared with other XSPEC models we used during the Xray spectral fitting procedure and we briefly discuss possible implications on the geometry of these systems. I critically discuss and compare the results presented in the thesis in the conclusion section.
Tipologia del documento:  Tesi di Dottorato (Tesi di Dottorato) 

Data:  15 Marzo 2012 
Relatore:  Titarchuk, Lev 
Coordinatore ciclo:  Guidi, Vincenzo 
Istituzione:  Università degli studi di Ferrara 
Dottorato:  XXIV Anno 2009 > FISICA 
Struttura:  Dipartimento > Fisica 
Soggetti:  Area 02  Scienze fisiche > FIS/05 Astronomia e astrofisica 
Parole chiave:  trasporto radiativo, Compton, campi magnetici, radiative transfer, magnetic fields 
Depositato il:  21 Feb 2013 15:33 
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