Modeling the pulsating emission for ULXs

Ultra-luminous X-ray sources, aka ULXs, are off-nuclear, extragalactic sources that shine in the X-rays with luminosities of Lx > 10^{39} erg s^{-1}. The huge energy output together with the X-ray variability suggest that ULXs are binary systems with a donor star transferring mass onto a compact object. ULXs luminosities far exceed the Eddington limit for a stellar-mass object, provided that the emission is assumed to be isotropic. This led to believe that these sources may host a black hole: either intermediate-mass black holes (IMBHs), or, if some degree of anisotropy is present in the emitted radiation, stellar-mass black holes (SMBHs). In 2014, observational evidences were found for pulsations in an ULX, revealing the existence of a new class of objects, dubbed Pulsating ULXs or PULXSs. Pulsations clearly point towards a the presence of a neutron star. The discovery of PULXs made the issue of how a super-Eddington flux may be produced even more irksome, since NS masses should not exceed ~ 2 solar masses. While invoking anisotropic emission is still an option, a further explanation may be provided by the presence of strong magnetic fields (B ~10^{14} G). In this thesis we discuss an emission model for PULXs, based on the effects of strong magnetic fields, which can account for their high luminosities. We focus on the modeling of the accretion disk and the accretion column close to the surface of the neutron star. The starting point is the commonly accepted picture according to which the accretion disk is truncated far from the star, due to the interaction between the disk itself and the magnetic field, and matter is then transported inwards along the field lines towards the poles, where a radiative shock and an accretion column are formed. Here the effects of the strong magnetic field on the electron scattering cross section starts to become important. In this thesis we built up on such a model, producing synthetic spectra and pulse profiles. A preliminary comparison with the observations of two PULXs show that indeed some observed features, like the increase of the pulsed fraction with energy, are indeed reproduced by the model.