Massive fast rotating highly magnetized White Dwarfs: theory and astrophysical applications

In this thesis the topic of magnetic white dwarfs is addressed. In the first part are considered the instability boundaries that allow us to ensure that white dwarfs with arbitrarily large magnetic fields cannot exist in nature. We show the impossibility of the existence of these objects considering different instabilities due to inverse beta-decay, pycnonuclear reactions, general relativity or restrictions coming out from the viral theorem. In this way we discard the existence of hypothetical ultra magnetic white dwarfs (with magnetic fields as large as 10^8 G), which have been recently proposed as an explanation for some peculiar super luminous type Ia supernovae. In the second part of this thesis we consider the possibility that magnetic fast rotating white dwarfs could behave like rotationally powered pulsars. We show that under certain physical conditions this pulsar-like behaviour is possible and we show some examples of applications of this hypothesis to explain the well known Anomalous X-ray pulsars (AXPs) and Soft Gamma-ray repeaters(SGRs). This White Dwarf hypothesis is an alternative to the current magnetar model that intends to explain AXPs/SGRs. Finally, in the third part of this work we consider again AXPs and SGRs and we consider the possibility that some of them could be modelled as traditional rotationally powered pulsars and not as ultra magnetic neutron stars (magnetar model). We show that using realistic neutron star configurations the number of potentially rotationally powered pulsars is increased with respect to the case when only fiducial parameters for the mass, radius and moment of inertia are considered. We consider also other observational features of these objects in the rotationally powered pulsar model such as glitches, outbursts or emission in radio wavelengths. Finally, general conclusions and proposals for future work are presented.