Hunting for high magnetic fields in different neutron star classes

Remnants of massive stars, neutron stars (Nss), are valuable laboratories to study matter under extreme densities and provide a unique environment with extreme temperature, magnetic and gravitational fields. Up to now almost 2500 NSs have been discovered and show different behaviours, leading astrophysicists to establish several classes. This thesis presents new results on isolated NSs. We investigate the possible manifestations of NS magnetic fields in order to find evidence for high-magnetic-field structures and analyze how these strong magnetic fields affect the star properties. Chapters 2 and 3 report the discovery of a phase-variable absorption feature in the spectra of two X-ray dim isolated NSs. If interpreted as being due to proton cyclotron resonant scattering in a magnetized bundle close to the surface, this feature provides the first indication for the existence of localized structures in these neutron stars. In Chapters 4 and 5, we study the source 1E 161348-5055. With its long periodicity (6.67 hours) and flux variability on a month/year timescale, it defied any classification until a new outburst was detected in 2016. The characteristics of the outburst and the following decay are consistent with 1E 161348-5055 being a magnetar. The emission of magnetars is powered by the instabilities and decay of their strong magnetic fields (up to 10^15 Gauss). The case of the magnetar CXOU J164710.2-455216 is discussed in Chapter 6. This object was revealed to be prolific, since two outbursts were detected in 2006 and 2011, with a third outburst triggered in May 2017.