Observing neutron stars at the magnetic extremes.

The thesis comprises new results on emission phenomena associated with neutron stars (NSs). It describes the long-term X-ray monitoring campaign of an extremely magnetic NS (magnetar) at the Galactic Centre and its extremely slow flux decay, which is challenging most of NS crustal cooling models. It reports on the identification of a source as the slowest NS ever following its recent outburst and also on the first observational demonstration for the existence of complex magnetic field configurations near the surface of X-ray dim isolated NSs. Results of the first systematic study of 19 magnetar outbursts, including searches for (anti-)correlations among different parameters (energetic, maximum flux increase, decay timescale, quiescent flux) are presented. Another chapter is devoted to the modelling of the multi-wavelength emission properties of the transitional pulsar (TMP) PSR J1023+0038 (J1023), i.e. a NS rotating at a period of a few milliseconds in a binary system showing rapid transitions between a bright X-ray pulsar regime powered by the accretion onto the NS surface of matter transferred by the companion star, a radio pulsar regime powered by the energy loss due to the fast rotation of the NS magnetic field and an intermediate accretion regime in which matter accretes only intermittently onto the NS surface. The thesis presents new observational properties of J1023 in the current state and a novel scenario explaining the observed rapid transitions