Abstract
Neutron stars are natural physical laboratories allowing us to study a plethora of phenomena in extreme conditions. In particular, these compact objects can have very strong magnetic fields with non-trivial origin and evolution. In many respects, its magnetic field determines the appearance of a neutron star. Thus, understanding the field properties is important for the interpretation of observational data. Complementing this, observations of diverse kinds of neutron stars enable us to probe parameters of electro-dynamical processes at scales unavailable in terrestrial laboratories. In this review, we first briefly describe theoretical models of the formation and evolution of the magnetic field of neutron stars, paying special attention to field decay processes. Then, we present important observational results related to the field properties of different types of compact objects: magnetars, cooling neutron stars, radio pulsars, and sources in binary systems. After that, we discuss which observations can shed light on the obscure characteristics of neutron star magnetic fields and their behaviour. We end the review with a subjective list of open problems.
Highlights
Magnetic fields play a significant role in many areas of modern astrophysics, manifesting themselves in different energetic phenomena ranging from solar flares to gamma-ray and fast radio bursts
The strongest magnetic fields are found in neutron stars (NSs), where they control whether NSs are seen as normal radio pulsars, magnetars, dim isolated cooling NSs, or something else [1,2]
Recent examples of population studies of magneto-rotational evolution of initially highly magnetised NSs can be found in [21,119]. These studies demonstrated that rapid field decay in magnetars prevents the formation of bright sources with spin periods longer than those already observed for these objects
Summary
Magnetic fields play a significant role in many areas of modern astrophysics, manifesting themselves in different energetic phenomena ranging from solar flares to gamma-ray and fast radio bursts. It is easy to derive that the differential Equation (1) has the following solution if the magnetic field and obliquity angle do not evolve with time: P2(t) 2. This age is called the spin-down (characteristic) age, and is often used to estimate the true age of a radio pulsar. This age has obvious disadvantages, related to assumptions that are made to derive it: (1) no evolution of the dipole field, (2) no evolution of the obliquity angle, and (3) a much smaller initial period than the modern period of radio pulsars.
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