Neutron Stars

Neutron Stars (NSs) are the most compact objects in the universe endowed with an internal structure. Proposed originally by Baade & Zwicky (1934) in the context of supernova explosions, they where discovered only in 1967 by Hewish et al. (1968) as radio pulsars. Today, NSs are among the most studied objects in high-energy astrophysics because they are known to power many astrophysical source of high energy emission. The extreme conditions characterizing their interior make them also interesting objects from the point of view of nuclear and condense matter physics, and future combined observations of both mass and radius of such compact objects may finally discriminate on the different equations of state (EoS) so far proposed .



It was immediately evident that NSs can also harbor very high magnetic fields, usually inferred to be in the range 108-12G for normal pulsars. It is indeed this very strong magnetic field that is responsible for most of their phenomenology and emission. The amplification of magnetic fields form the initial values prior to collapse to those enhanced values is believed to take place during the formation of the compact object itself: surely due to the compression associated with the collapse of the core of the progenitor star, it can be further increased by differential rotation in the core leading to the twisting of fieldlines, and to possible dynamo effects. In principle there is a large store of free energy available during and immediately following the collapse of the core and the formation of a proto-NS, such that a magnetic field as high as 1017-18G could be even reached.

Our further studies including radiation diagnostics opened then the way to some other puzzles: within axisymmetric MHD simulations, the morphology and emission spectrum of the Crab Nebula, the class prototype and one of the astrophysical objects for which data are most abundant, could not be simultaneously reproduced with a given value of the wind magnetization. More specifically the magnetization that could best reproduce the X-ray emission morphology would give origin to a nebular magnetic field well below the value inferred from multi-wavelength spectral modeling.

Magnetic fields are a key element in the physics and phenomenology of NS. Virtually nothing of their observed properties can be understood, without considering their effects. In particular the geometry of the magnetic field plays an important role, and even small differences can leads to changes in the physical processes that might be important for NS phenomenology.

Magnetars could be fundamental also to explain another class of objects typical of high-energy astrophysics, namely Gamma Ray Bursts (GRBs). The combination of a rapid millisecond-like rotation of a compact NS with a magnetic field of typical magnetar strength, can easily drive a relativistic outflow with energetics of the order of ~ 1049-50 erg s-1, enough to power a classical Long GRB. Short GRBs have been instead usually associated to merger events, rather than to core collapse of stellar objects, leading to the formation of a rotating Black Hole (BH), similarly to the collapsar scenario for Long GRBs


Our Group has contributed to the development of the millisecond magnetar model for long and short GRBs, carrying out the first relativistic MHD simulation of the interaction of the neutrino driven wind from a newly formed magnetar, spinning with millisecond period, with the surrounding environment, either the envelope of the progenitor for LGRBs or the low density merger ejecta for SGRBs, showing that efficient collimation and acceleration are achiavable.

Relevant Papers