Neutron stars (NSs) are for many reasons among the most fascinating astrophysical objects. The most extreme regimes of physics are combined in essentially perfect spheres of a dozen km in radius. These objects have enormous densities, ultrastrong magnetic fields and produce large curvatures in spacetime. We study these objects either when isolated or when in binary systems. To this end we have developed a fully three dimensional numerical code, Whisky, for carrying out simulations of general relativistic hydrodynamics.
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NS-NS binaries: Binary neutron stars are among the most promising sources of gravitational waves and are leading candidates as short-hard gamma-ray burst progenitors. The numerical investigation of their coalescence and merger within the framework of General Relativity is thus fundamental. Over the years, we have performed fully general-relativistic simulations of these systems, for various mass ratios and for both unmagnetized and magnetized neutron stars, reaching high accuracy standards. In doing this, major contributions to the development of the Whisky code were made. Particular attention has been put into the investigation of the rich late-time dynamics, the outcome of which includes the possible formation of tori and accretion disks, prompt collapse to a BH, and the chance of having the formation of hypermassive NS transients.
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BH-NS binaries: These binaries share with BH-BH and NS-NS binaries the importance of
being leading gravitational wave source candidates; they have also been indicated, along with binary neutron stars, as possible progenitors of a part of the short-hard gamma-ray bursts we observe. The major challenge with this type of binary is dealing simultaneously with the difficulties related to the BH and to the NS. Recently, the Numerical Relativity group has been investigating these systems in two directions: by performing accurate, general relativistic simulations of their merger, focusing especially on the effects of having a magnetized NS, and by elaborating semi-analytical models in order to predict the dependance on the binary physical parameters of poorly constrained key features of the outcome of its merger, such as the mass of the torus that may possibly form.
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Isolated neutron stars: During its life, it is likely that every neutron star undergoes an oscillatory phase: neutron star oscillations may be excited, for example, as matter is accreted from a companion, after a core collapse, during a starquake induced by the spin-down of the star or by a large phase transition, as transients during a gravitational collapse. The rich phenomenology associated with these oscillations includes, among the others, hydrodynamical instabilities, secular instabilities, gravitational wave emission, and critical phenomena, topics on which we have been extensively working, within the framework of general-relativistic simulations. We have also been addressing the stability of magnetars (young, isolated neutron stars with very strong magnetic fields), which are believed to be behind the observed soft gamma repeaters and anomalous X-ray pulsars.