Scientific context and activities of the various Research Units

The research field of compact stars and of dense nuclear matter has undergone a dramatic evolution after the event of August 2017 when the merger of two compact stars was observed for the first time in three different ways: as a Gravitational Wave (GW) signal GW170817, as a Gamma Ray Burst GRB170817A and as the first detected kilonova AT2017gfo.

The analysis of that event has provided new limits on the maximum mass of a compact star and on its radius, and now it is clear that this type of multi-messenger analysis will be key to finally unlock the mystery of the composition of such compact objects.  From the phenomenological viewpoint, fundamental breakthroughs have been achieved e.g. by relating the compactness of the star undergoing the merger to the kilonova signal, and by exploring in a much more systematic and consistent way the various possible Equations of State (EoSs) describing the matter inside a compact star. The EoS can be related to a variety of observables, e.g. the mass-radius relation, the tidal deformability (ApJ 860 (2018) 139), the process of cooling (MNRAS 484, 5162 (2019)) and the phenomenology of glitches (Nat.Astronomy 1 (2017) 0134).

The research activity aiming at investigating the relation between the GW signal and the EoS was hugely boosted by the detection of GW170817, and numerical simulations of mergers have been performed incorporating new ideas about the EoS (PRD 98 (2018) 043015), and testing new scenarios for quark deconfinement during the merger (ApJ 881 (2019) 2, 122). In particular, the process of quark deconfinement can play a crucial role in determining the duration of short GRBs and the delay between GW and EM emission (PRD93 (2016)103001), what could be tested by eXTP and THESEUS satellites. The possible existence of stars made totally (strange quark stars) or in part (hybrid stars) of deconfined quark matter has been at the center of many researches. In particular, the so-called two-families scenario has been proposed, in which strange quark stars can co-exist with stars made of hadrons, (EPJ A 52 (2016) 2, 40 and 41). This scenario has implications in a huge number of physical and astrophysical situations and its predictions are very easy to test, for instance by analyzing the data of mergers (ApJ 852 (2018) 2, L32).

The existence of long term collaborations among the different research units  allows us to outline common scientific objectives. A few activities are transversal among the various units, in particular at least the units of Catania, Ferrara and Pisa will contribute to developing the scientific case for eXTP (enhanced X-ray Timing Polarimetry) by providing the relevant tools for the modelling of NSs in the data analysis and by suggesting crucial signatures of the various theoretical scenarios. In particular the main common themes are :

Equations of state: to introduce in microscopically derived EoSs three-body forces for all the relevant hyperonic degrees of freedom, since that they can constitute a credible solution of the so-called hyperon puzzle (CT and PI). Another goal is to investigate the role of thermal effects on the EoS, especially on the maximum mass, which is crucial for determining the stability of the merger remnant (CT). A further common issue is the study of the role of Delta resonances, which needs to be introduced in microscopic calculations  (FE and PI).

Cooling: to compare the cooling in stars containing nucleons, resonances and hyperons with the cooling of strange quark stars. Also, to investigate the thermal evolution of the central object in LMXBs (CT and FE).

Mergers: to perform numerical simulations based on more realistic EoSs, including thermal effects and neutrino trapping. Also, to investigate the production of quark matter during the merger (CT, FE and PI). Another goal is to study the merger of a neutron star with a quark star in order to ascertain if the event of August 2017 was due to that type of merger (FE in collaboration with A.Bauswein).

Glitches: to investigate both the microphysics and the phenomenology of superfluid vortices in the crust of neutron stars (CT and MI). Also, to check if the two-family scenario is compatible with the analysis of the glitches, by assuming (at least initially) that strange quark stars do not glitch (FE and MI).

Quark matter phenomenology: to investigate the role of crystalline color superconducting phases, in particular concerning the damping of differential rotation (LNGS). In particular, EM emissions produced by the torsional oscillations of a crystalline quark phase have recently been suggested (ApJ 815 (2015) 81).

Strangelets and dark matter: to investigate if dark matter can be composed, at least in part, of strangelets and, in the case of other types of dark matter, to explore the role of dark matter in the structure of compact stars (CT and FE).

GWs and neutrinos: to study the emission of GWs and of neutrinos during core collapse SNe in order to improve the data analysis (LNGS).