A. Drago

Can very compact and very massive neutron stars both exist

The existence of neutron stars with masses of ∼2M⊙ requires a stiff equation of state at high densities. On the other hand, the necessary appearance also at high densities of new degrees of freedom, such as hyperons and Δ resonances, can lead to a strong softening of the equation of state with resulting maximum masses of ∼1.5M⊙ and radii smaller than ∼10 km. Hints for the existence of compact stellar objects with very small radii have been found in recent statistical analyses of quiescent low-mass X-ray binaries in globular clusters. We propose an interpretation of these two apparently contradicting measurements, large masses and small radii, in terms of two separate families of compact stars: hadronic stars, whose equation of state is soft, can be very compact, while quark stars, whose equation of state is stiff, can be very massive. In this respect an early appearance of Δ resonances is crucial to guarantee the stability of the branch of hadronic stars. Our proposal could be tested by measurements of radii with an error of ∼1 km, which is within reach of the planned Large Observatory for X-ray Timing satellite, and it would be further strengthened by the discovery of compact stars heavier than ∼2M⊙

The SPARC project: a high-brightness electron beam source at LNF to drive a SASE-FEL experiment

Abstract The Project Sorgente Pulsata e Amplificata di Radiazione Coerente (SPARC), proposed by a collaboration among ENEA–INFN–CNR–Universita’ di Tor Vergata–INFM–ST, was recently approved by the Italian Government and will be built at LNF. The aim of the project is to promote an R&D activity oriented to the development of a coherent ultra-brilliant X-ray source in Italy. This collaboration has identified a program founded on two main issues: the generation of ultra-high peak brightness electron beams and of resonant higher harmonics in the SASE-FEL process, as presented in this paper.

EMMI rapid reaction task force meeting on quark matter in compact stars

The recent measurement of two solar mass pulsars has initiated an intense discussion on its impact on our understanding of the high-density matter in the cores of neutron stars. A task force meeting was held from October 7-10, 2013 at the Frankfurt Institute for Advanced Studies to address the presence of quark matter in these massive stars. During this meeting, the recent observational astrophysical data and heavy-ion data was reviewed. The possibility of pure quark stars, hybrid stars and the nature of the QCD phase transition were discussed and their observational signals delineated.

Softening of the equation of state of matter at large densities and temperatures: chiral-symmetry restoration versus quark deconfinement.

We discuss two models in which a softening of the equation of state takes place due to the appearance of new degrees of freedom. The first is a hadronic model in which the softening is due to chiral symmetry restoration. In the second model the softening is associated with the formation of clusters of quarks in the mixed phase. We show that in the first case the bulk modulus is mainly dependent on the density, while in the mixed-phase model the bulk modulus strongly depends on the temperature and it is not vanishing due to the presence of two conserved charges, the baryon and the isospin one.

Combustion of a hadronic star into a quark star: the turbulent and the diffusive regimes

We argue that the full conversion of a hadronic star into a quark or a hybrid star occurs within two different regimes separated by a critical value of the density of the hadronic phase

Quark deconfinement and the duration of short Gamma Ray Bursts

\overline{n_h}

Conceptual design of a high-brightness linac for soft X-ray SASE-FEL source

. The first stage, occurring for

The SPARC/X SASE-FEL Projects

n_h>\overline{n_h}

Connection between distribution and fragmentation functions

, is characterized by turbulent combustion and lasts typically a few ms. During this short time-scale neutrino cooling is basically inactive and the star heats up thanks to the heat released in the conversion. In the second stage, occurring for

Generation of strong magnetic fields byr-modes in millisecond accreting neutron stars: Induced deformations and gravitational wave emission

n_h<\overline{n_h}