The first direct detection of gravitational waves has opened a new window to study the Universe and would probably start a new era: the gravitational wave Astronomy. Gravitational waves emitted by compact objects like neutron stars could provide significant information about their structure, composition and evolution.In this paper we calculate, using the relativistic Cowling approximation, the oscillations of compact stars focusing on hybrid stars, with and without a mixed phase in their cores. We study the existence of a possible hadron-quark phase transition in the central regions of neutron stars and the changes it produces on the gravitational modes frequencies emitted by these stars. We pay particular attention to the g-modes, which are extremely important as they could signal the existence of pure quark matter inside neutron stars. Our results show a relationship between the frequency of the g-modes and the constant speed of sound parametrization for the quark matter phase. We also show that the inclusion of color superconductivity produces an increase on the oscillation frequencies.We propose that observations of g-modes with frequencies f g between 1 kHz and 1.5 kHz should be interpreted as an evidence of a sharp hadron-quark phase transition in the core of a compact object.

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We study the influence of strong magnetic fields in hybrid stars, composed by hadrons and a pure quark matter core, and analyse their structure and stability as well as some possible evolution channels due to the magnetic field decay. Using an ad-hoc parametrisation of the magnetic field strength and taking into account Landau-quantization effects in matter, we calculate hybrid magnetised equations of state and some associated quantities, such as particle abundances and matter magnetisation, for different sets of parameters and different magnetic field strengths. Moreover, we compute the magnetised stable stellar configurations, the mass versus radius and the gravitational mass versus central energy density relationships, the gravitational mass versus baryon mass diagram, and the tidal deformability. Our results are in agreement with both, the ∼ 2M pulsars and the data obtained from GW170817. In addition, we study the stability of stellar configurations assuming that slow and rapid phase transitions occur at the sharp hadron-quark interface. We find that, unlike in the rapid transition scenario, where ∂ M/∂ c < 0 is a sufficient condition for instability, in the slow transition scenario there exists a connected extended stable branch beyond the maximum mass star, for which ∂ M/∂ c < 0. Finally, analysing the gravitational mass versus baryon mass relationship, we have calculated the energy released in transitions between stable stellar configurations. We find that the inclusion of the magnetic field and the existence of new stable branches allows the possibility of new channels of transitions that fulfil the energy requirements to explain Gamma Ray Bursts. From a model-theoretical point of view, magnetars are magnetised NSs (M ∼ 2 M , R ∼ 15 km (Glendenning 1997)) with an external solid crust and an internal extremely dense

The discovery of pulsars as heavy as 2 solar masses has led astrophysicists to rethink the core compositions of neutron stars, ruling out many models for the nuclear equations of state (EoS). We explore the hybrid stars that occur when hadronic matter is treated in a relativistic meanfield approximation and quark matter is modeled by three-flavor local and non-local Nambu−JonaLasinio (NJL) models with repulsive vector interactions. The NJL models typically yield equations of state that feature a first-order transition to quark matter. Assuming that the quark-hadron surface tension is high enough to disfavour mixed phases, and restricting to EoSs that allow starts to reach 2 solar masses, we find that the appearance of the quark matter core either destabilizes the star immediately (this is typical for non-local NJL models) or leads to a very short hybrid star branch in the mass-radius relation (this is typical for local NJL models). Using the Constant-SoundSpeed parametrization we can see that the reason for the near-absence of hybrid stars is that the transition pressure is fairly high and the transition is strongly

The recent direct observation of gravitational wave event GW 170817 and its GRB170817A signal has opened up a new window to study neutron stars and heralds a new era of Astronomy referred to as the Multimessenger Astronomy. Both gravitational and electromagnetic waves from a single astrophysical source have been detected for the first time. This combined detection offers an unprecedented opportunity to place constraints on the neutron star matter equation of state. The existence of a possible hadron-quark phase transition in the central regions of neutron stars is associated with the appearance of g-modes, which are extremely important as they could signal the presence of a pure quark matter core in the centers of neutron stars. Observations of g-modes with frequencies between 1 kHz and 1.5 kHz could be interpreted as evidence of a sharp hadron-quark phase transition in the cores of neutron stars. In this article, we shall review the description of the dense matter composing neutron stars, the determination of the equation of state of such matter, and the constraints imposed by astrophysical observations of these fascinating compact objects. arXiv:1907.04654v1 [astro-ph.HE]

In part one of this paper, we use a non-local extension of the 3-flavor Polyakov-Nambu-Jona-Lasinio model, which takes into account flavor-mixing, momentum dependent quark masses, and vector interactions among quarks, to investigate the possible existence of a spinodal region (determined by the vanishing of the speed of sound) in the QCD phase diagram and determine the temperature and chemical potential of the critical end point. In part two of the paper, we investigate the quark-hadron composition of baryonic matter at zero as well as non-zero temperature. This is of great topical interest for the analysis and interpretation of neutron star merger events such as GW170817. With this in mind, we determine the composition of proto-neutron star matter for entropies and lepton fractions that are typical of such matter. These compositions are used to delineate the evolution of proto-neutron stars to neutron stars in the baryon-mass versus gravitational-mass diagram. The hot stellar models turn out to contain significant fractions of hyperons and ∆-isobars but no deconfined quarks. The latter, are found to exist only in cold neutron stars. I. INTRODUCTIONExploring the thermodynamic behavior of the quark-gluon plasma and its associated equation of state (EoS) has become one of the forefront areas of modern physics. The properties of such matter are being probed with the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN, and great advances in our understanding of such matter are expected from the next generation of high density experiments at the Facility for Antiproton and Ion Research (FAIR at GSI) [1, 2], the Nuclotron-bases Ion Collider fAcility (NICA at JINR) [3, 4], the Japan Proton Accelerator Research Complex (J-PARC at Tokai campus of JAEA) [5], the Super Proton Synchrotron (SPS at CERN) [6] and the Beam Energy Scan (BES at BNL) [7].Depending on temperature T , and baryon chemical potential µ, the deconfined phase of quarks and gluons is believed to exist at two extreme regions in the phase diagram of quantum chromodynamics (QCD). The first regime corresponds to T >> µ, which was the case in the early Universe where the temperature was hundreds of MeV but the net baryon number density was very low. Secondly, it is theorized that quark deconfinement occurs also at low temperatures but very high chemical potential, T << µ, that is, at conditions which exist in the inner cores of (proto-) neutron stars [8]. Portions of the phase diagram lying between these two extreme physical regimes can be probed with relativistic collision experiments.Effective field-theoretical models such as the Nambu-Jona-Lasinio model and its extensions [9-13] as well as lattice QCD (LQCD) calculations [14-16] predict a smooth crossover of nuclear matter to quark matter in the low density but high temperature regime of the phase diagram. On the other hand, in the low temperature but high chemical potential regime the hadron-quark phase transition is likely be of first-order [17]. Some recent works [18,...

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In this short communication we present a simple internal mechanism that accounts for the recently observed anti-glitch in magnetar 1E 2259+586.We propose that the decay of an internal toroidal magnetic field component would de-stabilize an originally stable prolate star configuration. Then, the subsequent rearrangement of the stellar structure would give rise to a "more spherical" configuration, resulting in a sudden spin-down of the whole star.We present here some order of magnitude calculation to give confidence to this scenario, using the simplest analytical stellar model and let more detailed calculations for a more technical future paper.