GW170817 and GW190425 as Hybrid Stars of Dark and Nuclear Matters

We propose three scenarios for compact hybrid stars consisting of nuclear and dark matters which could possibly serve as alternative interpretations to the LIGO/Virgo events GW170817 and GW190425. To demonstrate our proposal, we adopt the SLy4 equation of state (EoS) for nuclear matter, and an EoS for a bosonic self-interacting dark matter (SIDM), which is simple and capable of yielding both reasonable halo density and compact stars. We study the mass-radius and tidal Love number (TLN)-mass relations for these compact hybrid stars, and also generalize the Bardeen-Thorne-Meltzer (BTM) criteria to discuss in details the possible saddle instability due to the nature of two-fluid model. Our results show that it is possible for our hybrid star scenarios to explain GW170817 and GW190425. Some of the hybrid stars can have compact neutron or mixed cores around 10km while possessing thick dark matter shells, which can then explain the astrophysical observations of neutron stars with compact photon radius and mass higher than 2 solar masses. Reversely, we also infer the dark matter model from the parameter estimation of GW190425. Our scenarios of compact hybrid stars can be further tested by the coming LIGO/Virgo O3 events.

Hybrid stars with sequential phase transitions: the emergence of the g$_2$ mode

Neutron stars are the densest objects in the Universe, with $M sim 1.4 M_{odot}$ and $R sim 12$ km, and the equation of state associated to their internal composition is still unknown. The extreme conditions to which matter is subjected inside neutron stars could lead to a phase transition in their inner cores, giving rise to a hybrid compact object. The observation of $2M_{odot}$ binary pulsars (PSR~J1614-2230, PSR~J0343$+$0432 and PSR~J0740$+$6620) strongly constraints theoretical models of the equation of state. Moreover, the detection of gravitational waves emitted during the binary neutron star merger, GW170817, and its electromagnetic counterpart, GRB170817A, impose additional constraints on the tidal deformability. In this work, we investigate hybrid stars with sequential phase transitions hadron-quark-quark in their cores. We assume that both phase transitions are sharp and analyse the rapid and slow phase conversion scenarios. For the outer core, we use modern hadronic equations of state. For the inner core we employ the constant speed of sound parametrization for quark matter. We analyze more than 3000 hybrid equations of state, taking into account the recent observational constraints from neutron stars. The effects of hadron-quark-quark phase transitions on the normal oscillation modes $f$ and $g$, are studied under the Cowling relativistic approximation. Our results show that, in the slow conversion regime, a second quark-quark phase transition gives rise to a new $g_2$~mode. We discuss the observational implications of our results associated to the gravitational waves detection and the possibility of detecting hints of sequential phase transitions and the associated $g_2$~mode.

Constraint on hybrid stars with gravitational wave events

Motivated by the recent discoveries of compact objects from LIGO/Virgo observations, we study the possibility of identifying some of these objects as compact stars made of dark matter called dark stars, or the mix of dark and nuclear matters called hybrid stars. In particular, in GW190814, a new compact object with 2.6 $M_{odot}$ is reported. This could be the lightest black hole, the heaviest neutron star, and a dark or hybrid star. In this work, we extend the discussion on the interpretations of the recent LIGO/Virgo events as hybrid stars made of various self-interacting dark matter (SIDM) in the isotropic limit. We pay particular attention to the saddle instability of the hybrid stars which will constrain the possible SIDM models.

Implications from GW170817 and I-Love-Q relations for relativistic hybrid stars

Gravitational wave observations of GW170817 placed bounds on the tidal deformabilities of compact stars allowing one to probe equations of state for matter at supranuclear densities. Here we design new parametrizations for hybrid hadron-quark equations of state, that give rise to low-mass twin stars, and test them against GW170817. We find that GW170817 is consistent with the coalescence of a binary hybrid star--neutron star. We also test and find that the I-Love-Q relations for hybrid stars in the third family agree with those for purely hadronic and quark stars within $sim 3%$ for both slowly and rapidly rotating configurations, implying that these relations can be used to perform equation-of-state independent tests of general relativity and to break degeneracies in gravitational waveforms for hybrid stars in the third family as well.

Effects of modified theories of gravity on neutrino pair annihilation energy deposition near neutron stars

We study the neutrino pairs annihilation into electron-positron pairs ($ u+{bar u}to e^- + e^+$) near the surface of a neutron star. The analysis is performed in the framework of extended theories of gravity. The latter induce a modification of the minimum photon-sphere radius ($R_{ph}$) and the maximum energy deposition rate near to $R_{ph}$, as compared to ones of General Relativity. These results might lead to an efficient mechanism for generating GRBs.