Combining the GW observations of merging systems of binary neutron stars and quasi-universal relations, we set constraints on the maximum mass that can be attained by nonrotating stellar models of neutron stars. More specifically, exploiting the recent observation of the GW event GW170817 and drawing from basic arguments on kilonova modeling of GRB 170817A together with the quasi-universal relation between the maximum mass of nonrotating stellar models and the maximum mass supported through uniform rotation , we set limits for the maximum mass to be , where the lower limit in this range comes from pulsar observations. Our estimate, which follows a very simple line of arguments and does not rely on the modeling of the electromagnetic signal in terms of numerical simulations, can be further refined as new detections become available. We briefly discuss the impact that our conclusions have on the equation of state of nuclear matter.

We explore in a parameterized manner a very large range of physically plausible equations of state (EOSs) for compact stars for matter that is either purely hadronic or that exhibits a phase transition. In particular, we produce two classes of EOSs with and without phase transitions, each containing one million EOSs. We then impose constraints on the maximum mass ($M<2.16M_{\odot }$) and on the dimensionless tidal deformability ($\tilde{\mathrm{\Lambda }}<800$) deduced from GW170817, together with recent suggestions of lower limits on $\tilde{\mathrm{\Lambda }}$. Exploiting more than $10^9$ equilibrium models for each class of EOSs, we produce distribution functions of all the stellar properties and determine, among other quantities, the radius that is statistically most probable for any value of the stellar mass. In this way, we deduce that the radius of a purely hadronic neutron star with a representative mass of $1.4M_{\odot }$ is constrained to be $12.00<R_{1.4}/\mathrm{km}<13.45$ at a $2\sigma$ confidence level, with a most likely value of ${\overline{R}}_{1.4}=12.39\mathrm{km}$; similarly, the smallest dimensionless tidal deformability is ${\tilde{\mathrm{\Lambda }}}_{1.4}>375$, again at a $2\sigma$ level. On the other hand, because EOSs with a phase transition allow for very compact stars on the so-called “twin-star” branch, small radii are possible with such EOSs although not probable, i.e., $8.53<R_{1.4}/\mathrm{km}<13.74$ and ${\overline{R}}_{1.4}=13.06\mathrm{km}$ at a $2\sigma$ level, with ${\tilde{\mathrm{\Lambda }}}_{1.4}>35.5$ at a $3\sigma$ level. Finally, since these EOSs exhibit upper limits on $\tilde{\mathrm{\Lambda }}$, the detection of a binary with a total mass of $3.4M_{\odot }$ and ${\tilde{\mathrm{\Lambda }}}_{1.7}>461$ can rule out twin-star solutions.

Merging binaries of neutron-stars are not only strong sources of gravitational waves, but also have the potential of revealing states of matter at densities and temperatures not accessible in laboratories. A crucial and long-standing question in this context is whether quarks are deconfined as a result of the dramatic increase in density and temperature following the merger. We present the first fully general-relativistic simulations of merging neutron-stars including quarks at finite temperatures that can be switched off consistently in the equation of state. Within our approach, we can determine clearly what signatures a quark-hadron phase transition would leave in the gravitational-wave signal. We show that if after the merger the conditions are met for a phase transition to take place at several times nuclear saturation density, they would lead to a postmerger signal considerably different from the one expected from the inspiral, that can only probe the hadronic part of the equations of state, and to an anticipated collapse of the merged object. We also show that the phase transition leads to a very hot and dense quark core that, when it collapses to a black hole, produces a ringdown signal different from the hadronic one. Finally, in analogy with what is done in heavy-ion collisions, we use the evolution of the temperature and density in the merger remnant to illustrate the properties of the phase transition in a QCD phase diagram.

The recent detection of GW190814 featured the merger of a binary with a primary having a mass of ∼23 M_⊙ and a secondary with a mass of ∼2.6 M_⊙. While the primary was most likely a black hole, the secondary could be interpreted as either the lightest black hole or the most massive neutron star ever observed, but also as the indication of a novel class of exotic compact objects. We here argue that although the secondary in GW190814 is most likely a black hole at merger, it needs not be an ab-initio black hole nor an exotic object. Rather, based on our current understanding of the nuclear-matter equation of state, it can be a rapidly rotating neutron star that collapsed to a rotating black hole at some point before merger. Using universal relations connecting the masses and spins of uniformly rotating neutron stars, we estimate the spin, |$0.49_{-0.05}^{+0.08} \lesssim \chi \lesssim 0.68_{-0.05}^{+0.11}$|⁠, of the secondary – a quantity not constrained so far by the detection – and a novel strict lower bound on the maximum mass, |$M_{_{\mathrm{TOV}}}> 2.08^{+0.04}_{-0.04}\, \, M_{\odot }$| and an optimal bound of |$M_{_{\mathrm{TOV}}}> 2.15^{+0.04}_{-0.04}\, \, M_{\odot }$|⁠, of nonrotating neutron stars, consistent with recent observations of a very massive pulsar. The new lower bound also remains valid even in the less likely scenario in which the secondary neutron star never collapsed to a black hole.

Context. Worldwide very long baseline radio interferometry (VLBI) arrays are expected to obtain horizon-scale images of supermassive black hole candidates and of relativistic jets in several nearby active galactic nuclei. This, together with the expected detection of electromagnetic counterparts of gravitational-wave signals, motivates the development of models for magnetohydrodynamic flows in strong gravitational fields. Aims. The Black Hole Accretion Code (BHAC) is a publicliy available code intended to aid with the modeling of such sources by performing general relativistic magnetohydrodynamical simulations in arbitrary stationary spacetimes. New additions to the code are required in order to guarantee an accurate evolution of the magnetic field when small and large scales are captured simultaneously. Methods. We discuss the adaptive mesh refinement (AMR) techniques employed in BHAC, which are essential to keep several problems computationally tractable, as well as staggered-mesh-based constrained transport (CT) algorithms to preserve the divergence-free constraint of the magnetic field. We also present a general class of prolongation operators for face-allocated variables compatible with them. Results. After presenting several standard tests for the new implementation, we show that the choice of the divergence-control method can produce qualitative differences in the simulation results for scientifically relevant accretion problems. We demonstrate the ability of AMR to decrease the computational costs of black hole accretion simulations while sufficiently resolving turbulence arising from the magnetorotational instability. In particular, we describe a simulation of an accreting Kerr black hole in Cartesian coordinates using AMR to follow the propagation of a relativistic jet while self-consistently including the jet engine, a problem set up for which the new AMR implementation is particularly advantageous. Conclusions. The CT methods and AMR strategies discussed here are currently being used in the simulations performed with BHAC for the generation of theoretical models for the Event Horizon Telescope collaboration.

We investigate the impact of using high-order numerical methods to study the merger of magnetized neutron stars with finite-temperature microphysics and neutrino cooling in full general relativity. By implementing a fourth-order accurate conservative finite-difference scheme we model the inspiral together with the early post-merger and highlight the differences to traditional second-order approaches at the various stages of the simulation. We find that even for finite-temperature equations of state, convergence orders higher than second order can be achieved in the inspiral and post-merger for the gravitational-wave phase. We further demonstrate that the second-order scheme overestimates the amount of proton-rich shock-heated ejecta, which can have an impact on the modelling of the dynamical part of the kilonova emission. Finally, we show that already at low resolution the growth rate of the magnetic energy is consistently resolved by using a fourth-order scheme.