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PHYSICAL REVIEW D (7)

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Univ Arizona, Dept Astron (7)

Paschalidis, Vasileios (6)Univ Arizona, Dept Phys (5)Ruiz, Milton (3)Shapiro, Stuart L. (3)Espino, Pedro L. (2)Tsokaros, Antonios (2)Baumgarte, Thomas W. (1)Bozzola, Gabriele (1)Kazanas, Demosthenes (1)View MoreTypes
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Revisiting the maximum mass of differentially rotating neutron stars in general relativity with realistic equations of state

Espino, Pedro L.; Paschalidis, Vasileios (AMER PHYSICAL SOC, 2019-04-30)

We study the solution space of general relativistic, axisymmetric, equilibria of differentially rotating neutron stars with realistic, nuclear equations of state. We find that different types of stars, which were identified by earlier works for polytropic equations of state, arise for realistic equations of state, too. Scanning the solution space for the sample of realistic equations of state we treat, we find lower limits on the maximum rest masses supported by cold, differentially rotating stars for each type of stars. We often discover equilibrium configurations that can support more than 2 times the mass of a static star. We call these equilibria "overmassive," and in our survey we find overmassive stars that can support up to 2.5 times the maximum rest mass that can be supported by a cold, nonrotating star with the same equation of state. This is nearly 2 times larger than what previous studies employing realistic equations of state had found, and which did not uncover overmassive neutron stars. Moreover, we find that the increase in the maximum rest mass with respect to the nonspinning stellar counterpart is larger for moderately stiff equations of state. These results may have implications for the lifetime and the gravitational wave and electromagnetic counterparts of hypermassive neutron stars formed following binary neutron star mergers.

Effects of spin on magnetized binary neutron star mergers and jet launching

Ruiz, Milton; Tsokaros, Antonios; Paschalidis, Vasileios; Shapiro, Stuart L. (AMER PHYSICAL SOC, 2019-04-17)

Events GW170817 and GRB 170817A provide the best confirmation so far that compact binary mergers where at least one of the companions is a neutron star can be the progenitors of short gamma-ray bursts (sGRBs). An open question for GW170817 remains the values and impact of the initial neutron star spins. The initial spins could possibly affect the remnant black hole mass and spin, the remnant disk, and the formation and lifetime of a jet and its outgoing electromagnetic Poynting luminosity. Here we summarize our general relativistic magnetohydrodynamic simulations of spinning, neutron star binaries undergoing merger, and delayed collapse to a black hole. The binaries consist of two identical stars, modeled as Gamma = 2 polytropes, in quasicircular orbit, each with spins chi(Ns) = -0.053, 0, 0.24, or 0.36. The stars arc endowed initially with a dipolar magnetic field extending from the interior into the exterior, as in a radio pulsar. Following the merger, the redistribution of angular momentum by magnetic braking and magnetic turbulent viscosity in the hypermassive neutron star (HMNS) remnant, along with the loss of angular momentum due to gravitational radiation, induces the formation of a massive, nearly uniformly rotating inner core surrounded by a magnetized Keplerian disklike envelope. The HMNS eventually collapses to a black hole, with spin a/M-BH similar or equal to 0.78 independent of the initial spin of the neutron stars, surrounded by a magnetized accretion disk. The larger the initial neutron star spin the heavier the disk. After Delta t similar to 3000M - 4000M similar to 45(M-Ns /1.625 M-circle dot) ms - 60(M-Ns /1.625 M-circle dot) ms following merger, a mildly relativistic jet is launched. The lifetime of the jet [Delta t similar to 100(M-Ns 11.625 M-circle dot) ms - 140(M-Ns /1.625 M-circle dot) ms] and its outgoing Poynting luminosity [L-Em similar to 10(51.5 +/- 1)erg /s] are consistent with typical sGRBs, as well as with the Blandford-Znajek mechanism for launching jets and their associated Poynting luminosities.

Are fast radio bursts the most likely electromagnetic counterpart of neutron star mergers resulting in prompt collapse?

Paschalidis, Vasileios; Ruiz, Milton (AMER PHYSICAL SOC, 2019-08-01)

Inspiraling and merging binary neutron stars (BNSs) are important sources of both gravitational waves and coincident electromagnetic counterparts. If the BNS total mass is larger than a threshold value, a black hole ensues promptly after merger. Through a statistical study in conjunction with recent LIGO/Virgo constraints on the nuclear equation of state, we estimate that up to ∼25% of BNS mergers may result in prompt collapse. Moreover, we find that most models of the BNS mass function we study here predict that the majority of prompt-collapse BNS mergers have q≳0.8. Prompt-collapse BNS mergers with mass ratio q≳0.8 may not be accompanied by detectable kilonovae or short gamma-ray bursts, because they unbind a negligible amount of mass and form negligibly small accretion disks onto the remnant black hole. We call such BNS mergers “orphan.” However, recent studies have found that 1041–43(Bp/1012 G)2 erg s−1 electromagnetic signals can be powered by magnetospheric interactions several milliseconds prior to merger. Moreover, the energy stored in the magnetosphere of an orphan BNS merger remnant will be radiated away in O(1 ms). Through simulations in full general relativity of BNSs endowed with an initial dipole magnetosphere, we find that the energy in the magnetosphere following black hole formation is EB∼1039–41(Bp/1012 G)2 erg. Radiating ∼1% of EB in 1 ms, as has been found in previous studies, matches the premerger magnetospheric luminosity. These magnetospheric signals are not beamed, and their duration and power agrees with those of nonrepeating fast radio bursts (FRBs). These results combined with our statistical study suggest that a nonrepeating FRB may be the most likely electromagnetic counterpart of prompt-collapse BNSs. Detection of a nonrepeating FRB coincident with gravitational waves from a BNS merger could settle the extragalactic origin of a fraction FRBs and could be used to place constraints on the nuclear equation of state. FRBs can also initiate triggered searches for weak signals in the LIGO/Virgo data.

Testing viable f(R) models with the angular-diameter distance to compact quasar cores

Sultana, Joseph; Melia, Fulvio; Kazanas, Demosthenes (AMER PHYSICAL SOC, 2019-05-06)

We consider here some popular f(R) models generally viewed as possible alternatives to the existence of dark energy in General Relativity. For each of these, we compute the redshift zmax at which the angular diameter distance dA(z) is expected to reach its maximum value. This turning point in dA(z) was recently measured in a model-independent way using compact quasar cores and was found to occur at zmax=1.70±0.20. We compare the predictions of zmax for the f(R) models with this observed value to test their viability at a deeper level than has been attempted thus far, thereby quantifying an important observational difference between such modified gravity scenarios and standard Lambda Cold Dark Matter (ΛCDM) cosmology. Our results show that, while the most popular f(R) models today are consistent with this measurement to within 1σ, the turning point zmax will allow us to prioritize these alternative gravity theories as the measurement precision continues to improve, particularly with regard to how well they mitigate the tension between the predictions of ΛCDM and the observations. For example, while the Hu-Sawicki version of f(R) increases this tension, the Starobinky model reduces it.

Effect of spin on the inspiral of binary neutron stars

Tsokaros, Antonios; Ruiz, Milton; Paschalidis, Vasileios; Shapiro, Stuart L.; Uryū, Kōji (AMER PHYSICAL SOC, 2019-07-29)

We perform long-term simulations of spinning binary neutron stars, with our highest dimensionless spin being chi similar to 0.32. To assess the importance of spin during the inspiral, we vary the spin and also use two equations of state, one that consists of plain nuclear matter and produces compact stars (SLy) and a hybrid one that contains both nuclear and quark matter and leads to larger stars (ALF2). Using high resolution that has grid spacing Delta x similar to 98 m on the finest refinement level, we find that the effects of spin in the phase evolution of a binary system can be larger than the one that comes from tidal forces. Our calculations demonstrate explicitly that although tidal effects are dominant for small spins (less than or similar to 0.1), this is no longer true when the spins are larger, but still much smaller than the Keplerian limit.

Dynamical stability of quasitoroidal differentially rotating neutron stars

Espino, Pedro L.; Paschalidis, Vasileios; Baumgarte, Thomas W.; Shapiro, Stuart L. (AMER PHYSICAL SOC, 2019-08-16)

We investigate the dynamical stability of relativistic, differentially rotating, quasitoroidal models of neutron stars through hydrodynamical simulations in full general relativity. We find that all quasitoroidal configurations studied in this work are dynamically unstable against the growth of nonaxisymmetric modes. Both one-arm and bar mode instabilities grow during their evolution. We find that very high rest mass configurations collapse to form black holes. Our calculations suggest that configurations whose rest mass is less than the binary neutron star threshold mass for prompt collapse to black hole transition dynamically to spheroidal, differentially rotating stars that are dynamically stable, but secularly unstable. Our study shows that the existence of extreme quasitoroidal neutron star equilibrium solutions does not imply that long-lived binary neutron star merger remnants can be much more massive than previously found. Finally, we find models that are initially supra-Kerr (J/M-2 > 1) and undergo catastrophic collapse on a dynamical timescale, in contrast to what was found in earlier works. However, cosmic censorship is respected in all of our cases. Our work explicitly demonstrates that exceeding the Kerr bound in rotating neutron star models does not imply dynamical stability.

Initial data for general relativistic simulations of multiple electrically charged black holes with linear and angular momenta

Bozzola, Gabriele; Paschalidis, Vasileios (AMER PHYSICAL SOC, 2019-05-17)

A general relativistic, stationary, and axisymmetric black hole in a four-dimensional asymptotically flat spacetime is fully determined by its mass, angular momentum, and electric charge. The expectation that astrophysically relevant black holes do not posses charge has resulted in a limited number of investigations of moving and charged black holes in the dynamical, strong-field gravitational (and electromagnetic) regime, in which numerical studies are necessary. Apart from having a theoretical interest, the advent of multimessenger astronomy with gravitational waves offers new ways to think about charged black holes. In this work, we initiate an exploration of charged binary black holes by generating valid initial data for general relativistic simulations of black hole systems that have generic electric charge and linear and angular momenta. We develop our initial data formalism within the framework of the conformal transversetraceless (Bowen-York) technique using the puncture approach and apply the theory of isolated horizons to attribute physical parameters (mass, charge, and angular momentum) to each hole. We implemented our formalism in the case of a binary system by modifying the publicly available TWOPUNCTURES and QUASILOCALMEASURES codes. We demonstrate that our code can recover existing solutions and that it has excellent self-convergence properties for a generic configuration of two black holes.

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