Rotational properties of hypermassive neutron stars from binary mergers (original) (raw)
Related papers
Astronomy & Astrophysics, 2017
Aims. We explore the implications of a strong first-order phase transition region in the dense matter equation of state in the interiors of rotating neutron stars, and the resulting creation of two disjoint families of neutron-star configurations (the so-called high-mass twins). Methods. We numerically obtained rotating, axisymmetric, and stationary stellar configurations in the framework of general relativity, and studied their global parameters and stability. Results. The instability induced by the equation of state divides stable neutron star configurations into two disjoint families: neutron stars (second family) and hybrid stars (third family), with an overlapping region in mass, the high-mass twin-star region. These two regions are divided by an instability strip. Its existence has interesting astrophysical consequences for rotating neutron stars. We note that it provides a natural explanation for the rotational frequency cutoff in the observed distribution of neutron star spins, and for the apparent lack of back-bending in pulsar timing. It also straightforwardly enables a substantial energy release in a mini-collapse to another neutron-star configuration (core quake), or to a black hole.
A New View on the Maximum Mass of Differentially Rotating Neutron Stars
The Astrophysical Journal
We study the main astrophysical properties of differentially rotating neutron stars described as stationary and axisymmetric configurations of a moderately stiff Γ = 2 polytropic fluid. The high level of accuracy and of stability of our relativistic multidomain pseudo-spectral code enables us to explore the whole solution space for broad ranges of the degree of differential rotation, but also of the stellar density and oblateness. Staying within an astrophysicaly motivated range of rotation profiles, we investigate the characteristics of neutron stars with maximal mass for all types of families of differentially rotating relativistic objects identified in a previous article (Ansorg et al. 2009). We find that the maximum mass depends on both the degree of differential rotation and on the type of solution. It turns out that the maximum allowed mass can be up to 4 times higher than what it is for non-rotating stars with the same equation of state. Such values are obtained for a modest degree of differential rotation but for one of the newly discovered type of solutions. Since such configurations of stars are not that extreme, this result may have important consequences for the gravitational wave signal to expect from coalescing neutron star binaries or from some supernovae events.
Properties of hypermassive neutron stars formed in mergers of spinning binaries
Physical Review D, 2015
We present numerical simulations of binary neutron star mergers, comparing irrotational binaries to binaries of NSs rotating aligned to the orbital angular momentum. For the first time, we study spinning BNSs employing nuclear physics equations of state, namely the ones of Lattimer and Swesty as well as Shen, Horowitz, and Teige. We study mainly equal mass systems leading to a hypermassive neutron star (HMNS), and analyze in detail its structure and dynamics. In order to exclude gauge artifacts, we introduce a novel coordinate system used for post-processing. The results for our equal mass models show that the strong radial oscillations of the HMNS modulate the instantaneous frequency of the gravitational wave (GW) signal to an extend that leads to separate peaks in the corresponding Fourier spectrum. In particular, the high frequency peaks which are often attributed to combination frequencies can also be caused by the modulation of the m = 2 mode frequency in the merger phase. As a consequence for GW data analysis, the offset of the high frequency peak does not necessarily carry information about the radial oscillation frequency. Further, the low frequency peak in our simulations is dominated by the contribution of the plunge and the first 1-2 bounces. The amplitude of the radial oscillations depends on the initial NS spin, which therefore has a complicated influence on the spectrum. Another important result is that HMNSs can consist of a slowly rotating core with an extended, massive envelope rotating close to Keplerian velocity, contrary to the common notion that a rapidly rotating core is necessary to prevent a prompt collapse. Finally, our estimates on the amount of unbound matter show a dependency on the initial NS spin, explained by the influence of the latter on the amplitude of radial oscillations, which in turn cause shock waves.
Numerical Models of Irrotational Binary Neutron Stars in General Relativity
Physical Review Letters, 1999
We report on general relativistic calculations of quasiequilibrium configurations of binary neutron stars in circular orbits with zero vorticity. These configurations are expected to represent realistic situations as opposed to corotating configurations. The Einstein equations are solved under the assumption of a conformally flat spatial 3-metric (Wilson-Mathews approximation). The velocity field inside the stars is computed by solving an elliptical equation for the velocity scalar potential. Results are presented for sequences of constant baryon number (evolutionary sequences). Although the central density decreases much less with the binary separation than in the corotating case, it still decreases. Thus, no tendency is found for the stars to individually collapse to black hole prior to merger.
arXiv: High Energy Astrophysical Phenomena, 2016
We explore the implications of a strong first-order phase transition region in the dense matter equation of state in the interiors of rotating neutron stars, and the resulting creation of two disjoint families of neutron-stars' configurations (the so-called high-mass twins). Rotating, axisymmetric and stationary stellar configurations are obtained numerically in the framework of general relativity, and their global parameters and stability are studied. The equation of state-induced instability divides stable neutron star configurations into two disjoint families: neutron stars (second family) and hybrid stars (third family), with an overlapping region in mass, the high-mass twin star region. These two regions are divided by an instability strip. Its existence has interesting astrophysical consequences for rotating neutron stars. We note that it provides a "natural" explanation for the rotational frequency cutoff in the observed distribution of neutron stars spins, and ...
Differentially-rotating neutron star models with a parametrized rotation profile
2011
We analyze the impact of the choice rotation law on equilibrium sequences of relativistic differentially-rotating neutron stars in axisymmetry. The maximum allowed mass for each model is strongly affected by the distribution of angular velocity along the radial direction and by the consequent degree of differential rotation. In order to study the wide parameter space implied by the choice of rotation law, we introduce a functional form that generalizes the so called "j-const. law" adopted in all previous work. Using this new rotation law we reproduce the angular velocity profile of differentially-rotating remnants from the coalescence of binary neutron stars in various 3-dimensional dynamical simulations. We compute equilibrium sequences of differentially rotating stars with a polytropic equation of state starting from the spherically symmetric static case. By analyzing the sequences at constant ratio, T/|W|, of rotational kinetic energy to gravitational binding energy, we find that the parameters that best describe the binary neutron star remnants cannot produce equilibrium configurations with values of T/|W| that exceed 0.14, the criterion for the onset of the secular instability.
Fast Rotating Neutron Stars with Realistic Nuclear Matter Equation of State
We construct equilibrium configurations of uniformly rotating neutron stars for selected relativis-tic mean-field nuclear matter equations of state (EOS). We compute in particular the gravitational mass (M), equatorial (Req) and polar (R pol) radii, eccentricity, angular momentum (J), moment of inertia (I) and quadrupole moment (M2) of neutron stars stable against mass-shedding and secular axisymmetric instability. By constructing the constant frequency sequence f = 716 Hz of the fastest observed pulsar, PSR J1748–2446ad, and constraining it to be within the stability region, we obtain a lower mass bound for the pulsar, Mmin = [1.2–1.4] M⊙, for the EOS employed. Moreover we give a fitting formula relating the baryonic mass (M b) and gravitational mass of non-rotating neutron stars, M b /M⊙ = M/M⊙ + (13/200)(M/M⊙) 2 [or M/M⊙ = M b /M⊙ − (1/20)(M b /M⊙) 2 ], which is independent on the EOS. We also obtain a fitting formula, although not EOS independent, relating the gravitational mass and the angular momentum of neutron stars along the secular axisymmetric instability line for each EOS. We compute the maximum value of the dimensionless angular momentum , a/M ≡ cJ/(GM 2) (or " Kerr parameter "), (a/M)max ≈ 0.7, found to be also independent on the EOS. We compare and contrast then the quadrupole moment of rotating neutron stars with the one predicted by the Kerr exterior solution for the same values of mass and angular momentum. Finally we show that, although the mass quadrupole moment of realistic neutron stars never reaches the Kerr value, the latter is closely approached from above at the maximum mass value, as physically expected from the no-hair theorem. In particular the stiffer the EOS is, the closer the Kerr solution is approached.
Modeling differential rotations of compact stars in equilibriums
Physical Review D, 2017
Outcomes of numerical relativity simulations of massive core collapses or binary neutron star mergers with moderate masses suggest formations of rapidly and differentially rotating neutron stars. Subsequent fall back accretion may also amplify the degree of differential rotation. We propose new formulations for modeling the differential rotation of those compact stars, and present selected solutions of differentially rotating, stationary, and axisymmetric compact stars in equilibrium. For the cases when rotating stars reach break-up velocities, the maximum masses of such rotating models are obtained.
Fast Rotation of Neutron Stars
The Astrophysical Journal, 1996
We show that for realistic equations of state of dense matter, the universal proportionality factor relating the maximum rotation rate of neutron stars due to mass-shedding limit to the mass and radius of maximum allowable mass configuration of non-rotating models results from a universal proportionality between masses and radii of static maximum-mass neutron stars and those of maximally rotating configurations. These empirical relations cannot be obtained in the slow rotation approximation.
Axially-symmetric Neutron stars: Implication of rapid rotation
arXiv (Cornell University), 2009
Models of relativistic rotating neutron star composed of hyperon rich matter is constructed in the framework of an effective field theory in the mean-field approach. The gross properties of compact star is calculated at both static and the mass-shedding limit in the axially symmetric basis. The effect of appearance and abundance of hyperons on equation of state of dense matter and stellar properties is lineated with particular emphasis on the underlying nuclear interactions. We find that the models can explain fast rotations, which supports the existence of millisecond pulsars. An important offshoot of the present investigation is that, irrespective of the model parameters and interaction taken, the star seems to sustain faster rotations (an increase in rotational frequency up to ≈ 50%) without any further deformation.