Equation of state effects in black hole–neutron star mergers

Abstract

Abstract. The merger dynamics of a black hole-neutron star (BHNS) binary is influenced by the neutron star equation of state (EoS) through the latter's effect on the neutron star's radius and on the character of the mass transfer onto the black hole. We study these effects by simulating a number of BHNS binaries in full general relativity using a mixed pseudospectral/finite difference code. We consider several models of the neutron star matter EoS, including Γ = 2 and Γ = 2.75 polytropes and the nuclear-theory based Shen EoS. For models using the Shen EoS, we consider two limits for the evolution of the composition: source-free advection and instantaneous β-equilibrium. To focus on EoS effects, we fix the mass ratio to 3:1 and the initial aligned black hole spin to a/m = 0.5 for all models. We confirm earlier studies which found that more compact stars create a stronger gravitational wave signal but a smaller postmerger accretion disk. We also vary the EoS while holding the compaction fixed. All mergers are qualitatively similar, but we find signatures of the EoS in the waveform and in the tail and disk structures.

Black-hole–neutron-star mergers at realistic mass ratios: Equation of state and spin orientation effects

“…Within the disk, we still have s $ 4-5. As the disk settles down over $10-20 ms, we would expect the entropy to exhibit a minimum at the peak of the surface density distribution, as was observed in lower mass ratio systems [22,26].…”

“…orbit N cycles eðt ¼ 0Þ [22], while the tidal deformability $ k 2 R 5 NS determines tidal effects during the inspiral [33] (k 2 being the tidal Love number of the neutron star). With respect to these parameters, the configurations considered here are within the range currently allowed for real neutron stars.…”

“…The quantities k hk, defined in Eq. (22), are the detectability criteria for optimally oriented events at 100 Mpc. We consider first the results for the numerical waveform limited to f > 0:8 kHz (NR only), then for the first-order PN expansion [Eq.…”

“…Smoothed particle hydrodynamics results predict ejected masses of up to $0:1M [21]. General relativistic simulations for low mass, low spin black holes find little ejected material (< 0:01M ) [22], but estimates for rapidly spinning black holes have not been offered yet. We show in this paper that more massive outflows (0:01M -0:05M ) are likely for high black-hole spins.…”

Enforcing causality in nonrelativistic equations of state at finite temperature

“…Large-scale computer simulations [6][7][8][9][10][11][12][13][14][15][16][17] of the astrophysical phenomena mentioned above employing the microscopic physics input of model EOS's from both nonrelativistic and relativistic approaches have indicated the ranges of n/n s , where n s 0.16 fm −3 is the nuclear saturation density, T , and the net electron fraction Y e = n e /n encountered. To enable simulations, EOS's that range over n/n s up to 10, T up to 200 MeV, and Y e up to 0.6 are required.…”