Great Impostors: Extremely Compact, Merging Binary Neutron Stars in the Mass Gap Posing as Binary Black Holes

Abstract

Can one distinguish a binary black hole undergoing a merger from a binary neutron star if the individual compact companions have masses that fall inside the so-called mass gap of 3 − 5 M ? For neutron stars, achieving such masses typically requires extreme compactness and in this work we present initial data and evolutions of binary neutron stars initially in quasiequilibrium circular orbits having a compactness C = 0.336. These are the most compact, nonvacuum, quasiequilibrium binary objects that have been constructed and evolved to date, including boson stars. The compactness achieved is only slightly smaller than the maximum possible imposed by causality, Cmax = 0.355, which requires the sound speed to be less than the speed of light. By comparing the emitted gravitational waveforms from the late inspiral to merger and post-merger phases between such a binary neutron star vs. a binary black hole of the same total mass we identify concrete measurements that serve to distinguish them. With that level of compactness, the binary neutron stars exhibit no tidal disruption up until merger, whereupon a prompt collapse is initiated even before a common core forms. Within the accuracy of our simulations the black hole remnants from both binaries exhibit ringdown radiation that is not distinguishable from a perturbed Kerr spacetime. However, their inspiral leads to phase differences of the order of ∼ 5 rads over an ∼ 81 km separation (1.7 orbits) while typical neutron stars exhibit phase differences of ≥ 20 rads. Although a difference of ∼ 5 rads can be measured by current gravitational wave laser interferometers (e.g. aLIGO/Virgo), uncertainties in the individual masses and spins will likely prevent distinguishing such compact, massive neutron stars from black holes.

Distinguishing double neutron star from neutron star-black hole binary populations with gravitational wave observations

“…While it has long been known that mass measurements are not sufficient to distinguish between BNS and NSBH systems [36,37], our work differs from similar recent proposals. Measurements of the tidal deformabilities Λ 1 and Λ 2 of the individual binary components could be consistent with a NSBH system even for large-SNR signals and large tidal effects if at least one of the two tidal deformabilities is consistent with zero at the 50% confidence level [38], therefore it is hard to distinguish BNS from NSBH systems with GWs alone 1 if we assume that Λ 1 and Λ 2 are independent [39].…”

Locating ergostar models in parameter space

“…We also note here that the maximum mass of the spherical solutions, as well as the maximum mass at the mass-shedding limit, increase considerably from the SLy EOS (by factors of 1.97 and 2.12, respectively). This has already been seen with the ALF2cc EOS employed in [8,37]. Given the fact that the SLycc1 and ALF2cc EOSs only differ in the crust (i.e., for ρ 0 ≤ ρ 0nuc ), it is not surprising that the differences in the TOV and Kepler lines are minute.…”

“…Instead of a parabolic type, the profile with SLycc1 starts from a smaller central density, diminishes somewhat all the way to the surface of the star, where it abruptly reduces to zero (see Fig. 1 in [37]). In this respect, stars with the SLycc1 or ALF2cc EOS resemble quark stars that have a finite surface density.…”

Multimessenger Binary Mergers Containing Neutron Stars: Gravitational Waves, Jets, and $\boldsymbolγ$-Ray Bursts

“…This value, which is even higher than the maximum possible compactness that can be achieved by solitonic boson stars (Palenzuela et al 2017), is slightly smaller than the limiting compactness C max = 0.355 set by causality (Lattimer & Prakash 2016). To build these binaries, Tsokaros et al (2020c) employed the ALF2 EOS (Alford et al 2005), but replaced the region where the rest-mass density satisfies ρ 0 ≥ ρ 0,s = ρ 0,nuc = 2.7 × 10 14 gr/cm 3 by the maximally stiff EOS…”