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 _ { \odot } ?
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 , C _ { max } = 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 distinguihable from a perturbed Kerr spacetime .
However , their inspiral leads to phase differences of the order of \sim 4 rads over an \sim 81 km separation ( 1.7 orbits ) while typical neutron stars exhibit phase differences of \geq 20 rads .
Although a difference of 4 rads can be measured by current gravitational wave laser interferometers ( e.g .
aLIGO/Virgo ) , uncertainties in the orbital parameters will likely prevent distinguishing such compact , massive neutron stars from black holes .