Distinguishing prompt-collapse binary neutron star mergers from binary black holes: Tidal effects and remnant properties

We study the properties of remnants formed in prompt-collapse binary neutron star mergers. We consider nonspinning neutron star binaries over a range of total masses and mass ratios across a set of 22 equations of state, totaling 107 numerical relativity simulations. We report the final mass and spin of the systems (including the accretion disk and ejecta) to be constrained in a narrow range— <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>0.98</a:mn> <a:mo>≲</a:mo> <a:msub> <a:mi>M</a:mi> <a:mi>f</a:mi> </a:msub> <a:mo>/</a:mo> <a:mi>M</a:mi> <a:mo>≲</a:mo> <a:mn>0.99</a:mn> </a:math> for the mass and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mn>0.85</c:mn> <c:mo>≲</c:mo> <c:msub> <c:mi>a</c:mi> <c:mi>f</c:mi> </c:msub> <c:mo>≲</c:mo> <c:mn>0.95</c:mn> </c:math> for the dimensionless spin—regardless of the binary configuration and matter effects. This sets them apart from binary black hole merger remnants. We assess the detectability of the postmerger signal in a future 40 km Cosmic Explorer observatory and find that the signal-to-noise ratio in the postmerger of an optimally located and oriented binary at a distance of 100 Mpc can range from <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mo>&lt;</e:mo> <e:mn>1</e:mn> </e:math> to 8, depending on the binary configuration and equation of state, with a majority of them greater than 4 in the set of simulations that we consider. We also consider the distinguishability between prompt-collapse binary neutron star and binary black hole mergers with the same masses and spins. We find that Cosmic Explorer will be able to distinguish such systems primarily via the measurement of tidal effects in the late inspiral. Neutron star binaries with <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mover accent="true"> <g:mi mathvariant="normal">Λ</g:mi> <g:mo stretchy="false">˜</g:mo> </g:mover> </g:math> as small as <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mo>∼</l:mo> <l:mn>3.5</l:mn> </l:math> can be identified up to a distance of 100 Mpc, while neutron star binaries with <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mover accent="true"> <n:mi mathvariant="normal">Λ</n:mi> <n:mo stretchy="false">˜</n:mo> </n:mover> <n:mo>∼</n:mo> <n:mn>22</n:mn> </n:math> can be identified to distances greater than 250 Mpc. This is larger than the distance up to which the postmerger will be visible. Finally, we discuss the possible implications of our findings for the equation of state of neutron stars from the gravitational wave event GW230529.