Filling the Mass Gap: How Kilonova Observations Can Unveil the Nature of the Compact Object Merging with the Neutron Star

Abstract

In this letter we focus on the peculiar case of a coalescing compact-object binary whose chirp mass is compatible both with a neutron star-neutron star and black hole-neutron star system, with the black hole in the ∼ 3 − 5 M range defined as the mass gap". Some models of core-collapse supernovae predict the formation of such low-mass black holes and a recent observation seems to confirm their existence. Here we show that the nature of the companion to the neutron star can be inferred from the properties of the kilonova emission once we know the chirp mass, which is the best constrained parameter inferred from the gravitational signal in low-latency searches. In particular, we find that the kilonova in the black hole-neutron star case is far more luminous than in the neutron star-neutron star case, even when the black hole is non spinning. The difference in the kilonovae brightness arises primarily from the mass ejected during the merger. Indeed, in the considered interval of chirp masses, the mass ejection in double neutron star mergers is at its worst as the system promptly forms a black hole. Instead mass ejection for black hole-neutron star case is at its best as the neutron stars have low mass/large deformability. The kilonovae from black hole-neutron star systems can differ by two to three magnitudes. The outcome is only marginally dependent on the equation of state. The difference is above the systematics in the modeling.

Optical follow-up of the neutron star–black hole mergers S200105ae and S200115j

“…The best limiting magnitudes correspond to absolute magnitudes −16 mag for both events, with typical observations ranging from M∼ −16.5 mag to M∼ −17.5 mag. AT2017gfo 12 , the optical counterpart to GW170817, peaked at M∼ −16 mag, and KNe from NSBH models are typically brighter than those from BNSs [13][14][15] , indicating that our observations are in the magnitude range required for detection.…”

“…Several works in the literature [13][14][15] have shown that KNe from NSBH mergers are generally brighter than those resulting from BNS mergers. A similar behaviour is found in NSBH and BNS models computed here and in Ref.…”

Neutron star mergers and how to study them

“…The predictions for the inferred ejecta mass, average velocity, and electron fraction differs for NSBH mergers and BNS mergers. Delineation between the progenitors and remnants will have to rely on combinations of ejecta mass, velocities, kilonova color, and multimessenger determination of inclination (Barbieri et al 2019(Barbieri et al , 2020. A self-consistent picture with GW-determined masses SGRB and kilonova observations will strengthen such claims.…”

On the Possibility of GW190425 Being a Black Hole–Neutron Star Binary Merger

“…If this heavy component is a low-mass black hole, it challenges existence of the mass gap between black holes and neutron stars (Kreidberg et al 2012;Özel et al 2012). Thus, the determination of the binary type is essential to obtain robust astrophysical knowledge (see Barbieri et al 2019, for related work in this direction).…”