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

We argue that the kilonova/macronova associated with the gravitational-wave event GW190425 could have been bright enough to be detected if it was caused by the merger of a low-mass black hole (BH) and a neutron star (NS). Although tidal disruption occurs for such a low-mass BH is generally expected, the masses of the dynamical ejecta are limited to ≲10−3 M⊙, which is consistent with previous work in the literature. The remnant disk could be as massive as 0.05–0.1 M⊙, and the disk outflow of ∼0.01–0.03 M⊙ is likely to be driven by viscous or magnetohydrodynamic effects. The disk outflow may not be neutron-rich enough to synthesize an abundance of lanthanide elements, even in the absence of strong neutrino emitter, if the ejection is driven on the viscous timescale of ≳0.3 s. If this is the case, the opacity of the disk outflow is kept moderate, and a kilonova/macronova at the distance of GW190425 reaches a detectable brightness of 20–21 mag at 1 day after merger for most viewing angles. If some disk activity ejects the mass within ∼0.1 s, instead, lanthanide-rich outflows would be launched and the detection of emission becomes challenging. Future possible detections of kilonovae/macronovae from GW190425-like systems will disfavor the prompt collapse of binary NSs and a non-disruptive low-mass BH–NS binary associated with a small NS radius, whose mass ejection is negligible. The host-galaxy distance will constrain the viewing angle and deliver further information about the mass ejection.