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 less than or similar to 10(-3)M(circle dot), which is consistent with previous work in the literature. The remnant disk could be as massive as 0.05-0.1M(circle dot), and the disk outflow of similar to 0.01-0.03M(circle dot) 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 greater than or similar to 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 similar to 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.