Distinguishing Kilonovae from Binary Neutron Star and Neutron Star-Black Hole Mergers

Kilonovae from compact binary mergers are most informative when accompanied by a gravitational-wave signal, which can help identify the source as a binary neutron star (BNS) or a neutron star-black hole (NSBH) merger. However, future events will also be discovered serendipitously or through follow-up of other transients, without a confident identification of the progenitor. Hence, we ask whether the kilonova light curve alone can distinguish between these two progenitor channels. Using simulated BNS and NSBH populations together with semi-analytic light curve models, we compare their post-peak evolution across the optical $ugrizy$ bands. The strongest contrast appears in the blue $u$ band 2 days after peak and in the redder $i$ band 10 days after peak. In the $u$ band, typical BNS kilonovae decline by only $\sim 1$ mag within 2 days of peak, whereas NSBH kilonovae typically decline by $\gtrsim 3$ mag over the same interval. In the $i$ band, the trend reverses for most of the population, with NSBH kilonovae evolving more slowly than BNS kilonovae. We attribute this behavior to differences in ejecta mass, opacity, and diffusion timescale between the two merger classes. Although the quantitative overlap is model-dependent, the qualitative distinction persists across model variations, identifying post-peak decline to be a viable diagnostic for inferring whether the source was a BNS or an NSBH merger.