Constraining the Properties of Kilonovae based on the Zwicky Transient Facility Searches for 13 Neutron Star Mergers.

In their third observing run (O3), LIGO and Virgo detected gravitational-wave (GW) candidates from several neutron star-black hole and binary neutron star mergers. The Zwicky Transient Facility (ZTF), an optical time-domain survey telescope, followed-up 13 of these GW events in search of kilonovae (KNe; electromagnetic counterparts to GW events). However, no KNe were found. To assess the implications on potential KN emission based on the upper limits, empirical limits on the KN peak magnitude and evolution rate were determined. One shortcoming of these analyses was the assumption that all peak magnitudes and evolution rates are equally likely. In this work, we present a method to improve upon this assumption by comparing to light curves generated using radiative-transport based KN models, parameterized by ejecta mass and inclination angle. Specifically, we construct priors informed by these KN models to identify regions of parameter space where particular KNe luminosities and evolution rates are improbable. We also factor color evolution into the decay rates, important as observations of GW170817 have shown that photometric behavior of a KN differs in different bands. Finally, we close with an application of this methodology to derive constraints on KN ejecta masses.