Uniform Modeling of Observed Kilonovae: Implications for Diversity and the Progenitors of Merger-driven Long Gamma-Ray Bursts

Abstract We present uniform modeling of eight kilonovae, five following short gamma-ray bursts (GRBs; including GRB 170817A) and three following long GRBs. We model their broadband afterglows to determine the relative contributions of afterglow and kilonova emission. We fit the kilonovae using a three-component model in MOSFiT , and report population median ejecta masses for the total, blue ( κ B = 0.5 cm 2 g −1 ), purple ( κ P = 3 cm 2 g −1 ), and red ( κ R = 10 cm 2 g −1 ) components. The kilonova of GW170817 is near the sample median in most derived properties. We investigate trends between the ejecta masses and the isotropic-equivalent and beaming-corrected γ -ray energies ( E γ ,iso , E γ ), as well as rest-frame durations ( T 90,rest ). We find long GRB kilonovae have higher median red ejecta masses ( M ej,R ≳ 0.05 M ⊙ ) compared to on-axis short GRB kilonovae ( M ej,R ≲ 0.02 M ⊙ ). We also observe a weak scaling between the total and red ejecta masses with E γ ,iso and E γ , though a larger sample is needed to establish a significant correlation. These findings imply a connection between merger-driven long GRBs and larger tidal dynamical ejecta masses, which may indicate that their progenitors are asymmetric compact object binaries. We produce representative kilonova light curves, and find that the planned depths and cadences of the Rubin and Roman Observatory surveys will be sufficient for order-of-magnitude constraints on M ej,B (and, for Roman, M ej,P and M ej,R ) of future kilonovae at z ≲ 0.1.