LIGO Document T2000392-v3

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

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T - Technical notes
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In their third observing run (O3), LIGO and Virgo detected gravitational-wave (GW) candidates
from several neutron star-black hole (NSBH) and binary neutron star (BNS) mergers. The Zwicky
Transient Facility (ZTF), an optical time-domain survey telescope, followed-up thirteen 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 luminosities and evolution rates are improbable. We factor color evolution into
the decay rates, important as observations of GW170817 have shown that photometric behavior of a
KN differs in different bands. Using our band-specific priors, more realistic constraints are placed on
KN source properties. Finally, we close with an application of this methodology to derive constraints
on KN ejecta masses.

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