Abstract
Abstract We present nimbus: a hierarchical Bayesian framework to infer the intrinsic luminosity parameters of kilonovae (KNe) associated with gravitational-wave (GW) events, based purely on nondetections. This framework makes use of GW 3D distance information and electromagnetic upper limits from multiple surveys for multiple events and self-consistently accounts for the finite sky coverage and probability of astrophysical origin. The framework is agnostic to the brightness evolution assumed and can account for multiple electromagnetic passbands simultaneously. Our analyses highlight the importance of accounting for model selection effects, especially in the context of nondetections. We show our methodology using a simple, two-parameter linear brightness model, taking the follow-up of GW190425 with the Zwicky Transient Facility as a single-event test case for two different prior choices of model parameters: (i) uniform/uninformative priors and (ii) astrophysical priors based on surrogate models of Monte Carlo radiative-transfer simulations of KNe. We present results under the assumption that the KN is within the searched region to demonstrate functionality and the importance of prior choice. Our results show consistency with simsurvey—an astronomical survey simulation tool used previously in the literature to constrain the population of KNe. While our results based on uniform priors strongly constrain the parameter space, those based on astrophysical priors are largely uninformative, highlighting the need for deeper constraints. Future studies with multiple events having electromagnetic follow-up from multiple surveys should make it possible to constrain the KN population further.
Highlights
Mergers of neutron stars and neutron star-black hole binaries (BNS and NSBH) present unique opportunities to probe multi-messenger astrophysics (e.g., Metzger 2019)
We show our methodology using a simple, two-parameter linear brightness model, taking the follow-up of GW190425 with the Zwicky Transient Facility (ZTF) as a single-event test case for two different prior choices of model parameters – (i) uniform/uninformative priors and (ii) astrophysical priors based on surrogate models of Monte Carlo radiative transfer simulations of KNe
While our results based on uniform priors strongly constrain the parameter space, those based on astrophysical priors are largely uninformative, highlighting the need for deeper constraints
Summary
Mergers of neutron stars and neutron star-black hole binaries (BNS and NSBH) present unique opportunities to probe multi-messenger astrophysics (e.g., Metzger 2019). Significant amounts of neutron-star (NS) matter are ejected at subrelativistic speeds due to either tidal or hydrodynamical forces; the radioactive decay of r-process elements synthesized in the neutron-rich merger ejecta powers a thermal ultraviolet, optical and near infrared transient, often referred to as a kilonova (KN) (Li & Paczynski 1998; Rosswog 2005; Metzger et al 2010; Tanaka & Hotokezaka 2013) Despite their color- and luminosityevolution being viewing-angle dependent (Kasen et al 2015; Bulla 2019; Kawaguchi et al 2020; Korobkin et al 2020; Zhu et al 2021), their (largely) isotropic emission makes KNe one of the promising targets for EM counterpart follow-up observations (Roberts et al 2011). These observations have highlighted the ability to test models of KNe and provide constraints on the ejecta mass and velocity (Abbott et al 2017a; Cowperthwaite et al 2017; Perego et al 2017; Pian et al 2017; Smartt et al 2017; Tanaka et al 2017; Waxman et al 2018; Coughlin et al 2019a; Kawaguchi et al 2020; Heinzel et al 2021; Raaijmakers et al 2021), r-process elemental abundances (Cote et al 2018; Hotokezaka et al 2018; Radice et al 2018; Tanaka et al 2018; Hotokezaka & Nakar 2020; Siegel 2019), the NS equation of state (Foucart et al 2018; Coughlin et al 2018a; Radice & Dai 2019; Hinderer et al 2019; Breschi et al 2021; Nicholl et al 2021) and Hubble Constant (Hotokezaka et al 2019; Dietrich et al 2020; Dhawan et al 2020)
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