Simulation Based Inference of BNS Kilonova Properties: A Case Study with AT2017gfo

Kilonovae are a class of astronomical transients observed as counterparts to mergers of compact binary systems, such as a binary neutron star (BNS) or black hole-neutron star (BHNS) inspirals. They serve as probes for heavy-element nucleosynthesis in astrophysical environments, while together with gravitational wave emission constraining the distance to the merger itself, they can place constraints on the Hubble constant. Obtaining the physical parameters (e.g. ejecta mass, velocity, composition) of a kilonova from observations is a complex inverse problem, usually tackled by sampling-based inference methods such as Markov-chain Monte Carlo (MCMC) or nested sampling techniques. These methods often rely on computing approximate likelihoods, since a full simulation of compact object mergers involve expensive computations such as integrals, the calculation of likelihood of the observed data given parameters can become intractable, rendering the likelihood-based inference approaches inapplicable. We propose here to use Simulation-based Inference (SBI) techniques to infer the physical parameters of BNS kilonovae from their spectra, using simulations produced with KilonovaNet. Our model uses Sequential Neural Posterior Estimation (SNPE) together with an embedding neural network to accurately predict posterior distributions from simulated spectra. We further test our model with real observations from AT2017gfo, the only kilonova with multi-messenger data, and show that our estimates agree with previous likelihood-based approaches.