Hierarchically modelling NGC 3147's trio of Type Ia supernova siblings: SNe 2021hpr, 1997bq and 2008fv

Metro Toronto Convention Centre 

We introduce the 'relative intrinsic scatter' parameter, σRel, to model Type Ia supernovae that exploded in the same host galaxy: 'SN siblings'. We define σRel as the dispersion of individual siblings distance estimates relative to one another, and show that marginalising over σRel is a robust and inexpensive way of combining these distances. We proceed to fit a newly trained BayeSN model to new Young Supernova Experiment grizy photometry of SN 2021hpr, together with photometry of its siblings in NGC 3147: SNe 1997bq and 2008fv. By hierarchically fitting these light curves simultaneously, we improve the estimates of distance and dust parameters, as compared to individual fits to each SN. Moreover, just as σRel affects the distance uncertainty, we find the dust parameter posteriors are also affected (in the opposite sense, with larger σRel values leading to larger dust parameter uncertainties). Applying our methods, we constrain a common dust law shape parameter: RV=2.62±0.67, and the Hubble constant: H0=78.4±6.5 km/s/Mpc. We conclude that σRel-marginalisation is important to robustly combine siblings distances for cosmology, and for investigating siblings-host correlations.