An improved standardization framework for Type II-P supernovae using a hierarchical Bayesian approach

The work uses Type II-P supernovae observed by the Nearby Supernova Factory (SNfactory) as standardizable candles, aiming to improve their cosmological standardization. The dataset consists of accurately flux-calibrated, host-subtracted spectrophotometry of low-redshift SNe II-P spanning 3200–10000 Å.

The authors develop an improved expansion-velocity measurement based on Gaussian Process regression, then extend the conventional standardized candle method (SCM) with an additional correction term based on the Hα absorption-to-emission ratio and a hierarchical Bayesian framework.

The added correction improves standardization quality, especially around 30–40 days after explosion. It reduces the scatter of Hubble-flow SNe II-P from 0.28 mag to 0.18 mag relative to previous work. As an application, they measure H0 using a pre-existing calibrator sample of SNe II with Cepheid- or TRGB-based distance moduli, obtaining H0 = 70.6^{+4.5}_{-4.3} km/s/Mpc including only statistical uncertainty, a precision of 6.2%. Hierarchical model variations shift H0 by 1.4 km/s/Mpc, well within the quoted statistical uncertainty. Without the extra correction term, they obtain H0 = 73.0^{+6.9}_{-6.1} km/s/Mpc (8.9% precision), highlighting the improved precision of the new methodology.

The authors also identify and discuss inconsistencies between the measured parent populations of the calibrator and Hubble-flow samples in their standardization characteristics—present in this and literature samples—that are not explained by a simple selection on magnitude. They conclude these will need to be improved upon as SN II-P samples grow.