Relative intrinsic scatter in hierarchical Type Ia supernova siblings analyses: Application to SNe 2021hpr, 1997bq & 2008fv in NGC 3147

Understand

We present Young Supernova Experiment $grizy$ photometry of SN 2021hpr, the third Type Ia supernova sibling to explode in the Cepheid calibrator galaxy, NGC 3147.

• Siblings are useful for improving SN-host distance estimates, and investigating the contributions towards the SN Ia intrinsic scatter (post-standardisation residual scatter in distance estimates).
• We thus develop a principled Bayesian framework for analyzing SN Ia siblings.
• At its core is the cosmology-independent relative intrinsic scatter parameter, $\sigma_{Rel}$: the dispersion of siblings distance estimates relative to one another within a galaxy.