SN 2021hpr and its two siblings in the Cepheid calibrator galaxy NGC
3147: A hierarchical BayeSN analysis of a Type Ia supernova trio, and a
Hubble constant constraint
To improve Type Ia supernova (SN Ia) standardisability, the consistency of
distance estimates to siblings -- SNe in the same host galaxy -- should be
investigated. We present Young Supernova Experiment Pan-STARRS-1 grizy
photometry of SN 2021hpr, the third spectroscopically confirmed SN Ia in the
high-stellar-mass Cepheid-calibrator galaxy NGC 3147. We analyse NGC 3147's
trio of SN Ia siblings: SNe 1997bq, 2008fv and 2021hpr, using a new version of
the BayeSN model of SN Ia spectral-energy distributions, retrained
simultaneously using optical-NIR BgVrizYJH (0.35--1.8 μm) data. The
distance estimates to each sibling are consistent, with a sample standard
deviation ≲0.01 mag, much smaller than the total intrinsic scatter in
the training sample: σ0≈0.09 mag. Fitting normal SN Ia siblings
in three additional galaxies, we estimate a ≈90% probability that the
siblings' intrinsic scatter is smaller than σ0. We build a new
hierarchical model that fits light curves of siblings in a single galaxy
simultaneously; this yields more precise estimates of the common distance and
the dust parameters. Fitting the trio for a common dust law shape yields
RV=2.69±0.52. Our work motivates future hierarchical modelling of more
siblings, to tightly constrain their intrinsic scatter, and better understand
SN-host correlations. Finally, we estimate the Hubble constant, using a Cepheid
distance to NGC 3147, the siblings trio, and 109 Hubble flow (0.01<zCMB<0.08) SNe Ia; marginalising over the siblings' and population's
intrinsic scatters, and the peculiar velocity dispersion, yields
H0=77.9±6.5 km s−1Mpc−1.Comment: Submitted to MNRAS; 30 pages, 22 figure