ZTF SN Ia DR2: Cosmology-independent constraints on Type Ia supernova standardisation from supernova siblings

Understanding Type Ia supernovae (SNe Ia) and the empirical standardisation relations that make them excellent distance indicators is vital to improving cosmological constraints. SN Ia ‘siblings, i.e. two or more SNe Ia in the same host or parent galaxy, offer a unique way to infer the standardisation relations and their scatter across the population. We analysed a sample of 25 SN Ia pairs observed homogeneously by the Zwicky Transient Facility (ZTF) to infer the SNe Ia light curve width-luminosity and colour-luminosity parameters, α and β. Using the pairwise constraints from siblings, which allow for a scatter in the standardisation relations, we found α = 0.218 ± 0.055 and β = 3.084 ± 0.312, respectively, with a dispersion in α and β of ≤0.195 and ≤0.923, respectively, at a 95% confidence level. While the median dispersion is large, the values within ∼1σ are consistent with no dispersion. Hence, fitting for a single global standardisation relation, we found α = 0.228 ± 0.029 and β = 3.160 ± 0.191. We also found a very small intrinsic scatter of the siblings sample σint ≤ 0.10 mag at a 95% confidence level compared to σint = 0.22 ± 0.04 mag when computing the scatter using the Hubble residuals without comparing them as siblings. When comparing to large samples used in cosmological measurements, we found an α that is ∼2-3 σ higher, while the β values are consistent. The high α is driven by low x1 pairs, potentially suggesting that the slow and fast declining SN Ia have different slopes for the width-luminosity relation. We found no difference in α and β when dividing the sample by host galaxy mass. The finding of a higher α with increased statistics can be confirmed or refuted through upcoming time-domain surveys. If confirmed, this finding can improve the cosmological inference from SNe Ia and be used to infer properties of the progenitors for subpopulations of SNe Ia.