2023A&A...672A.129C

Context. Type II supernovae offer a direct way of estimating distances via the expanding photosphere method, which is independent of the cosmic distance ladder. A Gaussian process-based method was recently introduced, allowing for a fast and precise modelling of spectral time series and placing accurate and computationally cheap Type II-based absolute distance determinations within reach.
Aims. The goal of this work is to assess the internal consistency of this new modelling technique coupled with the distance estimation in an empirical way, using the spectral time series of supernova siblings, that is, supernovae that exploded in the same host galaxy.
Methods. We used a recently developed spectral emulator code, trained on TARDIS radiative transfer models that is capable of a fast maximum-likelihood parameter estimation and spectral fitting. After calculating the relevant physical parameters of supernovae, we applied the expanding photosphere method to estimate their distances. Finally, we tested the consistency of the obtained values by applying the formalism of Bayes factors.
Results. The distances to four different host galaxies were estimated based on two supernovae in each. The distance estimates are not only consistent within the errors for each of the supernova sibling pairs, but in the case of two hosts, they are precise to better than 5%. The analysis also showed that the main limiting factor of this estimation is the number and quality of spectra available for the individual objects, rather than the physical differences of the siblings.
Conclusions. Even though the literature data we used was not tailored to the requirements of our analysis, the agreement of the final estimates shows that the method is robust and is capable of inferring both precise and consistent distances. By using high-quality spectral time series, this method can provide precise distance estimates independent of the distance ladder, which are of high value for cosmology.