Family dispute: do Type IIP supernova siblings agree on their distance?

Abstract

Context: Type II supernovae provide a direct way to estimate distances through the expanding photosphere method, which is independent of the cosmic distance ladder. A recently introduced Gaussian process-based method allows for a fast and precise modelling of spectral time series, which puts accurate and computationally cheap Type II-based absolute distance determinations within reach. Aims: The goal of the paper is to assess the internal consistency of this new modelling technique coupled with the distance estimation empirically, using the spectral time series of supernova siblings, i.e. supernovae that exploded in the same host galaxy. Methods: We use a recently developed spectral emulator code, which is trained on \textsc{Tardis} radiative transfer models and is capable of a fast maximum likelihood parameter estimation and spectral fitting. After calculating the relevant physical parameters of supernovae we apply the expanding photosphere method to estimate their distances. Finally, we test 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\%. Conclusions: Even though the literature data we used was not tailored for 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.