Family dispute: do Type IIP supernova siblings agree on their distance?
Content overview (TL;DR)
Are Type II supernova-based distances only precise or accurate as well? Are they internally consistent? Why are we even curious? In the era of the Hubble tension, small error bars are not enough. Those uncertainties have to be meaningful as well. When implementing new distance estimator methods, one must consider these questions before applying the new method.
Type II supernovae are versatile distance indicators when one applies the so-called Expanding Photosphere Method (EPM) to them. This method relates the angular diameter of the supernova, as measured through photometry, with the expansion velocity of the ejecta. This analysis was shown to result in distances that are precise to a few per cent. However, given the difficulties in the radiative transfer modelling, it is interesting to question whether the EPM modelling yields the same distance if the supernova is replaced by one with a different set of physical parameters.
The simplest way to test this question leads through sibling supernovae, or transients that exploded in the same galaxy. This property sets an extremely good constraint on their distance; whatever their physical parameters may be, the EPM analysis of the two siblings should yield the same distance since the objects reside in the same galaxy. This way, we can test the internal consistency of our method, an important test for Type II supernova cosmology.
To perform this test, we looked through the literature for such siblings; when all conditions were considered, we found that four suitable pairs exist.