Distance estimate to NGC 6951 from supernova siblings Type IIP SN 2020dpw and Type Ib SN 2021sjt

Context. Supernova (SN) siblings are powerful tools used to calibrate and improve distance measurement methods, and to make the systematic uncertainty to distances to their host galaxies considerably lower compared to other techniques. Aims. In this paper we present distance estimates to NGC6951, a galaxy that hosted the Type IIP SN 2020dpw, the Type Ib SN 2021sjt, and three other SNe. Methods. Photometric observations of the two objects were carried out using two 80cm Ritchey-Chretien telescopes located in Hungary, while spectra were obtained from the Las Cumbres Observatory (LCO) and the WiseRep database. After data reduction, the distances to the studied SNe were inferred. For the distance estimates, we applied the expanding photosphere method (EPM), which connects the observed angular radius ( θ ) of a SN to its physical radius and is related to the velocity of the photosphere ( v ph ). Although the EPM is mostly applied to derive the distance of Type IIP SNe, in the literature there are several examples of this technique being used for Type IIn and stripped-envelope SNe as well. Therefore, we made another attempt to infer the distance of the Type Ib SN 2021sjt by applying the EPM together with its Type IIP sibling SN 2020dpw. The θ values in different epochs for each studied supernova were estimated from photometric observations, while v ph was constrained by modeling the available spectra using SYN++. Results. Our analysis resulted in a distance of 25.76 ± 0.34(random)±5.51 (systematic) Mpc and 24.57 ± 1.27(random)±4.64 (systematic) Mpc for SN 2020dpw and SN 2021sjt, respectively. Systematic errors were estimated with respect to the used dilution factor, the interstellar reddening, and the date of the explosion (which was fixed to a value between the last non-detection and the first detection for each object). Conclusions. The obtained distance values agree with each other and with the literature, which shows the validity of the methods used. In this way, new and perhaps improved distance estimates to NGC 6951 were obtained, and the applicability of the EPM for Type Ib SNe was tested.