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

Abstract

{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.} {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.} {Photometric observations of the two objects were carried out using two 80cm RC telescopes located in Hungary, while spectra were obtained from the LCO and the WiseRep database. 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 ($v_{\rm ph}$). 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. } {Our analysis resulted in a distance of $25.76 \pm 0.34 (\rm random) \pm 5.51$ (systematic) Mpc and $24.57 \pm 1.27 (\rm random) \pm 4.64$ (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).} {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.}

Figures (10)

• Figure 1: The position of supernova siblings SNe 1999el, 2000E, 2015G, 2020dpw, and 2021sjt in NGC 6951 (http://simbad.u-strasbg.fr).
• Figure 2: Photometric data of SN 2020dpw (left) and SN 2021sjt (right) from different sources (see Section \ref{['sec:obs']}). Filled dots denote to RC80 and BRC80 data, respectively, while empty triangles code the LCO data. The purple-shaded regions in each panel refer to the time interval, where the expanding photosphere method was applied (see its explanation in \ref{['sec:distances']}.
• Figure 3: The observed spectra of SN 2021sjt corrected for interstellar reddening and redshift (black lines) compared to the synthetic spectra of 2015MNRAS.453.2189D created for Type Ib SNe (blue lines). Each observed spectrum is compared to a synthetic spectrum with the same phase from the explosion.
• Figure 4: The continuum-normalized, redshift- and extinction-corrected spectrum of SN 2020dpw taken at 21d phase post-explosion (black) and its best-fit model obtained in SYN++ (red). The contribution of each identified ion are plotted with orange color, and shifted vertically for clarification.
• Figure 5: The 4d, 7d, 10d, 11d and 19d phase spectrum of SN 2021sjt (black) with their best-fit SYN++ models (blue). The single-ion contribution to the model spectra are plotted in a similar way as in Figure \ref{['fig:20dpw_spec']}.