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

¹ HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Observatory, Konkoly Th. M. út 15-17, 1121 Budapest, Hungary
² CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³ Department of Experimental Physics, Institute of Physics, University of Szeged, Dóm tér 9, 6720 Szeged, Hungary
⁴ MTA-ELTE Lendület “Momentum” Milky Way Research Group, Szent Imre H. st. 112, 9700 Szombathely, Hungary
⁵ Baja Astronomical Observatory of University of Szeged, Szegedi út, Kt. 766, 6500 Baja, Hungary
⁶ HUN-REN–SZTE Stellar Astrophysics Research Group, Szegedi út, Kt. 766, 6500 Baja, Hungary
⁷ Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055, USA
⁸ Kavli Institute for Theoretical Physics, University of California, Santa Barbara, 552 University Road, Goleta, CA 93106-4030, USA
⁹ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102 Goleta, CA 93117, USA
¹⁰ Department of Physics, University of California, Santa Barbara, Broida Hall, Mail Code 9530, Santa Barbara, CA 93106-9530, USA
¹¹ University of Delaware, 210 S College Ave, Newark, DE 19716, USA
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400 Austin, Texas 78712-1205, USA
¹⁴ NASA’s Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹⁵ Adler Planetarium, 1300 S. DuSable Lake Shore Drive, Chicago, IL 60605, USA
¹⁶ UC Davis, One Shields Avenue, Davis, CA 95616, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 November 2025
Accepted: 18 March 2026

Abstract

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.