SN 2021tsz: A luminous, short photospheric phase Type II supernova in a low-metallicity host

¹ Millennium Institute of Astrophysics (MAS), Nuncio Monsenor Sòtero Sanz 100, Providencia, Santiago RM, Chile
² Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
³ Aryabhatta Research Institute of Observational SciencES, Manora Peak, Nainital 263001, India
⁴ Department of Applied Physics, Mahatma Jyotiba Phule Rohilkhand University, Bareilly 243006, India
⁵ Las Cumbres Observatory, 6740 Cortona Drive Suite 102, Goleta, CA 93117-5575, USA
⁶ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
⁷ Indian Institute of Astrophysics, Koramangala 2nd Block, Bangalore 560034, India
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, (Barcelona), Spain
⁹ Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹⁰ Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹¹ Department of Physics, Royal Holloway – University of London, Egham TW20 0EX, UK
¹² Instituto de Astrofísica, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
¹³ Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
¹⁴ University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA
¹⁵ Johns Hopkins University, Bloomberg Center for Physics and Astronomy, San Martin Dr, Baltimore, MD 21210, USA
¹⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
¹⁷ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
¹⁸ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, Cambridge, USA
¹⁹ Adler Planetarium, 1300 S Dusable Lk Shr Dr, Chicago, IL 60605, USA

^⋆ Corresponding author: rdastidr@gmail.com

Received: 23 June 2025
Accepted: 3 October 2025

Abstract

We present the analysis of the luminous Type II Supernova (SN) 2021tsz, which exploded in a low-luminosity galaxy. It reached a peak magnitude of −18.88 ± 0.13 mag in the r band and exhibited an initial rapid decline of 4.05 ± 0.14 mag (100 d)⁻¹ from peak luminosity till ∼30 d. The photospheric phase is short, with the SN displaying bluer colours and a weak Hα absorption component–features consistent with other luminous, short-photospheric phase Type II SNe. A distinct transition from the photospheric to the radioactive tail phase in the V band–as is common in hydrogen-rich Type II SNe–is not visible in SN 2021tsz, although a modest ∼1 mag drop is apparent in the redder filters. Hydrodynamic modelling suggests the luminosity is powered by ejecta-circumstellar material (CSM) interaction during the early phases (< 30 days). Interaction with 0.6 M_⊙ of dense CSM extending to 3100 R_⊙ reproduces the observed luminosity, with an explosion energy of 1.3 × 10⁵¹ erg. The modelling indicates a pre-SN mass of 9 M_⊙, which includes a hydrogen envelope of 4 M_⊙, and a radius of ∼1000 R_⊙. Spectral energy distribution analysis and strong-line diagnostics revealed that the host galaxy of SN 2021tsz is a low-metallicity, dwarf galaxy. The low-metallicity environment and the derived high mass loss from the hydrodynamical modelling strongly support a binary progenitor system for SN 2021tsz.