Eruptive mass loss less than a year before the explosion of superluminous supernovae

I. The cases of SN 2020xga and SN 2022xgc

¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden
² Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
³ Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁵ The Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, SE-106 91 Stockholm, Sweden
⁶ Nordita, Stockholm University and KTH Royal Institute of Technology, Hannes Alfvéns väg 12, 106 91 Stockholm, Sweden
⁷ Konkoly Observatory, Research Center for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly Th. M. út 15-17, H-1121 Budapest, Hungary
⁸ Department of Experimental Physics, Institute of Physics, University of Szeged, Dóm tér 9, Szeged 6720, Hungary
⁹ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁰ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
¹¹ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile
¹² Center for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, 5270 Ajdovščina, Slovenia
¹³ Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan
¹⁴ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
¹⁵ School of Physics, University College Dublin, L.M.I. Main Building, Beech Hill Road, Dublin 4 D04 P7W1, Ireland
¹⁶ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹⁸ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, (Barcelona), Spain
¹⁹ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl St, 76100 Rehovot, Israel
²⁰ GRANTECAN, Cuesta de San José s/n, 38712 Breña Baja, La Palma, Spain
²¹ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
²² Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
²³ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
²⁴ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
²⁵ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
²⁶ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139-4301, USA
²⁷ Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
²⁸ LPNHE, CNRS/IN2P3, Sorbonne Université, Université Paris-Cité, Laboratoire de Physique Nucléaire et de Hautes Énergies, 75005 Paris, France
²⁹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³⁰ Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN, UK
³¹ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
³² Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II, Via Cinthia 9, 80126 Naples, Italy
³³ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Naples, Italy
³⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
³⁵ Astrophysics sub-Department, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author; annagji1996@gmail.com

Received: 24 September 2024
Accepted: 19 December 2024

Abstract

We present photometric and spectroscopic observations of SN 2020xga and SN 2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) at z = 0.4296 and z = 0.3103, respectively, which show an additional set of broad Mg II absorption lines, blueshifted by a few thousands kilometer second⁻¹ with respect to the host galaxy absorption system. Previous work interpreted this as due to resonance line scattering of the SLSN continuum by rapidly expanding circumstellar material (CSM) expelled shortly before the explosion. The peak rest-frame g-band magnitude of SN 2020xga is −22.30 ± 0.04 mag and of SN 2022xgc is −21.97 ± 0.05 mag, placing them among the brightest SLSNe-I. We used high-quality spectra from ultraviolet to near-infrared wavelengths to model the Mg II line profiles and infer the properties of the CSM shells. We find that the CSM shell of SN 2020xga resides at ∼1.3 × 10¹⁶ cm, moving with a maximum velocity of 4275 km s⁻¹, and the shell of SN 2022xgc is located at ∼0.8 × 10¹⁶ cm, reaching up to 4400 km s⁻¹. These shells were expelled ∼11 and ∼5 months before the explosions of SN 2020xga and SN 2022xgc, respectively, possibly as a result of luminous-blue-variable-like eruptions or pulsational pair instability (PPI) mass loss. We also analyzed optical photometric data and modeled the light curves, considering powering from the magnetar spin-down mechanism. The results support very energetic magnetars, approaching the mass-shedding limit, powering these SNe with ejecta masses of ∼7 − 9 M_⊙. The ejecta masses inferred from the magnetar modeling are not consistent with the PPI scenario pointing toward stars > 50 M_⊙ He-core; hence, alternative scenarios such as fallback accretion and CSM interaction are discussed. Modeling the spectral energy distribution of the host galaxy of SN 2020xga reveals a host mass of 10^7.8 M_⊙, a star formation rate of 0.96_−0.26^+0.47 M_⊙ yr⁻¹, and a metallicity of ∼0.2 Z_⊙.