A long-lasting eruption heralds SN 2023ldh, a clone of SN 2009ip

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
³ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini, I-64100 Teramo, Italy
⁴ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
⁵ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences Kunming 650216, PR China
⁶ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, PR China
⁷ Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, FI-20014 Turku, Finland
⁸ Department of Physics and Astronomy, University of California, 1 Shields Avenue, Davis, CA 95616-5270, USA
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
¹⁰ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
¹¹ Las Cumbres Observatory, 6740 Cortona Dr #102, Goleta, CA 93117, USA
¹² Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
¹³ Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
¹⁴ Science and Technology Department, Parthenope University of Naples, Centro Direzionale, Isola C4, I-80143 Naples, Italy
¹⁵ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹⁶ Istitut d’Estudis Espacials de Catalunya (IEEC), E-08860 Castelldefels (Barcelona), Spain
¹⁷ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, USA
¹⁸ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
¹⁹ Astrophysics Research Institute, Liverpool John Moores University, IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁰ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
²¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone, Italy
²² Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
²³ INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
²⁴ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
²⁵ INAF – IASF Milano, Via A. Corti 12, I-20133 Milano, Italy
²⁶ Technical University of Munich, TUM School of Natural Sciences, Physics Department, James-Franck-Str. 1, 85741 Garching, Germany
²⁷ Department of Physics, University of Oxford, Denys Wikinson Building, Kable Road, Oxford OX1 3RH, UK
²⁸ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
²⁹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³⁰ Adler Planetarium, 1300 S. DuSable Lake Shore Drive, Chicago, IL 60605, USA
³¹ School of Physics and Astronomy, Beijing Normal University, Beijing 100875, China
³² Department of Physics, Faculty of Arts and Sciences, Beijing Normal University, Zhuhai 519087, China
³³ Università degli Studi dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, Via Valeggio 11, I-22100 Como, Italy
³⁴ Itagaki Astronomical Observatory, Yamagata 990-2492, Japan
³⁵ Variable Star Observers League in Japan, 7-1 Kitahatsutomi, Kamagaya, Chiba 273-0126, Japan
³⁶ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
³⁷ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

^⋆ Corresponding author: andrea.pastorello@inaf.it

Received: 29 March 2025
Accepted: 4 July 2025

Abstract

We discuss the results of the spectroscopic and photometric monitoring of the type IIn supernova (SN) 2023ldh. Survey archive data show that the SN progenitor experienced erratic variability in the years before exploding. Beginning May 2023, the source showed a general slow luminosity rise that lasted for over four months, with some superposed luminosity fluctuations. In analogy to SN 2009ip, we call this brightening ‘Event A’. During Event A, SN 2023ldh reached a maximum absolute magnitude of M_r = −15.52 ± 0.24 mag. The light curves then decreased by about 1 mag in all filters for about two weeks reaching a relative minimum, which was followed by a steep brightening (Event B) to an absolute peak magnitude of M_r = −18.53 ± 0.23 mag, replicating the evolution of SN 2009ip and similar to that of type IIn SNe. The three spectra of SN 2023ldh obtained during Event A show multi-component P Cygni profiles of H I and Fe II lines. During the rise to the Event B peak, the spectrum shows a blue continuum dominated by Balmer lines in emission with Lorentzian profiles, with a full width at half maximum velocity of about 650 km s⁻¹. Later, in the post-peak phase, the spectrum reddens, and broader wings appear in the Hα line profile. Metal lines with P Cygni profiles and velocities of about 2000 km s⁻¹ are clearly visible. Beginning around three months past maximum and until very late phases, the Ca II lines become among the most prominent features, while Hα is dominated by an intermediate-width component with a boxy profile. Although SN 2023ldh mimics the evolution of other SN 2009ip-like transients, it is slightly more luminous and has a slower photometric evolution. The surprisingly homogeneous observational properties of SN 2009ip-like events may indicate similar explosion scenarios and similar progenitor parameters.