The fast rise of the unusual type IIL/IIb SN 2018ivc

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
³ Instituto de Astrofísica, Universidad Andres Bello, Fernandez Concha 700, Las Condes, 8320000 Santiago RM, Chile
⁴ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, 8320000 Santiago RM, Chile
⁵ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁶ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁷ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, and Graduate Institute for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
⁹ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
¹⁰ Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Finland
¹¹ Instituto de Astrofísica de La Plata (IALP), CCT-CONICET-UNLP, Paseo del Bosque S/N, B1900FWA La Plata, Argentina
¹² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, B1900FWA La Plata, Argentina
¹³ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
¹⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, 8320000 Santiago, Chile
¹⁵ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹⁶ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
¹⁷ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
¹⁸ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
¹⁹ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
²⁰ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, PR China
²¹ Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
²² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
²³ INAF – Osservatorio Astronomioco di Roma, Via Frascati 33, I-00078 Monte Porzio Catone (RM), Italy
²⁴ Università degli Studi di Catania, Dip. di Fisica e Astronomia “Ettore Majorana”, Via S. Sofia 64, I-95123 Catania, Italy
²⁵ INAF - Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania, Italy
²⁶ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain

^⋆ Corresponding author; andrea.reguitti@inaf.it

Received: 20 May 2024
Accepted: 25 September 2024

Abstract

We present an analysis of the photometric and spectroscopic dataset of the type II supernova (SN) 2018ivc in the nearby (10 Mpc) galaxy Messier 77. Thanks to our high-cadence data, we observed the SN rising very rapidly by nearly three magnitudes in five hours (or 18 mag d⁻¹). The r-band light curve presents four distinct phases: the maximum light, which was reached in just one day, followed by a first, rapid linear decline and a short-duration plateau. Finally, the long, slower linear decline lasted for one year. Thanks to the ensuing radio re-brightening, we were able to detect SN 2018ivc four years after the explosion. The early spectra show a blue, nearly featureless continuum, but the spectra go on to evolve rapidly; after about ten days, a prominent Hα line starts to emerge, characterised by a peculiar profile. However, the spectra are heavily contaminated by emission lines from the host galaxy. The He I lines, namely λλ5876,7065, are also strong. In addition, strong absorption from the Na I doublet is evident and indicative of a non-negligible internal reddening. From its equivalent width, we derived a lower limit on the host reddening of A_V ≃ 1.5 mag. From the Balmer decrement and a match of the B − V colour curve of SN 2018ivc to that of the comparison objects, we obtained a host reddening of A_V ≃ 3.0 mag. The spectra are similar to those of SNe II, but with strong He lines. Given the peculiar light curve and spectral features, we suggest SN 2018ivc could be a transitional object between the type IIL and type IIb SNe classes. In addition, we found signs of an interaction with the circum-stellar medium (CSM) in the light curve, also making SN 2018ivc an interacting event. Finally, we modelled the early multi-band light curves and photospheric velocity of SN 2018ivc to estimate the physical parameters of the explosion and CSM.