SN 2016iog: A fast-declining Type II-L supernova with an ultra-faint tail persistently interacting with circumstellar material

¹ School of Electronic Science and Engineering, Chongqing University of Posts and Telecommunications Chongqing 400065, P.R. China
² INAF – Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 5 35122 Padova, Italy
³ Yunnan Observatories, Chinese Academy of Sciences Kunming 650216, P.R. China
⁴ International Centre of Supernovae, Yunnan Key Laboratory Kunming 650216, P.R. China
⁵ INAF – Osservatorio Astronomico di Brera Via E. Bianchi 46 23807 Merate (LC), Italy
⁶ INAF – Osservatorio Astronomico d’Abruzzo Via Mentore Maggini I-64100 Teramo, Italy
⁷ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
⁸ School of Astronomy and Space Science, University of Chinese Academy of Sciences Beijing 100049, P.R. China
⁹ National Astronomical Observatories, Chinese Academy of Sciences Beijing 100101, P.R. China
¹⁰ European Southern Observatory (ESO) Alonso de Córdova 3107 Vitacura Santiago, Chile
¹¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n E-08193 Barcelona, Spain
¹² CISAS – Centro di Ateneo di Studi e Attività Spaziali ”Giuseppe Colombo”, Università degli Studi di Padova Via Venezia 1 35131 Padova, Italy
¹³ Fabra Observatory, Royal Academy of Sciences and Arts of Barcelona (RACAB) 08001 Barcelona, Spain
¹⁴ Institute for Space Studies of Catalonia (IEEC), Campus UPC 08860 Castelldefels (Barcelona), Spain
¹⁵ INAF – Osservatorio Astronomico di Cagliari Via della Scienza 5 09047 Selargius (CA), Italy
¹⁶ Laboratory of Instrumentation and Experimental Particle Physics Av. Prof. Gama Pinto 2 – 1649-003 Lisboa, Portugal
¹⁷ Department of Physics and Astronomy, University of Padova Via F. Marzolo 8 I-35131 Padova, Italy
¹⁸ Universitat Politècnica de Catalunya, Departament de Física c/ Esteve Terrades 5 08860 Castelldefels, Spain
¹⁹ Adler Planetarium 1300 S. DuSable Lake Shore Drive Chicago IL 60605, USA
²⁰ Physics Department, Tsinghua University Beijing 100084, P.R. China
²¹ School of Physics and Electrical Engineering, Liupanshui Normal University, Liupanshui Guizhou 553004, P.R. China
²² Purple Mountain Observatory, Chinese Academy of Sciences Nanjing 210023, P.R. China

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Received: 9 July 2025
Accepted: 16 November 2025

Abstract

We present optical photometric and spectroscopic observations of the rapidly declining Type IIL supernova (SN) 2016iog. SN 2016iog reached its peak ∼14 days after explosion, with an absolute magnitude in the V band of −18.64 ± 0.15 mag, followed by a steep decline of 8.85 ± 0.15 mag (100 d)⁻¹ post-peak. Such a high decline rate makes SN 2016iog one of the fastest-declining Type IIL SNe observed to date. The rapid rise in the light curve, combined with the nearly featureless continuum observed in the spectrum at +9.3 days, suggests the presence of interaction. In the recombination phase, we observed broad Hα lines that persist at all epochs. In addition, the prominent double-peaked Hα feature observed in the late-time spectrum (+190.8 days) is likely attributable either to significant dust formation within a cool dense shell or to asymmetric circumstellar material. These features suggest the presence of a sustained interaction around SN 2016iog. We propose that the observed characteristics of SN 2016iog can be qualitatively explained by assuming a low-mass H-rich envelope surrounding a red supergiant progenitor star with low-density circumstellar material.