MNRAS 000,1–19 (2021) Preprint 24 August 2021 Compiled using MNRAS LATEX style file v3.0 A puzzle solved after two decades: SN 2002gh among the brightest of superluminous supernovae Régis Cartier,1¢ Mario Hamuy,2,3 Carlos Contreras,4 Joseph P. Anderson,5 Mark M. Phillips,4 Nidia Morrell,4 Maximilian D. Stritzinger,6 Emilio D. Hueichapan,1 Alejandro Clocchiatti,7,8 Miguel Roth,4 Joanna Thomas-Osip,9 and Luis E. González10 1Cerro Tololo Inter-American Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile 2VicePresident and Head of Mission of AURA-O in Chile, Avda. Presidente Riesco 5335 Suite 507, Santiago, Chile 3Hagler Institute for Advanced Studies, Texas A&M University, Texas, USA 4Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile 5European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile 6Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark 7Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile 8Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile 9Gemini Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile 10Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile 24 August 2021 ABSTRACT We present optical photometry and spectroscopy of the superluminous SN 2002gh from maximum light to ¸202 days, obtained as part of the Carnegie Type II Supernova (CATS) project. SN 2002gh is among the most luminous discovered supernovae ever, yet it remained unnoticed for nearly two decades. Using Dark Energy Camera archival images we identify the potential SN host galaxy as a faint dwarf galaxy, presumably having low metallicity, and in an apparent merging process with other nearby dwarf galaxies. We show that SN 2002gh is among the brightest hydrogen-poor SLSNe with "+ = −22.40 ± 0.02, with an estimated peak bolometric luminosity of 2.6 ± 0.2 × 1044 erg s−1. We discount the decay of radioactive nickel as the main SN power mechanism, and assuming that the SN is powered by the spin down of a magnetar we obtain two alternative solutions. The first case, is characterized by significant magnetar power leakage, and "ej between 0.8 and 1.6 " , %spin = 3.4 ms, and 퐵 = 5 × 1013 G. The second case does not require power leakage, resulting in a huge ejecta mass of about 30 " , a fast spin 14 period of %spin ∼ 1 ms, and 퐵 ∼ 1.6 × 10 G. We estimate a zero-age main-sequence mass between 16 and 19 " for the first case and of about 135 " for the second case. The latter case would place the SN progenitor among the most massive stars observed to explode as a SN. Key words: supernovae: general — supernovae: individual (SN 2002gh) 1 INTRODUCTION divided into subtypes based on the presence of hydrogen in their spectra as hydrogen-rich (Type II) and hydrogren-poor (Type I) (see The first half of the 21BC century will be remembered as a revolution- arXiv:2108.09828v1 [astro-ph.HE] 18 Aug 2021 Moriya et al. 2018; Gal-Yam 2019a). Some SLSNe-II show rela- ary epoch for time-domain astronomy. Thanks to large area and untar- tively narrow hydrogen emission lines on top of a broad component, geted transient surveys, astronomers are exploring the time-domain characteristic of SNe Type IIn (see Schlegel 1990), and evidence of Universe to an unprecedented level, discovering transient events that interaction between the energetic SN ejecta and a massive hydrogen- span a wide range of timescales and luminosities. One of the most rich circum-stellar medium, such as SN 2006gy (Smith et al. 2007; spectacular such transients are the rare class of supernovae (SNe) Ofek et al. 2007). These SLSNe-II are usually considered the bright characterized by their extremely bright peak luminosities (' −21 end of Type IIn SNe, although it is unclear whether an additional mag at optical wavelengths) powered by a non-standard source of power source other than the strong ejecta-CSM interaction and ra- energy, that have challenged our understanding of massive star evo- dioactive decay significantly contributes to their extreme luminosities lution and explosion. (see e.g., Woosley et al. 2007; Jerkstrand et al. 2020). The idea of These rare and bright SNe are commonly referred to as superlu- an additional power source is reinforced by some SLSNe-II that do minous SNe (SLSNe). Like their lower luminosity cousins they are not show clear signatures of strong ejecta-CSM interaction (Gezari et al. 2009; Miller et al. 2009; Inserra et al. 2018; Dessart 2018). A potential power source is the energy injection from the spin down ¢ [email protected] © 2021 The Authors 2 Cartier et al. of a fast rotating neutron star with an extremely powerful magnetic ter for Transients (MOSFiT; Guillochon et al. 2018). They applied field, a magnetar, as explained below. Other notable events of the their magnetar model to a large sample of hydrogen-poor SLSNe SLSN class, are a handful of objects initially classified as hydrogen- collected from the literature and showed that such magnetar model poor SLSNe, but several days after maximum light reveal hydrogen can explain the light curve evolution of many objects of this class. features and signatures of ejecta-CSM interaction (Yan et al. 2015, Another alternative mechanism to power SLSN-I luminosity, is the 2017), suggesting that some hydrogen-poor SLSNe lose their hydro- SN ejecta-CSM interaction with a hydrogen and helium free CSM. gen envelopes shortly before their final explosion. For complete recent reviews on the SLSNe properties and their power Objects such as SN 1999as (Knop et al. 1999), SN 2005ap sources see Gal-Yam(2019a) and Moriya et al.(2018). (Quimby et al. 2007), SCP 06F6 (Barbary et al. 2009), and SN 2007bi Recently, the light curves of the hydrogen-poor SLSNe discovered (Gal-Yam et al. 2009) were the first SLSN-I reported, several years in the course of the Palomar Transient Factory (PTF; De Cia et al. before they were distinguished as a separate SN class. Their bright 2018), the Pan-STARRS1 Medium Deep Survey (PS1; Lunnan et al. peak luminosities together with the identification of a common set 2018) and the Dark Energy Survey (DES; Angus et al. 2019) were of spectral features made it possible to distinguish them as members presented and analysed. These surveys show that the low-luminosity of a completely new SN class (Quimby et al. 2011). These SLSNe tail of the SLSN population extends and overlaps in luminosity with are characterized by their extreme peak luminosities, very blue and the normal stripped envelope core collapse SN (CCSN) population. nearly featureless optical spectral emission, the lack of hydrogen and Although there is a continuous distribution in brightness from SNe helium lines and the presence of O ii and C ii absorption lines before Ic, Ic-BL to SLSNe-I, and rare examples of non-SLSNe powered by and close to maximum light (Quimby et al. 2011). A few weeks af- a magnetar exist (e.g., Maeda et al. 2007; Grayling et al. 2021), the ter maximum light, when the ejecta are cool enough, SLSN-I spectra dominant power mechanism seems to be different in the different SN become similar to SNe Ic (e.g., Pastorello et al. 2010) or to broad line populations. The well-sampled multi-band light curves provided by SNe Ic (SNe Ic-BL; Liu et al. 2017) at an early phase. Their nebular these surveys, confirmed the large diversity in the morphology of spectra also show some similarity with the nebular spectra of SNe SLSN light curves, many of them showing bumps in their light curve Ic-BL (Milisavljevic et al. 2013; Jerkstrand et al. 2017; Nicholl et al. evolution (see e.g., Nicholl et al. 2015; Smith et al. 2016; Yan et al. 2016b, 2019). 2017). One of the first models proposed to account for the large luminosity The host galaxies of hydrogen-poor SLSNe are characterized by 9 displayed by SLSNe was the radioactive decay of several solar masses being low mass dwarf galaxies ("stellar < 2 × 10 M ; e.g., Neill of 56Ni (Quimby et al. 2007; Gal-Yam et al. 2009), synthesized in et al. 2011; Lunnan et al. 2014; Leloudas et al. 2015; Angus et al. a Pair Instability SN (PISN) (Barkat et al. 1967; Rakavy & Shaviv 2016; Perley et al. 2016; Schulze et al. 2018), usually of irregular 1967). PISNe are the theoretical explosions of low metallicity and morphology with roughly half of the galaxies exhibiting a morphol- very massive main-sequence stars (∼ 140−260 M ), ejecting several ogy that is either asymmetric, off-centre or consisting of multiple tens of solar masses of 56Ni (Heger & Woosley 2002). These objects peaks (Lunnan et al. 2015), the latter being a possible signature of are expected to be common in the early Universe. The large amount merging systems. SLSN host galaxies are clearly different from the of radioactive material ejected by PISNe can potentially power the galaxies hosting normal core-collapse SNe (CCSNe), which are more extreme luminosities observed in SLSNe, however, some hydrogen- massive and in general exhibit spiral structure. SLSN-I host galaxies poor SLSNe fade too fast after peak to be consistent with the 56Ni ! are also characterized by low metallicities (/ ≤ 0.5/ ; e.g., Lunnan 56Co ! 56Fe decay chain, arguing against this power mechanism. et al. 2014; Leloudas et al. 2015; Perley et al. 2016; Schulze et al. Also the large opacity of iron-peak elements is expected to shift 2018), and by high specific Star Formation Rates (sSFR ∼ 10−9 yr−1; the emission towards near-infrared wavelengths (NIR), which is in e.g., Neill et al.