Understanding the extreme luminosity of DES14X2fna

MNRAS 505, 3950–3967 (2021) https://doi.org/10.1093/mnras/stab1478 Advance Access publication 2021 May 24 Understanding the extreme luminosity of DES14X2fna M. Grayling ,1,2‹C. P. Guti´ errez ,1M. Sullivan ,1P. Wiseman ,1M. Vincenzi ,3 S. Gonz´ alez-Gait´ an ,4B. E. Tucker,5L. Galbany ,6L. Kelsey ,1C. Lidman ,5E. Swann,3 M. Smith ,1C. Frohmaier, D. Carollo,7K. Glazebrook,8G. F. Lewis ,9A. M¨ oller,10 S. R. Hinton ,11 S. A. Uddin,12 T. M. C. Abbott,13 M. Aguena ,14,15 S. Avila ,16 E. Bertin ,17,18 S. Bhargava,19 D. Brooks,20 A. Carnero Rosell ,21,22 M. Carrasco Kind ,23,24 J. Carretero ,25 M. Costanzi,26,27 L. N. da Costa,15,28 J. De Vicente ,29 S. Desai,30 H. T. Diehl,31 P. Doel,20 S. Everett,32 I. Ferrero ,33 P. Fosalba ,34,35 J. Frieman,31,36 J. Garc´ ıa-Bellido ,16 E. Gaztanaga ,34,35 D. Gruen ,37,38,39 R. A. Gruendl,23,24 J. Gschwend,15,28 G. Gutierrez ,31 B. Hoyle,40,41,42 K. Kuehn,43,44 N. Kuropatkin,31 M. Lima,14,15 N. MacCrann ,45,46 J. L. Marshall,47 P. Martini,45,48,49 R. Miquel,25,50 R. Morgan,51 A. Palmese ,31,36 F. Paz-Chinch´ on,24,52 A. A. Plazas ,53 A. K. Romer,19 C. S´ anchez ,54 E. Sanchez,29 V. Scarpine,31 S. Serrano,34,35 I. Sevilla-Noarbe,29 M. Soares-Santos ,55 E. Suchyta ,56 G. Tarle,55 D. Thomas ,3C. To ,37,38,39 T. N. Varga,41,42 A. R. Walker,13 and R. D. Wilkinson19 (DES Collaboration) Affiliations are listed at the end of the paper Accepted 2021 May 18. Received 2021 May 18; in original form 2020 September 22 ABSTRACT We present DES14X2fna, a high-luminosity, fast-declining Type IIb supernova (SN IIb) at redshift z=0.0453, detected by the Dark Energy Survey (DES). DES14X2fna is an unusual member of its class, with a light curve showing a broad, luminous peak reaching Mr≃−19.3 mag 20 d after explosion. This object does not show a linear decline tail in the light curve until ≃60 d after explosion, after which it declines very rapidly (4.30 ±0.10 mag 100 d−1in the rband). By fitting semi-analytic models to the photometry of DES14X2fna, we find that its light curve cannot be explained by a standard 56Ni decay model as this is unable to fit the peak and fast tail decline observed. Inclusion of either interaction with surrounding circumstellar material or a rapidly-rotating neutron star (magnetar) significantly increases the quality of the model fit. We also investigate the possibility for an object similar to DES14X2fna to act as a contaminant in photometric samples of SNe Ia for cosmology, finding that a similar simulated object is misclassified by a recurrent neural network (RNN)-based photometric classifier as an SN Ia in ∼1.1–2.4 per cent of cases in DES, depending on the probability threshold used for a positive classification. Key words: supernovae: general – supernovae: individual: DES14X2fna. 1 INTRODUCTION Core-collapse supernovae (SNe) are a diverse and heterogeneous populationofevents,withthevariety of observedsub-typesreflecting the complexity of their possible progenitor systems and astrophysics. Type II SNe (SNe II) are events displaying hydrogen lines in their photospheric spectra, SNe Ib lack hydrogen but do contain helium, while SNe Ic lack both. SNe IIb are an intermediate class, displaying hydrogen lines at early times before the appearance of helium lines as seen in SNe Ib (Filippenko 1997; Gal-Yam 2017; Modjaz, Guti´ errez & Arcavi 2019). The commonly accepted physical explanation for SNe IIb is that their progenitors have had their outer hydrogen envelope partially, but not fully, stripped away. SNe Ib have this envelope fully stripped, leading to spectra with helium but not hydrogen, while E-mail: [email protected] the progenitors of SNe Ic are stripped of both hydrogen and helium. A further class of stripped-envelope SNe, SN Ic with broad lines (SN Ic-BLs), shows similar spectroscopic features to SNe Ic, but with broader features indicating high expansion velocities and an energetic explosion. The exact mechanism driving the envelope stripping of these SNe is still open for debate, but proposed solutions include stellar winds (Woosley, Langer & Weaver 1993) and interaction with a binary companion (Nomoto, Iwamoto & Suzuki 1995) in the case of a binary progenitor system. Stellar winds require massive progenitors of 25–30 Min order to remove at least the majority of the hydrogen envelope (Eldridge & Tout 2004). Pre-explosion and latetime images of the SN environment of the well-studied SN IIb SN 1993J (Aldering, Humphreys & Richmond 1994; Fox et al. 2014) indicate the presence of a binary system, with evidence of a binary companion also found in SN 2001ig (Ryder et al. 2018)and SN 2011dh (Folatelli et al. 2014). However, deep imaging studies of the SN IIb remnant Cassiopeia A have not indicated a binary C 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 Extreme luminosity of DES14X2fna 3951 companion (Kochanek 2018; Kerzendorf et al. 2019), suggesting that both of these progenitor scenarios may occur. Inthe canonical picture of anSNIIb,the light curveisdrivenbythe radioactive decay chain of 56Ni synthesized in the explosion, which subsequently decays into 56Co and then stable 56Fe. Treatments of this radioactive decay model, for example, the commonly used ‘Arnett’ model from Arnett (1982) and more recently Khatami & Kasen (2019), allow for various properties of the explosion to be estimated. Some other types of core-collapse SNe are primarily driven by different physical processes (e.g. interaction with a surrounding circumstellar material (CSM) for SNe IIn; Moriya et al. 2013), although a 56Ni decay model can still be used to estimate some explosion properties (e.g. Prentice et al. 2016; Meza & Anderson 2020). For SNe with light curves driven by 56Ni decay such as SNe IIb, a more luminous SN indicates a higher synthesized mass of 56Ni to power the peak of the light curve. EnergeticSNeIc-BL,however,arenotwellfitbythismodel,which cannot reproduce both the luminous peaks and the late-time light curves of these objects. These objects have traditionally been fit with a two-component model, with the light-curve peak and broad spectral features powered by a fast-moving component and the exponential decline powered by a slower moving dense component (Maeda et al. 2003). However, more recently, magnetar models have also proved successful in fitting the light curves of these objects. In this scenario, the light curve is powered by a combination of radioactive decay and energyinjectedinto the system by a centralengine,thespin-downofa rapidlyrotating neutronstar(Kasen& Bildsten 2010;Woosley2010). Wang et al. (2017) show that a combination of a magnetar with 56Ni decay successfully fits the light curves of the SNe Ic-BL SN 1998bw and SN 2002ap, with the magnetar able to explain the deviation of the late-time light curve from the intermediate exponential decline. In addition to this, some SNe IIb (e.g. 1993J, Richmond et al. 1994; 2016gkg, Arcavi et al. 2017b; Bersten et al. 2018) exhibit an initial peak in their light curves, which has been attributed to postshock-breakout cooling in the case of a progenitor with a compact core surrounded by extended, low-mass material (Bersten et al. 2012; Nakar & Piro 2014). This typically occurs over a short period of a few days, and is not observed in all SNe IIb (e.g. SN 2008ax; Pastorello et al. 2008), potentially because the SN is not discovered until after this phase. This pre-max bump can help infer properties of the progenitor including radius and binarity using hydrodynamic simulations (e.g. Bersten et al. 2012; Piro 2015; Sapir & Waxman 2017). As the spectroscopic properties of a stripped-envelope SN differ primarily due to the degree of stripping of the progenitor star, it is an open question as to whether SNe IIb, Ib, Ic, and Ic-BL are distinct classes or part of a continuum (Modjaz et al. 2019). Galbany et al. (2018) find that SNe IIb have unusual host properties compared with other core-collapse SN hosts, having particularly low metallicity and star formation rate (SFR). However, Schulze et al. (2020) finds that the host properties of SNe IIb are consistent with those of SNe II. SNe Ib, Ic and Ic-BL have been previously observed with peak absolute magnitudes from −16 up to and even brighter than −20, as shown in figs 2 and 3 of Modjaz et al. (2019). Historically, SNe IIb have exhibited less diversity in peak luminosity, ranging from a peak r/R-band absolute magnitude of roughly −16.5 to −18. However, the recent discovery of ASASSN-18am with a peak MV∼−19.7 (Bose et al. 2020) demonstrated that SNe IIb can reach considerable luminosities. In this paper, we present photometry and spectroscopy of DES14X2fna, an unusual and very luminous SN IIb discovered by DarkEnergySurvey(DES) under the Dark EnergySurveySupernova Programme (DES-SN; Bernstein et al. 2012) and exhibiting very different properties to those shown by previously observed SNe IIb. InSection 2, wedetailourobservationsofDES14X2fna.InSection 3, we analyse the spectroscopic and photometric properties of both the SN and its host, and compare to samples of historic SNe. We consider a variety of semi-analytic models to explain the luminosity and evolution of DES14X2fna in Section 4. Next, we discuss the possible mechanisms that could drive the unusual light curve of DES14X2fna and consider the possibility that a similar object could act as a contaminant in photometric samples of SNe Ia in Section 5, before concluding in Section 6. Throughout this analysis, we have assumed a flat CDM cosmology with M=0.3, =0.7, and H0=70 km s−1Mpc−1. 2 OBSERVATIONS DES14X2fna was discovered by DES-SN in an r-band image captured by Dark Energy Camera (DECam; Flaugher et al. 2015) at an apparent magnitude of mr=19.1 mag. This discovery was on 2014 October 1 (MJD 56931.2), with a previous non-detection on 2014 September 24 (MJD 56924.2) at mz∼23.7. The transient was located in a faint host galaxy with Mr∼−16 at z=0.0453,1 at position α=02h23m15. s64, δ=−07◦0520. 8 (J2000). Based on the epochs of first detection and last non-detection, we adopt an explosion date of MJD 56927.7 ±3.5d. After discovery, griz photometric coverage was acquired by DESSN until 2015 January. Photometric measurements were made using the pipeline outlined in Papadopoulos et al. (2015) and Smith et al. (2016), which uses template subtraction to remove the host galaxy contribution to the image using a point spread function (PSF) matching routine. From this difference image, PSF-fitting is used to measure the SN photometry. We correct the photometry for Milky Way extinction using dust maps from Schlafly & Finkbeiner (2011), assuming RV=3.1. We assume negligible host galaxy extinction – we verify this by comparing Hαand H βfluxratiosinspectroscopy of the host, which we find to be consistent with E(B−V)host ∼0 (Osterbrock 1989). Photometric data were then K-corrected into the rest frame. We do this using the SED templates of DES14X2fna from Hounsell et al. (in preparation), which we interpolate to epochs where we have observations and calibrate (‘mangle’) to match our photometry. The mangling process required simultaneous observations in each photometric band – although the DES observations were near simultaneous across different bands, in a few instances, data for a given band were missing. To complete our data and give fully simultaneous data, we interpolate the observed light curves using Gaussian processes (GP; Rasmussen & Williams 2005). These were implemented using the PYTHON package GEORGE (Ambikasaran et al. 2015), following the process outlined in Angus et al. (2019). The observed photometry without any corrections is detailed in Table 1, and the corrected restframe light curves are shown in Fig. 1and also detailed in Table 1. The quoted uncertainties for the observer-frame photometry are purely statistical without incorporating any systematic uncertainties. Due to the high signal-to-noise observations of DES14X2fna, these uncertainties are very small and reach millimag levels at peak. In practice, the uncertainties will be larger than this – for our analysis, we add the statistical errors in quadrature with a value of 0.05 mag to represent systematic uncertainty. This value was selected as the smallest statistical error we could apply to obtain a stable GP fit 1Obtained from narrow host galaxy emission features. MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 3952 DES Collaboration Table 1. Observed and rest-frame photometry of DES14X2fna, quoted in AB magnitudes in the natural DECam system. MJD UT date Rest-frame g(rest) r(rest) i(rest) z(rest) phase (d) (mag) (mag) (mag) (mag) (mag) (mag) (mag) (mag) 56923.3 20140923 −4.2 >23.7 – >23.5 – – – – 56924.2 20140924 −3.3 >23.9 – – – >23.9 – >23.7 – 56931.1 20141001 3.3 18.980 ±0.005 18.87 ±0.07 19.13 ±0.01 18.98 ±0.06 19.30 ±0.01 19.14 ±0.06 19.44 ±0.01 19.24 ±0.05 56934.4 20141004 6.4 18.115 ±0.003 17.89 ±0.05 18.281 ±0.004 18.06 ±0.05 18.42 ±0.01 18.21 ±0.05 18.60 ±0.01 18.35 ±0.05 56936.3 20141006 8.3 17.504 ±0.002 17.51 ±0.05 17.727 ±0.002 17.69 ±0.05 17.911 ±0.002 17.86 ±0.05 18.077 ±0.003 18.02 ±0.05 56943.2 20141013 14.9 17.137 ±0.001 17.03 ±0.05 17.278 ±0.001 17.16 ±0.05 – 17.33 ±0.05 17.569 ±0.002 17.39 ±0.05 56949.1 20141019 20.5 17.240 ±0.002 17.12 ±0.05 17.291 ±0.002 17.17 ±0.05 17.40 ±0.01 17.28 ±0.05 17.524 ±0.003 17.39 ±0.05 56956.2 20141026 27.3 17.655 ±0.002 17.56 ±0.05 17.604 ±0.002 17.47 ±0.05 17.688 ±0.002 17.53 ±0.05 17.782 ±0.002 17.63 ±0.05 56960.2 20141030 31.1 17.986 ±0.002 17.92 ±0.06 17.826 ±0.002 17.69 ±0.05 17.906 ±0.002 17.77 ±0.05 – 17.89 ±0.05 56973.0 20141112 43.4 19.405 ±0.005 19.43 ±0.07 18.726 ±0.003 18.64 ±0.06 18.716 ±0.003 18.57 ±0.06 18.780 ±0.004 18.67 ±0.05 56980.0 20141119 50.1 20.55 ±0.01 20.63 ±0.07 19.57 ±0.01 19.51 ±0.06 19.49 ±0.01 19.37 ±0.05 19.43 ±0.01 19.38 ±0.05 56987.0 20141126 56.8 21.53 ±0.02 21.66 ±0.07 20.32 ±0.01 20.25 ±0.06 20.28 ±0.01 20.17 ±0.06 20.08 ±0.01 20.11 ±0.05 56990.1 20141129 59.7 21.85 ±0.14 21.95 ±0.19 20.51 ±0.06 20.51 ±0.08 20.51 ±0.17 20.50 ±0.09 20.37 ±0.04 20.35 ±0.06 56991.1 20141130 60.7 – 21.84 ±0.11 20.81 ±0.04 20.67 ±0.06 20.73 ±0.04 20.61 ±0.06 20.41 ±0.04 20.41 ±0.05 56992.1 20141201 61.6 21.83 ±0.06 21.94 ±0.10 20.65 ±0.02 20.60 ±0.06 20.65 ±0.02 20.57 ±0.05 20.36 ±0.02 20.44 ±0.05 57001.3 20141210 70.4 22.46 ±0.15 22.63 ±0.21 21.07 ±0.03 20.99 ±0.07 21.13 ±0.03 21.04 ±0.06 20.72 ±0.03 20.82 ±0.05 57005.0 20141214 74.0 22.55 ±0.08 22.72 ±0.12 21.26 ±0.03 21.15 ±0.07 – 21.35 ±0.07 – 21.09 ±0.06 57005.1 20141214 74.1 – – – – 21.44 ±0.05 – 20.93 ±0.03 – 57012.0 20141221 80.7 23.14 ±0.20 23.34 ±0.25 21.63 ±0.05 23.34 ±0.25 21.77 ±0.07 23.34 ±0.25 21.24 ±0.05 23.34 ±0.25 57014.0 20141223 82.6 22.67 ±0.07 22.76 ±0.12 21.71 ±0.03 22.76 ±0.12 21.79 ±0.04 22.76 ±0.12 21.29 ±0.03 22.76 ±0.12 57019.1 20141228 87.5 22.93 ±0.16 23.05 ±0.22 21.84 ±0.06 23.05 ±0.22 22.07 ±0.07 23.05 ±0.22 21.54 ±0.07 23.05 ±0.22 57026.1 20150104 94.2 23.12 ±0.32 23.15 ±0.39 22.21 ±0.11 23.15 ±0.39 22.36 ±0.11 23.15 ±0.39 21.90 ±0.07 23.15 ±0.39 57033.0 20150111 100.8 23.80 ±0.29 23.98 ±0.33 22.40 ±0.07 23.98 ±0.33 22.76 ±0.12 23.98 ±0.33 22.17 ±0.08 23.98 ±0.33 57040.0 20150118 107.5 23.42 ±0.21 23.39 ±0.24 22.76 ±0.09 23.39 ±0.24 22.90 ±0.12 23.39 ±0.24 22.34 ±0.09 23.39 ±0.24 57045.1 20150123 112.3 24.22 ±0.39 24.32 ±0.42 22.89 ±0.13 24.32 ±0.42 23.19 ±0.26 24.32 ±0.42 22.92 ±0.26 24.32 ±0.42 57052.1 20150130 119.0 23.77 ±0.46 23.68 ±0.49 23.58 ±0.34 23.68 ±0.49 23.78 ±0.34 23.68 ±0.49 22.86 ±0.15 23.68 ±0.49 Notes. Note that this observer-frame photometry has not been corrected for Milky Way extinction and quoted uncertainties are purely statistical. For our analysis, these have been added in quadrature with an error of 0.05 mag to represent systematic uncertainty, and these are included in the rest-frame photometry. Phases are given with respect to explosion. Figure 1. griz light curves of DES14X2fna, corrected for Milky Way extinction and k-corrected to the rest frame. Missing epochs of data have been interpolated using GP-interpolation. The dashed vertical lines indicate the epochs of spectral coverage, and the triangles prior to explosion denote upper limits. Diamond markers for data points indicate that they were reconstructed based on GP-interpolation of the full observer-frame light curve. across the full light curve with minimal unphysical undulations. This error is reflected in the rest-frame photometry in Table 1.TheGPinterpolated light curve used to reconstruct missing data was applied in the observer-frame – as such, it is not plotted with the rest-frame data in this figure. Instead, the GP-interpolation shown is obtained from this rest-frame data and is included to illustrate fits obtained from GP-interpolation. The length-scale of the GP fit was determined by maximizing the likelihood of the interpolation. Spectroscopy of DES14X2fna was taken between +17.5 and +52 d (all phases stated in this paper are in the rest frame and with respect to explosion epoch). These spectra were obtained with threedifferent instruments: the AAOmegaspectrographat the AngloAustralian Telescope (AAT) as part of the OzDES spectroscopic follow-up program, the Kast Double Spectrograph at Lick Observatory (LO), and the Blue Channel Spectrograph at the MMT Observatory. Details of the spectroscopic observations are in Table 2. MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 Extreme luminosity of DES14X2fna 3953 Table 2. Details of spectroscopy available for DES14X2fna. UT date MJD Rest-frame phase Telescope Range (d) +instrument (Å) 20141016 56946 +17.5 MMT +BCS 3340–8550 20141017 56947 +18.5 LICK +Kast 3400–10000 20141028 56958 +29 AAT +2dF/AAOmega 3740–8950 20141120 56981 +51 AAT +2dF/AAOmega 3740–8940 20141121 56982 +52 AAT +2dF/AAOmega 3740–8950 Notes. BCS – Blue Channel Spectrograph on MMT 6.5-m telescope; Kast – Kast Double Spectrograph on the 3m Shane telescope at Lick Observatory; 2dF/AAOmega – 2dF fibre positioner and AAOmega spectrograph on the 3.9-m AAT. Phases are given with respect to explosion. Figure 2. Optical spectra of DES14X2fna. Each spectrum has been smoothed using a Savitzky–Golay filter. Spectra have been offset by an arbitrary amount for clarity, and corrected for redshift and Milky Way reddening using the extinction model of Fitzpatrick & Massa (2007). Spectroscopic reductions were performed using standard procedures; the AAT spectrum was reduced following the procedure outlined in Childress et al. (2017). The spectral evolution of DES14X2fna is shown in Fig. 2.Note that as these spectra are only used for classification and calculating line velocities, they have not been calibrated to match photometry. Based on the presence of hydrogen at 18.5 d, it was initially classified as an SN II (Graham et al. 2014). The appearance of helium at 52 d led to a reclassification as an SN IIb (Kuehn et al. 2014). 3 CHARACTERIZING DES14X2FNA 3.1 Host galaxy DES14X2fna was located in an anonymous host galaxy at a redshift of 0.0453. Assuming peculiar velocity dispersion of 200 km s−1, this corresponds in our assumed cosmology to a distance of Figure 3. A composite gri-band image of the host galaxy of DES14X2fna, from the stacked templates of the DES-SN field of Wiseman et al. (2020a). The location of the SN is indicated by the blue markers. The adjacent galaxy, shown by the purple circle, is at higher redshift and not in proximity to the host. 200.7 ±3.0 Mpc or a distance modulus of μ=36.51 ±0.03 mag. To infer global properties of the host, we use griz-band photometry from the deep stacked templates of the DES-SN fields described in Wiseman et al. (2020a). A composite gri-band image of the host galaxy from these templates is shown in Fig. 3. The neighbouring galaxy is at z=0.207. This photometry corresponds to an absolute magnitude of Mr=−16.35 ±0.03. To estimate the stellar mass (Mstellar) and SFR, we fit stellar population synthesis models based on the templates of Bruzual & Charlot (2003) with a Chabrier (2003)initial mass function (IMF), as per Wiseman et al. (2020b). We measure log10(M∗/M)=8.13+0.16 −0.07 and log10(SFR/Myr−1)=−1.53+0.23 −0.47.We find consistent results when fitting with P´ EGASE.2 templates and a Kroupa IMF using the method of Smith et al. (2020). We also derive abundance measurements from nebular emission lines in the host galaxy spectrum2from the OzDES survey (Lidman et al. 2020), a spectroscopic redshift follow-up programme for DES. Emission-line measurements and abundance calculations are performed using the method outlined in Wiseman et al. (2020b). 2Host spectrum of the host of DES14X2fna was taken in 2018 September, by which time the SN had completely faded from view. MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 3954 DES Collaboration Table 3. Properties of the host galaxy of DES14X2fna. Property Host value Mg−16.01 ±0.04 mag Mr−16.34 ±0.04 mag Mi−16.50 ±0.05 mag Mz−16.56 ±0.05 mag Redshift 0.0453 ±0.0005 E(B−V)MW 0.0225 mag log10(M∗/M)8.13+0.16 −0.07 log10(SFR/Myr−1)−1.53+0.26 −0.48 log10(sSFR/yr−1)−9.66+0.10 −0.41 Metallicity: S2N2 D16 7.76+0.31 −0.54 dex Metallicity: PP04 N2 8.17+0.11 −0.17 dex Metallicity: PP04 O3N2 8.19+0.09 −0.14 dex Metallicity: KK04 R23 8.17+0.20 −0.23 dex Briefly, we subtract the stellar continuum as well as any Balmer line absorption using the Penalized PiXel-Fitting software (PPXF; Cappellari & Emsellem 2004; Cappellari et al. 2012; Cappellari 2017) with the MILES library of single stellar populations (Vazdekis et al. 2010), and fit the resulting gas spectrum with Gaussian profiles. We use the line fluxes to derive metallicities based on a number of different calibrations: S2N2 (Dopita et al. 2016), N2 and O3N2 (Pettini & Pagel 2004), and R23 (Kewley, Geller & Jansen 2004) (Table 3). Comparing our calculated mass of the host of DES14X2fna with previous studies of stellar masses of core-collapse SN hosts (e.g. fig. 4 of Galbany et al. 2018; fig. 11 of Wiseman et al. 2020a; fig. 14 of Schulze et al. 2020) indicates that this is a relatively low-mass host, although not unusually low for an SN IIb. The PISCO sample of host galaxies presented in Galbany et al. (2018) contains 13 SNe IIb host environments – this sample varies in PP04 O3N2 metallicity between approximately 8.4 and 8.7 and in log10(sSFR/yr−1) between approximately −11.2 and −9.5. DES14X2fna is lower metallicity than any of these hosts but falls in the upper end of the distribution in terms of sSFR. The PTF sample presented in Schulze et al. (2020) contains 61 SNe IIb hosts that extend down to 106M. For this sample, log10(sSFR/yr−1) varies from approximately −12 to −8. The host of DES14X2fna sits towards the upper end of this distribution although it is not as standout as compared to the PISCO sample. In summary, the host of DES14X2fna is a low-mass, lowmetallicity but relatively highly star-forming galaxy. The majority of core-collapse SNe are observed across a wide range of star-forming hosts (Anderson et al. 2010). However, the most energetic form of stripped envelope SN, SNe Ic-BL, are observed preferentially in low-mass, low-metallicity, star-forming environments similar to DES14X2fna (Japelj et al. 2018; Modjaz et al. 2020). 3.2 Photometry The rest-frame griz light curves of DES14X2fna are shown in Fig. 1. After explosion, DES14X2fna rises to a peak g-band absolute magnitude of ≃−19.5, and ≃−19.3, ≃−19.2, and ≃−19.1 in riz bands, respectively. After peak, the gband declines by ∼5 mag in ∼40 d, while riz decline by ∼3 mag over the same period. After approximately 60 d, each band appears to show a roughly linear decline. GP-interpolation can be used to estimate the rise time and peak absolute magnitude, but does not directly provide us with Table 4. Peak absolute magnitudes of DES14X2fna estimated from GP interpolation. Filter Peak absolute Rise time magnitude (mag) (d) g−19.47 ±0.06 16.67 ±0.53stat ±3.35sys r−19.37 ±0.05 18.00 ±0.56stat ±3.35sys i−19.23 ±0.06 18.90 ±0.58stat ±3.35sys z−19.14 ±0.05 18.35 ±0.57stat ±3.35sys Note. Statistical errors in rise time estimated from MC approach, systematic errors correspond to uncertainty in explosion epoch between last nondetection and first detection in rest frame. uncertainties on rise time. We estimate these using a Monte Carlo approach, randomizing the rest-frame photometry within error bars and GP-interpolating this to estimate the rise time and peak absolute magnitude. This is repeated 1000 times, with the mean and standard deviation taken as the final values and uncertainties. These values are shown in Table 4. To characterize the light curve of DES14X2fna relative to the population of SNe IIb, we have gathered a comparison sample of spectroscopically confirmed SNe IIb. We have selected objects with publicly-available photometry with good coverage around peak and a well-constrained explosion epoch, either through light-curve modelling or a short period between last non-detection and first detection (15 d is the maximum in our sample). This leaves us with a sample of 22 SNe IIb (see Table 5). The top panel of Fig. 4shows the rest-frame r-band light curve of DES14X2fna along with r/R-band light curves of our SNe comparison sample. Note that SN 2009mg, SN 2011ei and SN 2013cu lack sufficient r/R-band coverage and so we have used V-band photometry as the closest wavelength band available to R. For each object we use GP-interpolation to estimate the rise time and peak absolute magnitude. We estimate the epoch at which the linear decline phase of the light curve begins for each SN, ttail and the absolute magnitude at this epoch, Mtail, by eye, where these could be inferred from the data. In addition, we perform linear fits to calculate the tail decline rate where possible. Details and parameter values of each SN in our comparison sample are given in Table 5.Wehave corrected the light curves for redshift and Milky Way extinction based on values reported in the respective papers outlined in this table. However, as these objects are all at low redshift, we do not K-correct these light curves to the rest frame since the effect will be small. Uncertainties in quoted peak absolute magnitude (Mpeak, MW) incorporate uncertainty in GP-interpolation to observed photometry and Milky Way extinction correction. Distances are estimated by cross-matching SNe with their hosts using the NASA Extragalactic Database (NED3) and obtaining a luminosity distance in our assumed cosmology corrected for Virgo infall. We choose this approach for consistency as some of these hosts do not have redshift-independent distance estimates and where they do these are often calculated for different cosmologies. The exception to this is SN 1993J, as the negative redshift of the host means that a redshift distance is not appropriate – instead, we adopt the literature value of 2.9 ±0.4 Mpc from Lennarz, Altmann & Wiebusch (2012). Distance uncertainties are included in this analysis. Where data are available, we correct 3The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (http://ned.ipac.cal tech.edu) MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 Extreme luminosity of DES14X2fna 3955 Table 5. Details and light-curve properties of our SN IIb comparison sample. SN Redshift E(B−V)MW E(B−V)host Rise time Mpeak, MW Mpeak, host ttail Mtail Tail decline rate References (mag) (mag) (d) (mag) (mag) (d) (mag) (mag 100d−1) DES14X2fna 0.0453 0.0225 ±0.0003 Negligible 18.00 ±0.56stat ±3.35sys −19.37 ±0.05 −60 −16 4.30 ±0.10 − ASASSN-18am 0.0301 0.0086 ±0.0011 Negligible 18.89 ±0.42stat ±0.39sys −19.53 ±0.04 −60 −17.3 3.03 ±0.06 (1) SN 2016gkg 0.0049 0.0166 ±0.0002 0.09+0.08 −0.07 20.39 ±1.11stat ±0.16sys −16.69 ±0.06 −16.97+0.26 −0.22 43 −16 1.63 ±0.08 (2), (3), (4), (5), (6), (7) SN 2013cua0.0252 0.0105 ±0.0003 N/A 9.79 ±1.08stat ±0.97sys −18.51 ±0.07 – – – – (2), (8) SN 2013df 0.0024 0.0168 ±0.0002 0.081 ±0.016 22.32 ±0.68stat −15.54 ±0.04 −15.80 ±0.06 40 −14.8 1.96 ±0.02 (2), (9), (10), (11), (12) SN 2012P 0.0045 0.0437 ±0.0005 0.29+0.08 −0.05 20.32 ±0.19stat −15.93 ±0.03 −16.83+0.25 −0.16 [50,60] [−14,−14.2] 1.44 ±0.02 (2), (13) SN 2011dh 0.0016 0.0309 ±0.0017 <0.05 21.89 ±0.19stat ±0.88sys −17.47 ±0.03 – 48 −15.8 2.02 ±0.03 (2), (9), (14), (15), (16) SN 2011eia0.0093 0.0505 ±0.0008 0.18 11.84 ±0.70stat −16.14 ±0.07 −16.70 ±0.07 – – – (2), (17) SN 2011fu 0.0185 0.0648 ±0.0008 0.15 ±0.11 20.47 ±0.54stat −18.01 ±0.03 −18.48 ±0.34 [40,45] [−17.2,−17.4] 1.69 ±0.05 (18), (19) SN 2011hs 0.0057 0.0104 ±0.0004 0.16 ±0.07 17.91 ±0.29stat −16.35 ±0.03 −16.85 ±0.22 35 −15.1 2.14 ±0.02 (2), (20) SN 2009K 0.0117 0.0491 ±0.0014 N/A 27.46 ±0.41stat ±1.21sys −17.61 ±0.03 – – – – (21), (22), (23), (24) SN 2009mga0.0076 0.0388 ±0.0005 0.09 ±0.02 21.06 ±1.00stat −16.70 ±0.05 −16.98 ±0.08 – – – (2), (25) SN 2008aq 0.008 0.0386 ±0.0009 0.027 20.11 ±0.44stat −16.91 ±0.04 −17.00 ±0.04 47 −15.7 2.18 ±0.04 (2), (21), (22), (26) SN 2008ax 0.0019 0.0188 ±0.0002 0.38 ±0.1 23.92 ±1.07stat −16.32 ±0.04 −17.50 ±0.31 40 −15.3 2.13 ±0.03 (2), (22), (27), (28) SN 2006el 0.017 0.0975 ±0.0012 N/A 22.21 ±1.00stat −17.14 ±0.09 – – – – (22), (29) SN 2006T 0.0081 0.0643 ±0.0007 N/A 22.83 ±0.23stat ±7.63sys −17.15 ±0.03 – 48 −15.9 1.79 ±0.02 (2), (21), (22) SN 2004ex 0.018 0.0184 ±0.0016 N/A 25.67 ±0.56stat ±10.43sys −17.10 ±0.03 – 54 −15.9 2.06 ±0.08 (22) SN 2004ff 0.023 0.0274 ±0.001 N/A 11.82 ±0.37stat −17.27 ±0.03 – 40 −16 1.88 ±0.08 (21), (29), (30) SN 2003bg 0.0046 0.018 ±0.001 Negligible 49.41 ±0.81stat −16.36 ±0.04 – – – 1.29 ±0.06 (31) SN 2001ig 0.0031 0.0089 ±0.0004 <0.09 30.14 ±3.86stat −18.01 ±0.07 – – – – (32) SN 1996cb 0.0024 0.0261 ±0.0005 <0.12 24.09 ±0.67stat −16.10 ±0.05 – 44 −15.6 2.21 ±0.03 (33) SN 1993J −0.0001 0.069 ±0.0001 0.12 ±0.07 20.99 ±4.42stat −15.75 ±0.04 −16.12 ±0.22 40 −14.7 2.18 ±0.02 (9), (34), (35), (36), (37), (38), (39), (40), (41), (42), (43) References. (1) Bose et al. (2020); (2) Brown et al. (2014); (3) Bersten et al. (2018); (4) Tonry et al. (2016); (5) Arcavi et al. (2017b); (6) Nicholls et al. (2016); (7) Kilpatrick et al. (2016); (8) Ofek et al. (2014); (9) Tinyanont et al. (2016); (10) Szalai et al. (2016); (11) Maeda et al. (2015); (12) Morales-Garoffolo et al. (2014); (13) Fremling et al. (2016); (14) Ergon et al. (2015); (15) Sahu, Anupama & Chakradhari (2013); (16) Helou et al. (2013); (17) Drake et al. (2009); (18) Kumar et al. (2013); (19) Ciabattari & Mazzoni (2011); (20) Bufano et al. (2014); (21) Stritzinger et al. (2018); (22) Bianco et al. (2014); (23) Hicken et al. (2017); (24) Pignata et al. (2009); (25) Monard (2009); (26) Chu et al. (2008); (27) Tsvetkov et al. (2009); (28) Pastorello et al. (2008); (29) Drout et al. (2011); (30) Pugh, Park & Li (2004); (31) Galbany et al. (2016); (32) Kato et al. (2004); (33) Qiu et al. (1999); (34) Richmond et al. (1996); (35) Barbon et al. (1995); (36) Metlova et al. (1995); (37) Richmond et al. (1994); (38) Lewis et al. (1994); (39) Benson et al. (1994); (40) van Driel et al. (1993); (41) Okyudo et al. (1993); (42) Mikuz, Dintinjana & Zwitter (1993); (43) Zhou (1993). Missing host extinction values have been labelled as either negligible based on spectroscopy or N/A if no host extinction data was available. All presented magnitudes are in AB system. Where SN explosion dates were calculated using the dates of first detection and last non-detection, rise time uncertainties have quoted systematic errors as in Table 4. These values are not quoted where explosion epochs were sourced from literature using other methods. aThese properties were calculated using r/R-band photometry with the exception of SNe, for which V-band light curves were used due to a lack of r/R-band data. for host extinction and factor this into the peak absolute magnitude Mpeak, host. However, as these data were not available for all objects and only an upper limit, in some cases, we do not correct for host extinction when considering bolometric luminosities in order to be consistent across all objects. The most noticeable feature of DES14X2fna is its very high peak luminosity, 0.2 mag fainter than the very luminous ASASSN18am at peak but nearly one mag brighter than the next brightest object in the sample, SN 2013cu. It is important to note that host extinction can make a significant difference to the luminosity of each object, as in the case of SN 2008ax that shows an increase in peak luminosity of greater than 1 mag. However, as most objects have a host extinction considerably less than this and DES14X2fna is significantly more luminous than most of the sample, DES14X2fna still stands out for its considerable luminosity even when accounting for this possibility. DES14X2fna rises to peak in the rband in 18 d, similar to ASASSN-18am and SN 2011hs and fairly typical for SNe IIb based on the sample of SN IIb rise times presented in fig. 4 of Pessi et al. (2019). After rising to maximum, DES14X2fna declines at a similar rate to ASASSN-18am until ∼30dafter explosion, after which DES14X2fna begins to decline far more rapidly. DES14X2fna has a single-peaked light curve similar to SN 2008ax andunlikethe well-studied SN 1993J.However, as thereis aperiod of ≃7 d between the last non-detection and first detection it is possible that an initial peak did occur but was not observed, as this has been observed to last only a few days in previous SNe (Okyudo et al. 1993; Helou et al. 2013; Tartaglia et al. 2017). Motivated by the bright peak luminosity of DES14X2fna, we also comparetoSNeIc-BLtostudyanyresemblance to this veryenergetic class of stripped-envelope SNe. Following the method for our SN IIb sample, we form a similar comparison sample of SNe Ic-BL (Table 6), and light curves shown in the lower panel of Fig. 4. Overall, the peak of DES14X2fna resembles that of an SN Ic-BL more closely than a typical SN IIb. Most notably, in the rband, DES14X2fna matches SN 1998bw well until ≃30 d after explosion. After this time DES14X2fna declines more rapidly than SN 1998bw. As is evident from both panels of Fig. 4, after peak, DES14X2fna exhibits a very rapid tail decline compared to both SNe IIb and SNe Ic-BL. The top panel of Fig. 5focuses on this decline phase for each SN in our SN IIb comparison sample where a tail is apparent. For each SN in the plot, we fit a line to the tail to estimate the decline rate (Table5).DES14X2fnahas anr-band decline rate of 4.30 ±0.10 mag (100d)−1, significantly faster than ASASSN-18am, the next fastest decliner in the sample (3.03 ±0.06 mag (100 d)−1). The lower panel of Fig. 5also shows a histogram of the estimated decline rates for our sample, where DES14X2fna is a clear outlier. It is clear that DES14X2fna exhibits both a very luminous peak and a faster tail evolution than other previously observed SNe IIb. If we are to assume that this object follows the canonical 56Ni decay model for an SN IIb, high 56Ni and ejecta masses would be required to power the high maximum luminosity and broad peak MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 3956 DES Collaboration Figure 4. Upper panel: r-band light curve of DES14X2fna alongside the r/R-band light curves of our SN IIb comparison sample. The dashed line indicates the decline rate expected for fully trapped 56Co decay. Objects denoted with ∗in the legend show V-band photometry due to a lack of available r/R-band data. Lower panel: same as upper panel but for a comparison sample of SNe Ic-BL. All light curves are corrected for MW extinction, and DES14X2fna is k-corrected to the rest frame. Table 6. Our SN Ic-BL comparison sample. SN Redshift E(B−V)MW References SN 2016coi 0.003 646 0.0737 (1) SN 2014ad 0.0057 0.038 (2) SN 2010bh 0.0593 0.1004 (3) SN 2009bb 0.0104 0.0847 (4), (5) SN 2007ru 0.0155 0.2217 (6), (7) SN 2006aj 0.033023 0.1261 (6) SN 2003jd 0.019 0.0378 (6) SN 2002ap 0.002 108 0.0616 (6), (8), (9), (10), (11) SN 1998bw 0.0085 0.0494 (12), (13), (14), (15) Referenes. (1) Prentice et al. (2018); (2) Sahu et al. (2018); (3) Cano et al. (2011); (4) Stritzinger et al. (2018); (5) Pignata et al. (2011); (6) Bianco et al. (2014); (7) Sahu et al. (2009); (8) Foley et al. (2003); (9) Yoshii et al. (2003); (10) Pandey et al. (2003); (11) Gal-Yam, Ofek & Shemmer (2002); (12) Sollerman et al. (2002); (13) Patat et al. (2001); (14) Sollerman et al. (2000); (15) Galama et al. (1998). of DES14X2fna. However, a high ejecta mass would be expected to lead to complete trapping of the γ-rays produced in 56Co decay at late times as there would be significant surrounding material to absorb these γ-rays. Not only does DES14X2fna decline at a rate far faster than expected for fully trapped 56Co decay, it declines more than 1.0 mag (100 d)−1faster than any other SN IIb in our comparison sample. This raises the question as to whether the light curve of DES14X2fna is consistent with a 56Ni decay model or if an alternative mechanism to power the light curve is required. 3.3 Bolometric luminosity We next consider the bolometric and pseudo-bolometric light curves of DES14X2fna. Observed griz light curves are converted to luminosities by fitting with a blackbody curve to compute a spectral energydistribution(SED).Theexactvaluesanduncertainties inbolometric luminosities were estimated using a Monte Carlo approach: (i) An initial blackbody fit is carried out to estimate photospheric radius and temperature along with their uncertainties. (ii) Randomized radius and temperature values are then drawn from a Gaussian distribution using the best-fitting values and uncertainties and used to generate a randomized blackbody SED. (iii) Each randomized SED is integrated over the wavelength range covered by griz bands to produce a pseudo-bolometric luminosity, Lgriz. A bolometric luminosity Lbol is estimated by integrating the fitted SED over all wavelengths. MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 Extreme luminosity of DES14X2fna 3957 Figure 5. Upper panel: r-band light curve of DES14X2fna and the r/R-band light curves of the SNe in the SN IIb comparison sample that exhibit a linear decline. The solid lines show a linear fit to the post-peak linear decline of each SN. Lower panel: histogram showing r/R-band decline rates estimated from the linear fits in upper panel, with DES14X2fna annotated. (iv) This process is then repeated, with the final values of Lgriz and Lbol as well as their uncertainties calculated from the mean and standard deviation of values from all of the randomized blackbody SEDs. We also considered estimating bolometric luminosity using our mangled SED models and the bolometric corrections outlined in Lyman, Bersier & James (2013), but find that these obtain consistent results with our blackbody approach. As a result, we use blackbody fits since these provide information on photospheric temperature and radius as well as luminosity. Of course, the blackbody approximation is not valid at later times as the SN enters the nebular phase; hence, we do not estimate bolometric luminosity more than 120 d after explosion, after our last observation of DES14X2fna. At later times the decreasing quality of a blackbody fit is reflected in larger fit uncertainties and hence much larger uncertainties in Lbol and Lgriz. In addition to this, further uncertainty in Lbol arises from the lack of flux information beyond optical wavelengths, particularly in the UV. Without observations at these wavelengths, we are not able to quantify this effect but acknowledge that it will serve to increase our uncertainties. To calculate bolometric light curves for our SNe IIb comparison sample, we make cuts to select only SNe with observations in at least three photometric bands around peak luminosity to allow reasonable blackbody fits. Unlike DES14X2fna, our sample of SNe IIb includes objects with photometric data at very different epochs and phases of the light curve, which restricts the epochs at which we can calculate bolometric luminosities. At each epoch with available photometry, other observed bands were GP-interpolated to provide simultaneous photometry, after which the same procedure outlined for DES14X2fna was carried out to estimate blackbody fit parameters and bolometric luminosities. To ensure consistency with our estimates of the pseudo-bolometric luminosity of DES14X2fna, we use the same wavelengths limits in our integration regardless of which photometric bands were available for each SN. The bolometric luminosities, photospheric radii, and temperatures obtained from our blackbody fits are shown in Fig. 6. Aswiththe r/R-band photometric data, aside fromASASSN-18am DES14X2fna is the most luminous object in the SN IIb sample and has a relatively broad light-curve peak. We can estimate the true peak bolometric luminosity and rise time using GP-interpolation, which gives a peak luminosity of Lbol =(2.44 ±0.41) ×1043 and Lgriz =(1.17 ±0.08) ×1043 erg s−1, with a rise time to peak, tp, of 15.38 d. With the exception of ASASSN-18am and SN 2013cu, DES14X2fna is brighter at peak than any other SN in the sample by more than a factor of 2. This reiterates our findings from broad-band photometry: DES14X2fna is very luminous at maximum, comparable only to ASASSN-18am, and has a relatively broad light-curve peak. The photospheric radius of DES14X2fna shows an initial fast rise, reaching (1.86 ±0.08) ×1015 cm after 20.5 d, suggesting a photospheric velocity of ∼7900 km s−1. The radius then remains roughly constant for ∼30 d, before slowly decreasing again. This radius evolution is fairly typical for an SN IIb, most closely resembling SN 2011dh and SN 2004ff. ASASSN-18am also shows a similar radius evolution to DES14X2fna at early times, although declines more slowly after peak. DES14X2fna and ASASSN-18am both show a similar temperature evolution, rising to ∼12 000 K at peak after ∼8–9 d, before declining roughly linearly to ∼5000 K after ∼50dandstaying MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 3958 DES Collaboration Figure 6. Upper panel: bolometric light curves of DES14X2fna and our SN IIb comparison sample, estimated from blackbody fits to observed photometry. Middle panel: photospheric radii for DES14X2fna and our SN IIb comparison sample estimated from blackbody fits. Lower panel: effective temperatures for DES14X2fna and our SN IIb comparison sample. roughly constant thereafter, although ASASSN-18am declines more rapidly between ∼10 and 20 d. While SN 2016gkg and SN 2013cu do reach comparable temperatures, this occurs very soon after explosion due to shock cooling while DES14X2fna and ASASSN-18am show temperatures in excess of the rest of the SNe IIb sample until 50 d after explosion, with a far more prolonged temperature decline. 3.4 Spectroscopy Spectroscopy of DES14X2fna is shown in Fig. 2.After∼17.5 d, weak H βand HeI5876 lines become visible. Approximately one day later, a more noticeable broad H αfeature is visible. From ∼29 d, the features become more prominent, with H α,Hβ,andHeI5876 all visible. These are still present in the later spectra at ∼50 d, albeit with Hβbecoming increasingly noisy, and further He Ifeatures at 6678 and 7065 Å also appear in these later spectra. Where possible, we have estimated the expansion velocities of these lines from their P-Cygni profiles, fitting a Gaussian with a pseudo-continuum to estimate the minimum and using this to infer velocity. We follow the process outlined in Maguire et al. (2012), fitting to a small wavelength range around each feature and considering a 30-Å range for the cutoff on either side of the feature when searching for the best fit. The velocity evolution of DES14X2fna is shown in Table 7. Overall, Hαand H βmaintain a relatively constant velocity throughout all of our spectra of roughly 9000km s−1.HeI5876 has a velocity of 9254 ±517 km s−1after 17.5 d, comparable to the H lines, before decreasing gradually in each spectrum and reaching 6074 ±119 km s−1after 52 d. HeI7065 is not visible in the MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 Extreme luminosity of DES14X2fna 3965 funding-source, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundac¸˜ ao Carlos Chagas Filho de Amparo ` a Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Cient´ ıfico e Tecnol´ ogico, and the Minist´ erio da Ciˆ encia, Tecnologia e Inovac¸˜ ao, the Deutsche Forschungsgemeinschaft, and the Collaborating Institutions in the Dark Energy Survey. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Energ´ eticas, Medioambientales y Tecnol´ ogicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgen¨ ossische Technische Hochschule (ETH) Z¨ urich, Fermi NationalAcceleratorLaboratory,the University of Illinois at UrbanaChampaign, the Institut de Ci` encies de l’Espai (IEEC/CSIC), the Institut de F´ ısica d’Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universit¨ at M¨ unchen and the associated Excellence Cluster Universe, the University of Michigan, the National Optical Astronomy Observatory, the University of Nottingham, The Ohio State University, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, Texas A&M University, and the OzDES Membership Consortium. This paper is based in part on observations at Cerro Tololo InterAmerican Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy(AURA)underacooperativeagreement withthe National Science Foundation. The DES data management system is supported by the National Science Foundation under Grant Numbers AST-1138766 and AST-1536171. The DES participants from Spanish institutions are partially supported by MINECO under grants AYA201571825, ESP2015-66861, FPA2015-68048, SEV-2016-0588, SEV2016-0597, and MDM-2015-0509, some of which include ERDF funds from the European Union. IFAE is partially funded by the CERCA program of the Generalitat de Catalunya. Research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013) including ERC grant agreements 240672, 291329, and 306478. We acknowledge support from the Brazilian Instituto Nacional de Ciˆ encia e Tecnologia (INCT) e-Universe (CNPq grant 465376/2014-2). This paper has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the US Department of Energy, Office of Science, Office of High Energy Physics. Thispaperisbasedinpart on data acquired at the Anglo-Australian Telescope, under program A/2013B/012. We acknowledge the traditional owners of the land on which the AAT stands, the Gamilaraay people, and pay our respects to elders past and present. This paper has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the US Department of Energy, Office of Science, Office of High Energy Physics. 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MNRAS 505, 3950–3967 (2021) Downloaded from https://academic.oup.com/mnras/article/505/3/3950/6283733 by Universidad de Granada - Biblioteca user on 03 September 2021 Extreme luminosity of DES14X2fna 3967 Any queries (other than missing material) should be directed to the corresponding author for the article. 1School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK 2DISCnet Centre for Doctoral Training, University of Southampton, Southampton SO17 1BJ, UK 3Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK 4CENTRA, Instituto Superior T´ ecnico, Universidade de Lisboa, Av. Rovisco Pais 1, PL-1049-001 Lisboa, Portugal 5The Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2601, Australia 6Departamento de F´ ısica Te´ orica y del Cosmos, Universidad de Granada, E-18071 Granada, Spain 7INAF, Astrophysical Observatory of Turin, I-10025 Pino Torinese, Italy 8Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia 9Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, NSW 2006, Australia 10Universit´ e Clermont Auvergne, CNRS/IN2P3, LPC, F-63000 ClermontFerrand, France 11School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia 12McDonald Observatory, The University of Texas at Austin, Fort Davis, TX 79734, USA 13Cerro Tololo Inter-American Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile 14Departamento de F´ ısica Matem´ atica, Instituto de F´ ısica, Universidade de S˜ ao Paulo, CP 66318, S˜ ao Paulo, SP, 05314-970, Brazil 15Laborat´ orio Interinstitucional de e-Astronomia – LIneA, Rua Gal. Jos´ e Cristino 77, Rio de Janeiro, RJ – 20921-400, Brazil 16Instituto de Fisica Teorica UAM/CSIC, Universidad Autonoma de Madrid, E-28049 Madrid, Spain 17CNRS, UMR 7095, Institut d’Astrophysique de Paris, F-75014 Paris, France 18Sorbonne Universit´ es, UPMC Univ Paris 06, UMR 7095, Institut d’Astrophysique de Paris, F-75014 Paris, France 19Department of Physics and Astronomy, Pevensey Building, University of Sussex, Brighton BN1 9QH, UK 20Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK 21Instituto de Astrofisica de Canarias, E-38205 La Laguna, Tenerife, Spain 22Universidad de La Laguna, Dpto. Astrof´ ısica, E-38206 La Laguna, Tenerife, Spain 23Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 W. Green Street, Urbana, IL 61801, USA 24National Center for Supercomputing Applications, 1205 West Clark St, Urbana, IL 61801, USA 25Institut de F´ ısica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, E-08193 Bellaterra (Barcelona), Spain 26INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, I-34143 Trieste, Italy 27Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy 28Observat´ orio Nacional, Rua Gal. Jos´ e Cristino 77, Rio de Janeiro, RJ – 20921-400, Brazil 29Centro de Investigaciones Energ´ eticas, Medioambientales y Tecnol´ ogicas (CIEMAT), E-28040 Madrid, Spain 30Department of Physics, IIT Hyderabad, Kandi, Telangana 502285, India 31Fermi National Accelerator Laboratory, PO Box 500, Batavia, IL 60510, USA 32Santa Cruz Institute for Particle Physics, Santa Cruz, CA 95064, USA 33Institute of Theoretical Astrophysics, University of Oslo. PO Box 1029 Blindern, NO-0315 Oslo, Norway 34Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain 35Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain 36Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA 37Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA 38Kavli Institute for Particle Astrophysics and Cosmology, PO Box 2450, Stanford University, Stanford, CA 94305, USA 39SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA 40Faculty of Physics, Ludwig-Maximilians-Universit¨ at, Scheinerstr. 1, D81679 Munich, Germany 41Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, D85748 Garching, Germany 42Universit¨ ats-Sternwarte, Fakult¨ at f¨ ur Physik, Ludwig-Maximilians Universit¨ at M¨ unchen, Scheinerstr. 1, D-81679 M¨ unchen, Germany 43Australian Astronomical Optics, Macquarie University, North Ryde, NSW 2113, Australia 44Lowell Observatory, 1400 Mars Hill Rd, Flagstaff, AZ 86001, USA 45Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA 46Department of Physics, The Ohio State University, Columbus, OH 43210, USA 47George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA 48Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA 49Radcliffe Institute for Advanced Study, Harvard University, Cambridge, MA 02138, USA 50Instituci´ o Catalana de Recerca i Estudis Avanc¸ats, E-08010 Barcelona, Spain 51Physics Department, 2320 Chamberlin Hall, University of WisconsinMadison, 1150 University Avenue, Madison, WI 53706-1390, USA 52Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 53Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA 54Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA 55Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA 56Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA This paper has been typeset from a TEX/L A TEX file prepared by the author. 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