progenitor mass and the low metallicity being consequences of this.

SN 2020zbf has been characterized as type Ic due to the absence of H and He in the spectra, which is associated with H and possibly He envelope losses in the progenitor star by stellar winds or mass transfer in a binary system before the explosion. In the case of a magnetar-powered event, the dominance of C instead of O in the spectra would potentially imply an initially high fraction of C in the progenitor star. The best-fit 5p11Bx2 model (see Sect. 6) from Dessart (2019) corresponds to an initial massive 60 $M_\odot$ ZAMS star in a binary system that has stripped the hydrogen envelope through mass transfer during core hydrogen burning. It has lost the He envelope, and its surface is within the C-rich part of the CO core (Yoon et al. 2010; Dessart et al. 2015). This progenitor could be associated with a carbon-type WC star that ends with a final mass of 5.11 $M_\odot$ and it can create a SN-Ic with 3.63 $M_\odot$ ejecta from which 0.89 $M_\odot$ is C and 1.40 $M_\odot$ is O. The appearance of strong C II lines in the spectra of SN 2020zbf around peak and their vanishing in the late-time spectra is consistent with the stratification of the 5p11 model, in which the outer ejecta are enriched in C. The modeling of the light curve with MOSFiT results in a progenitor pre-SN star of 3.1 $M_\odot$ ($M_{ej} + M_{NS}$). Both the light curve modeling and the spectral comparison show a low-mass progenitor star at explosion with masses between 3 and 5 $M_\odot$, lower than the median value of $6.83_{-2.45}^{+4.04} M_\odot$ in Chen et al. (2023b) but not unprecedented since there are a few progenitor systems in the ZTF sample with similar values to SN 2020zbf.

In the case of the CSM interaction, the early photospheric spectra could point toward an interaction with CO material expelled before the explosion. The comparison with different CSM models shows that the progenitor star ($M_{ej} + M_{CSM} + 1.4 M_\odot$, a typical value for a neutron star; Lattimer & Prakash 2007) is between 6 and $10 M_\odot$ (6.4 $M_\odot$; Chatzopoulos et al. 2013, 5.4 $M_\odot$; Suzuki et al. 2020, <10 $M_\odot$; Khatami & Kasen 2023), lower than the value of $17.92_{-9.82}^{+24.11} M_\odot$ in Chen et al. (2023b) but still not extraordinary. However, taking into account the preference of all the models in low ejecta mass (0.22–3 $M_\odot$), the fast rise time in the SN 2020zbf’s light curve, and the fact that the CSM might have been expelled well in advance and should not be considered in the mass of the pre-SN progenitor, we conclude that the progenitor star at explosion is estimated to be in the range of 2–5 $M_\odot$.

Anderson et al. (2018) studied the SLSN-I SN 2018bsz in particular for the strong C II and they concluded that these lines are produced by a magnetar-powered explosion of a massive C-rich WR progenitor (11.4 $M_\odot$ pre-SN mass) in a binary system by comparing qualitatively with magnetar models from Dessart (2019). Considering the good match of the 5p11Bx2 model with the observed spectrum of SN 2020zbf, a possible progenitor system for SN 2020zbf could be as well a WR in a binary system. Binary interaction could also remove the H-envelope and form a H-rich CSM around SN 2020zbf, with which the ejecta will interact at later times.

## 9.3. Implications of C-rich SLSNe-I

In this work, we have compared SN 2020zbf with H-poor SLSNe that show C II in emission. The comparison is not with a statistically complete sample of all C-dominated objects but rather with a careful selection of published C-rich objects. Anderson et al. (2018) suggest that the C II lines in SN 2018bsz stem from the large fraction of C in the progenitor model. Even though we reach the same conclusion for SN 2020zbf, our SN presents

stronger lines of C II, possibly resulting from the lower ejecta mass. On the other hand, PTF10aagc seems to be the best spectral match for SN 2020zbf, but no study for the powering mechanism of that SN has been done so far.

Fiore et al. (2021) fit SN 2017gci photometry to synthetic light curves and model the nebular spectra and found that SN 2017gci can be fit well by both a magnetar model and CSM interaction with an ejecta mass of $10 M_\odot$, although they do not comment on the progenitor’s composition. In SN 2020wnt, the high ejecta mass (~26 $M_\odot$; Tinyanont et al. 2023) hides the magnetar at early times and it was not revealed until the nebular phase. The progenitor star for that SN was suggested to be a massive rotating star with high abundances of C in its outer layers (Tinyanont et al. 2023, their Fig. A.2).

The difference in the properties of the SLSN powering mechanisms reflects the large diversity in the light curves and the spectra of these objects. Carbon is always present in the outermost CO layers of the ejecta, and thus, we expect to see it in the spectra of SLSNe-I. However, the presence of lines stronger than for the typical SLSNe-I could imply a distinct mechanism and/or a specific composition in the ejecta. In any case, the presence of strong C II could indicate a higher amount of C in the ejecta. A detailed analysis of a statistically meaningful C-rich SLSN-I sample is required in the future to understand the mechanisms for the formation of C II lines and the properties of their progenitor systems.

## 10. Conclusions

In this work we have studied the H-poor SLSN SN 2020zbf, analyzing photometric data from ATLAS, LCO, and UVOT and spectra taken with NTT+EFOSC and the VLT+X-shooter instrument. Our main conclusions are as follows:

- SN 2020zbf is a fast-rising SLSN-Ic with a rise time of $\le 26.4$ days from the explosion and a peak magnitude of $M_g = -21.2$ mag. The rise time is on the faster end of the distribution for SLSNe-I.

- The early spectra of SN 2020zbf present three strong C II lines that are not typically seen in normal SLSNe-I.

- Both the light curve modeling and the comparison with synthetic spectra are consistent with a magnetar-powered SN of a C-rich star with an ejecta mass of about 1.5–3 $M_\odot$.

- Alternatively and/or additionally, we argue that the shape and the strength of the C II lines can also be attributed to an interaction between low-mass ejecta and a dense CO CSM.

- Based on the above modeling, the progenitor is estimated to have a pre-SN mass of between 2 and 5 $M_\odot$.

- A potential late-time $H\alpha$ emission that is accompanied by a knee in the LCO light curve could be an indication of interaction with H-rich CSM.

- The host galaxy has a mass of $\log(M/M_\odot) = 8.68_{-0.22}^{+0.18}$, a star formation rate of $0.24_{-0.12}^{+0.41} M_\odot \text{yr}^{-1}$, and a metallicity of $0.4 Z_\odot$, similar to typical SLSN-I host galaxies.

- There is large variety in the light curves and the spectra of SLSNe with strong C II, suggesting that many different configurations can result in such a spectral signature.

This object illustrates the challenges in classifying SLSNe-I by demonstrating how diverse even the SLSN class can be, as well as the implications for both progenitor populations and explosion mechanisms. More sophisticated tools for light curve and spectra modeling are required to explain the peculiarities of similar objects.
**Acknowledgements.** The authors would like to thank the anonymous referee for their comments and suggestions. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, as part of ePESSTO+ (the advanced Public ESO Spectroscopic Survey for Transient Objects Survey). ePESSTO+ observations were obtained under ESO program IDs 106.216 and 108.220C (PI: Inserra). LCO data have been obtained via an OPTICON proposal (IDs: OPTICON 20A/015, 20B/003, 21B/001, 22B/ 002) and as part of the Global Supernova Project. The OPTICON project has received funding from the European Union's Horizon 2020 research and inno- vation programme under grant agreement No 730890. This work makes use of data from the Las Cumbres Observatory global network of telescopes. The LCO group is supported by NSF grants AST-1911151 and AST-1911225. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contribu- tions of the University of Hawaii Institute for Astronomy, the Queen's Uni- versity Belfast, the Space Telescope Science Institute, and the South African Astronomical Observatory. R. Lunnan is supported by the European Research Council (ERC) under the European Union's Horizon Europe research and inno- vation programme (grant agreement No. 10104229 - TransPire). S. Schulze acknowledges support from the G.R.E.A.T. research environment, funded by Vetenskapsrådet, the Swedish Research Council, project number 2016-06012. M. Nicholl is supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 948381) and by UK Space Agency Grant No. ST/Y000692/1. T.-W. Chen acknowledges the Yushan Young Fellow Program by the Min- istry of Education, Taiwan for the financial support. M. Pursiainen acknowl- edges support from a UK Research and Innovation Fellowship (MR/T020784/1). T. E. Müller-Bravo acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033, and the European Union Next Generation EU/PRTR funds under the 2021 Juan de la Cierva program FJC2021-047124-I and the PID2020-115253GA-100 HOSTFLOWS project, from Centro Superior de Inves- tigaciones Científicas (CSIC) under the PIE project 20215AT016, and the pro- gram Unidad de Excelencia María de Maeztu CEX2020-001058-M. This work was funded by ANID, Millennium Science Initiative, ICN12_009. G. Pignata acknowledges support from ANID through Millennium Science Initiative Pro- grams ICN12_009. C. P. Gutiérrez acknowledges financial support from the Secretary of Universities and Research (Government of Catalonia) and by the Horizon 2020 Research and Innovation Programme of the European Union under the Marie Skłodowska-Curie and the Beatriu de Pinós 2021 BP 00168 pro- gramme, from the Spanish Ministerio de Ciencia e Innovación (MCIN) and the Agencia Estatal de Investigación (AEI) 10.13039/501100011033 under the PID2020-115253GA-100 HOSTFLOWS project, and the program Unidad de Excelencia María de Maeztu CEX2020-001058-M.

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## Appendix A: Photometric data

<table><caption>Table A.1. Photometric data of SN 2020zbf.</caption><thead><tr><th>MJD</th><th>Phase<sup>a</sup><br>(days)</th><th>Filter</th><th>Magnitude<br>(mag)</th><th>Instrument<sup>b</sup></th></tr></thead><tbody><tr><td>59137.5</td><td>-22.60</td><td>c</td><td>20.89 ± 0.35</td><td>ATLAS</td></tr><tr><td>59145.4</td><td>-15.90</td><td>c</td><td>19.65 ± 0.10</td><td>ATLAS</td></tr><tr><td>59147.3</td><td>-14.23</td><td>o</td><td>19.73 ± 0.21</td><td>ATLAS</td></tr><tr><td>59157.4</td><td>-5.86</td><td>o</td><td>18.96 ± 0.18</td><td>ATLAS</td></tr><tr><td>59161.4</td><td>-2.51</td><td>o</td><td>18.87 ± 0.08</td><td>ATLAS</td></tr><tr><td>59163.4</td><td>-0.84</td><td>o</td><td>18.93 ± 0.07</td><td>ATLAS</td></tr><tr><td>59164.8</td><td>0</td><td>B</td><td>18.64 ± 0.20</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>g</td><td>18.60 ± 0.07</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>V</td><td>18.66 ± 0.06</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>r</td><td>18.75 ± 0.10</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>i</td><td>18.90 ± 0.07</td><td>LCO</td></tr><tr><td>59169.3</td><td>4.19</td><td>c</td><td>18.79 ± 0.05</td><td>ATLAS</td></tr><tr><td>59170.8</td><td>5.02</td><td>B</td><td>18.75 ± 0.14</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>g</td><td>18.76 ± 0.09</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>V</td><td>18.85 ± 0.14</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>r</td><td>18.97 ± 0.04</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>i</td><td>19.28 ± 0.13</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>B</td><td>18.87 ± 0.13</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>g</td><td>18.99 ± 0.06</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>V</td><td>18.98 ± 0.11</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>r</td><td>19.04 ± 0.08</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>i</td><td>19.14 ± 0.08</td><td>LCO</td></tr><tr><td>59178.01</td><td>11.73</td><td>UVW1</td><td>20.50 ± 0.15</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>U</td><td>19.04 ± 0.11</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>B</td><td>18.99 ± 0.14</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>UVW2</td><td>21.67 ± 0.20</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>V</td><td>18.90 ± 0.24</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>UVM2</td><td>21.27 ± 0.15</td><td>Swift/UVOT</td></tr><tr><td>59179.66</td><td>13.11</td><td>UVW1</td><td>20.62 ± 0.16</td><td>Swift/UVOT</td></tr><tr><td>59179.66</td><td>13.11</td><td>U</td><td>19.14 ± 0.11</td><td>Swift/UVOT</td></tr><tr><td>59179.66</td><td>13.11</td><td>B</td><td>19.12 ± 0.15</td><td>Swift/UVOT</td></tr><tr><td>59179.67</td><td>13.11</td><td>UVW2</td><td>21.33 ± 0.17</td><td>Swift/UVOT</td></tr><tr><td>59179.67</td><td>13.11</td><td>V</td><td>18.81 ± 0.22</td><td>Swift/UVOT</td></tr><tr><td>59179.67</td><td>13.11</td><td>UVM2</td><td>21.36 ± 0.16</td><td>Swift/UVOT</td></tr><tr><td>59181.0</td><td>14.23</td><td>o</td><td>18.89 ± 0.17</td><td>ATLAS</td></tr><tr><td>59183.0</td><td>15.90</td><td>o</td><td>19.09 ± 0.22</td><td>ATLAS</td></tr><tr><td>59185.0</td><td>17.58</td><td>o</td><td>19.16 ± 0.16</td><td>ATLAS</td></tr><tr><td>59185.38</td><td>17.90</td><td>UVW1</td><td>21.20 ± 0.24</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>U</td><td>19.31 ± 0.13</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>B</td><td>19.27 ± 0.18</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>UVW2</td><td>22.62 ± 0.38</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>V</td><td>18.66 ± 0.22</td><td>Swift/UVOT</td></tr><tr><td>59185.39</td><td>17.90</td><td>UVM2</td><td>21.52 ± 0.19</td><td>Swift/UVOT</td></tr><tr><td>59186.1</td><td>18.41</td><td>B</td><td>19.16 ± 0.09</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>g</td><td>19.16 ± 0.07</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>V</td><td>19.14 ± 0.07</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>r</td><td>19.26 ± 0.06</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>i</td><td>19.38 ± 0.05</td><td>LCO</td></tr><tr><td>59187.3</td><td>19.25</td><td>o</td><td>19.29 ± 0.14</td><td>ATLAS</td></tr><tr><td>59191.3</td><td>22.60</td><td>o</td><td>19.65 ± 0.14</td><td>ATLAS</td></tr><tr><td>59193.9</td><td>24.27</td><td>B</td><td>19.28 ± 0.11</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>g</td><td>19.30 ± 0.16</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>V</td><td>19.30 ± 0.13</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>r</td><td>19.11 ± 0.15</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>i</td><td>19.34 ± 0.19</td><td>LCO</td></tr><tr><td>59193.27</td><td>24.50</td><td>UVW1</td><td>21.34 ± 0.38</td><td>Swift/UVOT</td></tr><tr><td>59193.27</td><td>24.50</td><td>U</td><td>19.58 ± 0.22</td><td>Swift/UVOT</td></tr></tbody></table>

<table><caption>Table A.1. continued.</caption><thead><tr><th>MJD</th><th>Phase<sup>a</sup><br>(days)</th><th>Filter</th><th>Magnitude<br>(mag)</th><th>Instrument<sup>b</sup></th></tr></thead><tbody><tr><td>59193.27</td><td>24.50</td><td>B</td><td>19.47 ± 0.30</td><td>Swift/UVOT</td></tr><tr><td>59193.28</td><td>24.50</td><td>UVW2</td><td>22.46 ± 0.53</td><td>Swift/UVOT</td></tr><tr><td>59193.28</td><td>24.50</td><td>V</td><td>18.73 ± 0.34</td><td>Swift/UVOT</td></tr><tr><td>59193.28</td><td>24.50</td><td>UVM2</td><td>21.85 ± 0.33</td><td>Swift/UVOT</td></tr><tr><td>59193.3</td><td>24.27</td><td>c</td><td>19.27 ± 0.10</td><td>ATLAS</td></tr><tr><td>59195.3</td><td>25.95</td><td>c</td><td>19.50 ± 0.10</td><td>ATLAS</td></tr><tr><td>59197.3</td><td>27.62</td><td>c</td><td>19.43 ± 0.12</td><td>ATLAS</td></tr><tr><td>59201.2</td><td>30.97</td><td>B</td><td>19.57 ± 0.08</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>g</td><td>19.47 ± 0.04</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>V</td><td>19.35 ± 0.07</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>r</td><td>19.39 ± 0.08</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>i</td><td>19.66 ± 0.07</td><td>LCO</td></tr><tr><td>59203.3</td><td>32.64</td><td>o</td><td>19.45 ± 0.09</td><td>ATLAS</td></tr><tr><td>59205.3</td><td>34.32</td><td>o</td><td>19.59 ± 0.17</td><td>ATLAS</td></tr><tr><td>59207.5</td><td>35.99</td><td>B</td><td>19.65 ± 0.19</td><td>LCO</td></tr><tr><td>59207.5</td><td>35.99</td><td>V</td><td>19.34 ± 0.22</td><td>LCO</td></tr><tr><td>59207.5</td><td>35.99</td><td>o</td><td>19.41 ± 0.12</td><td>ATLAS</td></tr><tr><td>59211.3</td><td>39.34</td><td>o</td><td>19.73 ± 0.27</td><td>ATLAS</td></tr><tr><td>59213.3</td><td>41.01</td><td>o</td><td>19.49 ± 0.28</td><td>ATLAS</td></tr><tr><td>59215.2</td><td>42.69</td><td>g</td><td>19.60 ± 0.14</td><td>LCO</td></tr><tr><td>59215.2</td><td>42.69</td><td>V</td><td>19.49 ± 0.17</td><td>LCO</td></tr><tr><td>59215.2</td><td>42.69</td><td>r</td><td>19.48 ± 0.12</td><td>LCO</td></tr><tr><td>59215.2</td><td>42.69</td><td>i</td><td>19.42 ± 0.12</td><td>LCO</td></tr><tr><td>59219.3</td><td>46.04</td><td>c</td><td>19.52 ± 0.09</td><td>ATLAS</td></tr><tr><td>59221.1</td><td>47.71</td><td>B</td><td>19.83 ± 0.15</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>g</td><td>19.65 ± 0.28</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>V</td><td>19.52 ± 0.09</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>r</td><td>19.53 ± 0.20</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>i</td><td>19.60 ± 0.10</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>B</td><td>19.98 ± 0.14</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>g</td><td>19.87 ± 0.10</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>V</td><td>19.66 ± 0.13</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>r</td><td>19.65 ± 0.15</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>i</td><td>19.74 ± 0.21</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>c</td><td>19.87 ± 0.12</td><td>ATLAS</td></tr><tr><td>59229.2</td><td>54.40</td><td>c</td><td>19.75 ± 0.13</td><td>ATLAS</td></tr><tr><td>59231.3</td><td>56.08</td><td>c</td><td>19.43 ± 0.10</td><td>ATLAS</td></tr><tr><td>59233.1</td><td>57.76</td><td>B</td><td>20.12 ± 0.15</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>g</td><td>20.01 ± 0.11</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>V</td><td>19.79 ± 0.13</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>r</td><td>19.77 ± 0.19</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>i</td><td>19.85 ± 0.25</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>g</td><td>20.18 ± 0.21</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>V</td><td>19.96 ± 0.22</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>r</td><td>19.90 ± 0.18</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>i</td><td>20.10 ± 0.12</td><td>LCO</td></tr><tr><td>59245.2</td><td>67.80</td><td>o</td><td>20.07 ± 0.26</td><td>ATLAS</td></tr><tr><td>59246.8</td><td>68.64</td><td>B</td><td>20.54 ± 0.28</td><td>LCO</td></tr><tr><td>59246.8</td><td>68.64</td><td>g</td><td>20.38 ± 0.15</td><td>LCO</td></tr><tr><td>59246.8</td><td>68.64</td><td>V</td><td>20.19 ± 0.22</td><td>LCO</td></tr><tr><td>59258.8</td><td>78.68</td><td>V</td><td>20.39 ± 0.28</td><td>LCO</td></tr><tr><td>59258.8</td><td>78.68</td><td>r</td><td>20.29 ± 0.21</td><td>LCO</td></tr><tr><td>59266.4</td><td>85.38</td><td>r</td><td>20.36 ± 0.20</td><td>LCO</td></tr><tr><td>59377.4</td><td>178.29</td><td>r</td><td>21.62 ± 0.23</td><td>LCO</td></tr><tr><td>59377.4</td><td>178.29</td><td>i</td><td>21.79 ± 0.26</td><td>LCO</td></tr></tbody></table>

Notes. The photometry is reported on the AB system and is not corrected for reddening. This table is available in machine readable form. Multiple exposures on any given night are averaged to give the values presented here. <sup>a</sup>Rest-frame relative to the g-band maximum (MJD 59164.8). <sup>b</sup>UVOT photometry is not host galaxy subtracted.
# Appendix A: Photometric data

<table><caption>Table A.1. Photometric data of SN 2020zbf.</caption><thead><tr><th>MJD</th><th>Phase<sup>a</sup><br>(days)</th><th>Filter</th><th>Magnitude<br>(mag)</th><th>Instrument<sup>b</sup></th></tr></thead><tbody><tr><td>59137.5</td><td>-22.60</td><td>c</td><td>20.89 ± 0.35</td><td>ATLAS</td></tr><tr><td>59145.4</td><td>-15.90</td><td>c</td><td>19.65 ± 0.10</td><td>ATLAS</td></tr><tr><td>59147.3</td><td>-14.23</td><td>o</td><td>19.73 ± 0.21</td><td>ATLAS</td></tr><tr><td>59157.4</td><td>-5.86</td><td>o</td><td>18.96 ± 0.18</td><td>ATLAS</td></tr><tr><td>59161.4</td><td>-2.51</td><td>o</td><td>18.87 ± 0.08</td><td>ATLAS</td></tr><tr><td>59163.4</td><td>-0.84</td><td>o</td><td>18.93 ± 0.07</td><td>ATLAS</td></tr><tr><td>59164.8</td><td>0</td><td>B</td><td>18.64 ± 0.20</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>g</td><td>18.60 ± 0.07</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>V</td><td>18.66 ± 0.06</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>r</td><td>18.75 ± 0.10</td><td>LCO</td></tr><tr><td>59164.8</td><td>0</td><td>i</td><td>18.90 ± 0.07</td><td>LCO</td></tr><tr><td>59169.3</td><td>4.19</td><td>c</td><td>18.79 ± 0.05</td><td>ATLAS</td></tr><tr><td>59170.8</td><td>5.02</td><td>B</td><td>18.75 ± 0.14</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>g</td><td>18.76 ± 0.09</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>V</td><td>18.85 ± 0.14</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>r</td><td>18.97 ± 0.04</td><td>LCO</td></tr><tr><td>59170.8</td><td>5.02</td><td>i</td><td>19.28 ± 0.13</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>B</td><td>18.87 ± 0.13</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>g</td><td>18.99 ± 0.06</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>V</td><td>18.98 ± 0.11</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>r</td><td>19.04 ± 0.08</td><td>LCO</td></tr><tr><td>59176.1</td><td>10.04</td><td>i</td><td>19.14 ± 0.08</td><td>LCO</td></tr><tr><td>59178.01</td><td>11.73</td><td>UVW1</td><td>20.50 ± 0.15</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>U</td><td>19.04 ± 0.11</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>B</td><td>18.99 ± 0.14</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>UVW2</td><td>21.67 ± 0.20</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>V</td><td>18.90 ± 0.24</td><td>Swift/UVOT</td></tr><tr><td>59178.01</td><td>11.73</td><td>UVM2</td><td>21.27 ± 0.15</td><td>Swift/UVOT</td></tr><tr><td>59179.66</td><td>13.11</td><td>UVW1</td><td>20.62 ± 0.16</td><td>Swift/UVOT</td></tr><tr><td>59179.66</td><td>13.11</td><td>U</td><td>19.14 ± 0.11</td><td>Swift/UVOT</td></tr><tr><td>59179.66</td><td>13.11</td><td>B</td><td>19.12 ± 0.15</td><td>Swift/UVOT</td></tr><tr><td>59179.67</td><td>13.11</td><td>UVW2</td><td>21.33 ± 0.17</td><td>Swift/UVOT</td></tr><tr><td>59179.67</td><td>13.11</td><td>V</td><td>18.81 ± 0.22</td><td>Swift/UVOT</td></tr><tr><td>59179.67</td><td>13.11</td><td>UVM2</td><td>21.36 ± 0.16</td><td>Swift/UVOT</td></tr><tr><td>59181.0</td><td>14.23</td><td>o</td><td>18.89 ± 0.17</td><td>ATLAS</td></tr><tr><td>59183.0</td><td>15.90</td><td>o</td><td>19.09 ± 0.22</td><td>ATLAS</td></tr><tr><td>59185.0</td><td>17.58</td><td>o</td><td>19.16 ± 0.16</td><td>ATLAS</td></tr><tr><td>59185.38</td><td>17.90</td><td>UVW1</td><td>21.20 ± 0.24</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>U</td><td>19.31 ± 0.13</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>B</td><td>19.27 ± 0.18</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>UVW2</td><td>22.62 ± 0.38</td><td>Swift/UVOT</td></tr><tr><td>59185.38</td><td>17.90</td><td>V</td><td>18.66 ± 0.22</td><td>Swift/UVOT</td></tr><tr><td>59185.39</td><td>17.90</td><td>UVM2</td><td>21.52 ± 0.19</td><td>Swift/UVOT</td></tr><tr><td>59186.1</td><td>18.41</td><td>B</td><td>19.16 ± 0.09</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>g</td><td>19.16 ± 0.07</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>V</td><td>19.14 ± 0.07</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>r</td><td>19.26 ± 0.06</td><td>LCO</td></tr><tr><td>59186.1</td><td>18.41</td><td>i</td><td>19.38 ± 0.05</td><td>LCO</td></tr><tr><td>59187.3</td><td>19.25</td><td>o</td><td>19.29 ± 0.14</td><td>ATLAS</td></tr><tr><td>59191.3</td><td>22.60</td><td>o</td><td>19.65 ± 0.14</td><td>ATLAS</td></tr><tr><td>59193.9</td><td>24.27</td><td>B</td><td>19.28 ± 0.11</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>g</td><td>19.30 ± 0.16</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>V</td><td>19.30 ± 0.13</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>r</td><td>19.11 ± 0.15</td><td>LCO</td></tr><tr><td>59193.9</td><td>24.27</td><td>i</td><td>19.34 ± 0.19</td><td>LCO</td></tr><tr><td>59193.27</td><td>24.50</td><td>UVW1</td><td>21.34 ± 0.38</td><td>Swift/UVOT</td></tr><tr><td>59193.27</td><td>24.50</td><td>U</td><td>19.58 ± 0.22</td><td>Swift/UVOT</td></tr></tbody></table>

<table><caption>Table A.1. continued.</caption><thead><tr><th>MJD</th><th>Phase<sup>a</sup><br>(days)</th><th>Filter</th><th>Magnitude<br>(mag)</th><th>Instrument<sup>b</sup></th></tr></thead><tbody><tr><td>59193.27</td><td>24.50</td><td>B</td><td>19.47 ± 0.30</td><td>Swift/UVOT</td></tr><tr><td>59193.28</td><td>24.50</td><td>UVW2</td><td>22.46 ± 0.53</td><td>Swift/UVOT</td></tr><tr><td>59193.28</td><td>24.50</td><td>V</td><td>18.73 ± 0.34</td><td>Swift/UVOT</td></tr><tr><td>59193.28</td><td>24.50</td><td>UVM2</td><td>21.85 ± 0.33</td><td>Swift/UVOT</td></tr><tr><td>59193.3</td><td>24.27</td><td>c</td><td>19.27 ± 0.10</td><td>ATLAS</td></tr><tr><td>59195.3</td><td>25.95</td><td>c</td><td>19.50 ± 0.10</td><td>ATLAS</td></tr><tr><td>59197.3</td><td>27.62</td><td>c</td><td>19.43 ± 0.12</td><td>ATLAS</td></tr><tr><td>59201.2</td><td>30.97</td><td>B</td><td>19.57 ± 0.08</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>g</td><td>19.47 ± 0.04</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>V</td><td>19.35 ± 0.07</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>r</td><td>19.39 ± 0.08</td><td>LCO</td></tr><tr><td>59201.2</td><td>30.97</td><td>i</td><td>19.66 ± 0.07</td><td>LCO</td></tr><tr><td>59203.3</td><td>32.64</td><td>o</td><td>19.45 ± 0.09</td><td>ATLAS</td></tr><tr><td>59205.3</td><td>34.32</td><td>o</td><td>19.59 ± 0.17</td><td>ATLAS</td></tr><tr><td>59207.5</td><td>35.99</td><td>B</td><td>19.65 ± 0.19</td><td>LCO</td></tr><tr><td>59207.5</td><td>35.99</td><td>V</td><td>19.34 ± 0.22</td><td>LCO</td></tr><tr><td>59207.5</td><td>35.99</td><td>o</td><td>19.41 ± 0.12</td><td>ATLAS</td></tr><tr><td>59211.3</td><td>39.34</td><td>o</td><td>19.73 ± 0.27</td><td>ATLAS</td></tr><tr><td>59213.3</td><td>41.01</td><td>o</td><td>19.49 ± 0.28</td><td>ATLAS</td></tr><tr><td>59215.2</td><td>42.69</td><td>g</td><td>19.60 ± 0.14</td><td>LCO</td></tr><tr><td>59215.2</td><td>42.69</td><td>V</td><td>19.49 ± 0.17</td><td>LCO</td></tr><tr><td>59215.2</td><td>42.69</td><td>r</td><td>19.48 ± 0.12</td><td>LCO</td></tr><tr><td>59215.2</td><td>42.69</td><td>i</td><td>19.42 ± 0.12</td><td>LCO</td></tr><tr><td>59219.3</td><td>46.04</td><td>c</td><td>19.52 ± 0.09</td><td>ATLAS</td></tr><tr><td>59221.1</td><td>47.71</td><td>B</td><td>19.83 ± 0.15</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>g</td><td>19.65 ± 0.28</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>V</td><td>19.52 ± 0.09</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>r</td><td>19.53 ± 0.20</td><td>LCO</td></tr><tr><td>59221.1</td><td>47.71</td><td>i</td><td>19.60 ± 0.10</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>B</td><td>19.98 ± 0.14</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>g</td><td>19.87 ± 0.10</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>V</td><td>19.66 ± 0.13</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>r</td><td>19.65 ± 0.15</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>i</td><td>19.74 ± 0.21</td><td>LCO</td></tr><tr><td>59227.1</td><td>52.73</td><td>c</td><td>19.87 ± 0.12</td><td>ATLAS</td></tr><tr><td>59229.2</td><td>54.40</td><td>c</td><td>19.75 ± 0.13</td><td>ATLAS</td></tr><tr><td>59231.3</td><td>56.08</td><td>c</td><td>19.43 ± 0.10</td><td>ATLAS</td></tr><tr><td>59233.1</td><td>57.76</td><td>B</td><td>20.12 ± 0.15</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>g</td><td>20.01 ± 0.11</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>V</td><td>19.79 ± 0.13</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>r</td><td>19.77 ± 0.19</td><td>LCO</td></tr><tr><td>59233.1</td><td>57.76</td><td>i</td><td>19.85 ± 0.25</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>g</td><td>20.18 ± 0.21</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>V</td><td>19.96 ± 0.22</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>r</td><td>19.90 ± 0.18</td><td>LCO</td></tr><tr><td>59239.1</td><td>62.78</td><td>i</td><td>20.10 ± 0.12</td><td>LCO</td></tr><tr><td>59245.2</td><td>67.80</td><td>o</td><td>20.07 ± 0.26</td><td>ATLAS</td></tr><tr><td>59246.8</td><td>68.64</td><td>B</td><td>20.54 ± 0.28</td><td>LCO</td></tr><tr><td>59246.8</td><td>68.64</td><td>g</td><td>20.38 ± 0.15</td><td>LCO</td></tr><tr><td>59246.8</td><td>68.64</td><td>V</td><td>20.19 ± 0.22</td><td>LCO</td></tr><tr><td>59258.8</td><td>78.68</td><td>V</td><td>20.39 ± 0.28</td><td>LCO</td></tr><tr><td>59258.8</td><td>78.68</td><td>r</td><td>20.29 ± 0.21</td><td>LCO</td></tr><tr><td>59266.4</td><td>85.38</td><td>r</td><td>20.36 ± 0.20</td><td>LCO</td></tr><tr><td>59377.4</td><td>178.29</td><td>r</td><td>21.62 ± 0.23</td><td>LCO</td></tr><tr><td>59377.4</td><td>178.29</td><td>i</td><td>21.79 ± 0.26</td><td>LCO</td></tr></tbody></table>

Notes. The photometry is reported on the AB system and is not corrected for reddening. This table is available in machine readable form. Multiple exposures on any given night are averaged to give the values presented here. <sup>a</sup>Rest-frame relative to the g-band maximum (MJD 59164.8). <sup>b</sup>UVOT photometry is not host galaxy subtracted.
## Appendix B: Spectroscopic data

<table><caption>**Table B.1.** SN 2020zbf spectroscopic observations.</caption><thead><tr><th>UT date</th><th>MJD<br>(days)</th><th>Phase<sup>a</sup></th><th>Telescope +<br>Instrument</th><th>Exposure<br>(s)</th><th>Grism</th><th>Wavelength range<br>(Å)</th></tr></thead><tbody><tr><td>20201109</td><td>59162</td><td>-2.37</td><td>NTT + EFOSC2</td><td>900</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20201116</td><td>59168</td><td>2.65</td><td>NTT +EFOSC2</td><td>1800</td><td>Gr#11 + Gr#16</td><td>3345–9995</td></tr><tr><td>20201118</td><td>59170.8</td><td>4.33</td><td>VLT + X-shooter</td><td>2400</td><td>–</td><td>3000–24800</td></tr><tr><td>20201208</td><td>59190</td><td>21.93</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20201223</td><td>59205</td><td>33.62</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20210104</td><td>59217</td><td>43.67</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20210123</td><td>59233.1</td><td>57.06</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr></tbody></table>

**Notes.** <sup>a</sup>Rest-frame relative to the g-band maximum (MJD 59164.8).
Appendix B: Spectroscopic data

<table><caption>**Table B.1.** SN 2020zbf spectroscopic observations.</caption><thead><tr><th>UT date</th><th>MJD<br>(days)</th><th>Phase<sup>a</sup></th><th>Telescope +<br>Instrument</th><th>Exposure<br>(s)</th><th>Grism</th><th>Wavelength range<br>(Å)</th></tr></thead><tbody><tr><td>20201109</td><td>59162</td><td>-2.37</td><td>NTT + EFOSC2</td><td>900</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20201116</td><td>59168</td><td>2.65</td><td>NTT +EFOSC2</td><td>1800</td><td>Gr#11 + Gr#16</td><td>3345–9995</td></tr><tr><td>20201118</td><td>59170.8</td><td>4.33</td><td>VLT + X-shooter</td><td>2400</td><td>–</td><td>3000–24800</td></tr><tr><td>20201208</td><td>59190</td><td>21.93</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20201223</td><td>59205</td><td>33.62</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20210104</td><td>59217</td><td>43.67</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr><tr><td>20210123</td><td>59233.1</td><td>57.06</td><td>NTT + EFOSC2</td><td>2700</td><td>Gr#13</td><td>3650–9250</td></tr></tbody></table>

Notes. <sup>a</sup>Rest-frame relative to the g-band maximum (MJD 59164.8).
Appendix C: MOSFiT results

<figure><img src="image_20.png" /><figcaption>Fig. C.1. 1D and 2D posterior distributions of the DEFAULT <sup>56</sup>Ni model parameters from the MOSFiT model. Median and 1σ of the best fit values are marked and labeled.</figcaption></figure>
Appendix C: MOSFiT results

<figure><img src="image_21.png" /><figcaption>Fig. C.1. 1D and 2D posterior distributions of the DEFAULT <sup>56</sup>Ni model parameters from the MOSFiT model. Median and 1σ of the best fit values are marked and labeled.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. C.2.** 1D and 2D posterior distributions of the SLSN magnetar model parameters from the MOSFiT model. Median and $1\sigma$ of the best fit values are marked and labeled.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. C.2.** 1D and 2D posterior distributions of the SLSN magnetar model parameters from the MOSFiT model. Median and $1\sigma$ of the best fit values are marked and labeled.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. C.3.** 1D and 2D posterior distributions of the csm model parameters from the MOSFiT model. Median and $1\sigma$ of the best fit values are marked and labeled.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. C.3.** 1D and 2D posterior distributions of the csm model parameters from the MOSFiT model. Median and $1\sigma$ of the best fit values are marked and labeled.</figcaption></figure>