identify the epoch of first light, we built custom-made nightly stacks and
visually inspected each coadded image. The earliest $3\sigma$ detection was
recovered from data obtained on 31 August 2018 ($t_{\rm max}$$-$80.1 days).
The brightness was $o=19.13\pm 0.21$ mag. The last non-detection before the
first detection was on 17 August 2018 ($t_{\rm max}$$-$92.1 days) and had a
depth of $o\approx 19.9$ mag at $3\sigma$ confidence. The full light curve
covers the period up until $t_{\rm max}$+386 days.

##### ZTF

— The Zwicky Transient Facility uses the Samuel Oschin 48-inch (1.22 m)
Schmidt telescope at Mount Palomar (USA) equipped with a 47-square-degree
camera (Dekany et al. 2020). Since 17 March 2018, the public ZTF Northern Sky
Survey monitors the northern hemisphere every 3 days in $g$ and $r$ band to a
depth of $\sim 20.7$ mag ($5\sigma$; Bellm et al. 2019a, b). The field of SN
2018ibb was observed for the first time on 29 August 2018.

ZTF started its science operations only in March 2018. This resulted in all
reference images between September and November 2018 being contaminated by SN
2018ibb. Hence, the difference photometry in the ZTF alert packages only
provided a partial event history, and the measurements were unusable. Owing to
that, we obtained the science-ready but not-template-subtracted images from
the NASA/IPAC Infrared Science Archive424242https://irsa.ipac.caltech.edu/.
(Their data reduction is described in Masci et al. 2019.) We measured the
brightness of SN 2018ibb using the aperture photometry
tool434343https://github.com/steveschulze/Photometry presented in Schulze et
al. (2018). Once an instrumental magnitude was established, it was calibrated
against the brightness of several stars from a cross-matched PanSTARRS-Gaia
catalogue. The full light curve covers the interval from $t_{\rm max}$$-$81.9
to $t_{\rm max}$+306 days.

##### Gaia

— The Gaia satellite has two $\rm 1.45\,m\,\times\,0.5\,m$ telescopes pointing
in two directions separated by an angle of 106$\aas@@fstack{\circ}$5 and
merged into a single focal plane. It monitors the sky with a $\gtrsim 30$ day
cadence in the unfiltered, white-light Gaia ‘G’ band (3320–10515 Å; Gaia
Collaboration et al. 2016). We retrieved the light curve from the Gaia
Photometric Science Alerts
database444444http://gsaweb.ast.cam.ac.uk/alerts/home. The light curve covers
the period from $t_{\rm max}$$-$93 to $t_{\rm max}$+259 days.

##### Catalina Sky Survey

— The data were taken by the Catalina Sky Survey (Larson et al. 2003) using
the 0.7 m Schmidt telescope454545https://catalina.lpl.arizona.edu/telescopes.
All observations were taken unfiltered using 30 s exposures and typically
reach $V\sim 19.5$ mag. The photometry was performed using Source Extractor
(Bertin & Arnouts 1996) and is calibrated to a pseudo-$V$ based on a
preselected set of calibrator stars as described in Drake et al. (2011). The
light curve covers the period from $t_{\rm max}$$-$72 to $t_{\rm max}$+259
days. Owing to the scatter in the data, we show the data in Figure 36.

Figure 36:  The light curve from the Catalina Sky Survey in pseudo-$V$ band
together with the $r$-band light curve of SN 2018ibb.  Table 12: Log of
photometric observations

Telescope/ | Filter | MJD | Brightness
---|---|---|---
Instrument |  | (day) | (mag)
Swift/UVOT | $uvw2$ | 58464.766 | $21.07\pm 0.16$
Swift/UVOT | $uvw2$ | 58472.770 | $21.36\pm 0.19$
Swift/UVOT | $uvw2$ | 58476.387 | $21.26\pm 0.19$
Swift/UVOT | $uvw2$ | 58481.176 | $21.40\pm 0.24$
Swift/UVOT | $uvw2$ | 58484.828 | $21.76\pm 0.23$
Swift/UVOT | $uvw2$ | 58493.922 | $21.69\pm 0.25$
Swift/UVOT | $uvw2$ | 58497.102 | $21.55\pm 0.22$
Swift/UVOT | $uvw2$ | 58500.664 | $21.84\pm 0.36$
Swift/UVOT | $uvw2$ | 58501.531 | $21.92\pm 0.34$
Swift/UVOT | $uvw2$ | 58505.371 | $21.75\pm 0.21$
Swift/UVOT | $uvw2$ | 58507.598 | $21.80\pm 0.21$
Swift/UVOT | $uvw2$ | 58516.656 | $22.02\pm 0.25$
Swift/UVOT | $uvw2$ | 58521.641 | $21.80\pm 0.22$
Swift/UVOT | $uvw2$ | 58528.613 | $22.22\pm 0.34$
464646All measurements are reported in the AB system. An s-correction was
applied to all measurements but no correction for reddening. A machine-
readable table is available online on WISeREP.

###  A.2 Large observing campaigns

##### 2.2 m MPG

— Between 16 December 2018 and 29 October 2019 ($t_{\rm max}$+11 – $t_{\rm
max}$+283 days), we monitored the light-curve evolution with the seven-channel
imager GROND (Gamma-Ray Burst Optical/Near-Infrared Detector; Greiner et al.
2008) from 4500 to 22,000 Å at the 2.2 m Max Planck Gesellschaft telescope at
La Silla Observatory (Chile) as a part of the GREAT survey (Chen et al. 2018).

The images were reduced with the GROND pipeline (Krühler et al. 2008), which
applies bias and flat-field corrections, stacks images and provides
astrometric calibration. The photometry was extracted similarly to the ZTF
data. To establish the absolute flux scale, we used a local sequence of stars
from a cross-matched PanSTARRS-Gaia catalogue ($griz$) and 2MASS ($JHK$;
Skrutskie et al. 2006). SN 2018ibb evaded detection in $K_{s}$ band. These
images are very shallow, and we omit them in this paper.

##### 3.58 m ESO/NTT

— At the end of October 2019, we switched from the 2.2 m MPG/ESO telescope to
the 3.58 m ESO New Technology Telescope at La Silla Observatory (Chile). We
obtained photometry in $BVgriz$ between 22 October 2019 and 11 November 2020
($t_{\rm max}$+277 – $t_{\rm max}$+619 days) with the EFOSC2 instrument (ESO
Faint Object Spectrograph and Camera version 2; Buzzoni et al. 1984).
Furthermore, we obtained a few epochs of $JHK$ photometry with the SOFI
(Moorwood et al. 1998). All observations were carried out as a part of the
ePESSTO survey (Smartt et al. 2015). On 1 March 2022 ($t_{\rm max}$+1023
days), we obtained a final image in $B$ band. At this phase, the brightness of
SN 2018ibb was well below the host level in the $B$ band. We use these data to
expand the host galaxy observations to shorter wavelengths.

The data were reduced with the PESSTO
pipeline474747https://github.com/svalenti/pessto (Smartt et al. 2015), which
applies bias and flat-field corrections, and provides astrometry calibration.
We utilised the tool from Schulze et al. (2018) to extract the photometry. For
the SN photometry, we used circular apertures and an elliptical aperture for
the host galaxy whose size was matched to that of the VLT/FORS2 data. The
$BVgriz$ photometry was calibrated with a local sequence of stars from a
cross-matched PanSTARRS-Gaia catalogue and the $JHK$ photometry with the stars
from the 2MASS point source catalogue.

##### 8.2 m ESO/VLT

— Between September 2020 and March 2021, we obtained photometry in $gRIz$ with
the FOcal Reducer/low dispersion Spectrograph 2 (FORS2, Appenzeller et al.
1998) at the 8.2 m Very Large Telescope at Paranal Observatory (Chile),
covering the time interval from $t_{\rm max}$+564 days to $t_{\rm max}$+706
days. These data suffered from a progressively increasing contamination by the
host galaxy. To remove the host contribution, we obtained a final set of
$gRIz$ photometry in January/February 2022 ($t_{\rm max}$+991 days). These
reference images were obtained under similar observing conditions, but their
integration time was a factor of 2 larger to ensure a reliable host
subtraction.

We reduced the data with IRAF in the same way as the GROND and NTT data. The
world coordinate system was calibrated with the software package
astrometry.net (Lang et al. 2010). To remove the host contribution, we aligned
all images in a given filter to the corresponding template image and
subtracted the images with the High Order Transform of Psf ANd Template
Subtraction code version 5.11 (HOTPANTS; Becker 2015). We measured the
brightness in the difference images using aperture photometry. The photometry
was calibrated against a set of stars identified in archival $griz$ CTIO/DECam
images from the Dark Energy Survey (Dark Energy Survey Collaboration et al.
2016) and DESI Legacy Imaging Surveys Dey et al. (2019)

##### Neil Gehrels Swift Observatory

— We observed the field with the 30 cm Ultraviolet/Optical Telescope (UVOT;
Roming et al. 2005) aboard the Swift satellite (Gehrels et al. 2004) between
$t_{\rm max}$+8.4 and $t_{\rm max}$+224 days in $w2$, $m2$, $w1$, $u$, $b$,
$v$. While SN 2018ibb was not observable from the ground between April and
August 2019, we made strategic use of Swift’s orbit to reduce the seasonal gap
of our ground-based campaigns from 100 to 54 rest-frame days.

We retrieved the science-ready data from the Swift
archive484848https://www.swift.ac.uk/swift_portal. We co-added all sky
exposures for a given epoch and filter to boost the S/N using uvotimsum in
HEAsoft494949https://heasarc.gsfc.nasa.gov/docs/software/heasoft/ version
6.26.1. Afterwards, we measured the brightness of SN 2018ibb with the Swift
tool uvotsource. The source aperture had a radius of $5^{\prime\prime}$ while
the background region had a significantly larger radius. The photometry was
calibrated with the latest calibration files from September 2020.

###  A.3 Supplementary observations

We augmented the campaigns mentioned above with targeted observations using

  * •

the Alhambra Faint Object Spectrograph and Camera
(ALFOSC)505050http://www.not.iac.es/instruments/alfosc on the 2.56 m Nordic
Optical Telescope (NOT) at the Observatorio del Roque de los Muchachos on La
Palma (Spain) in $griz$

  * •

the 0.76 m Katzman Automatic Imaging Telescope (KAIT) in $BVRI$ and ‘clear’ as
part of the Lick Observatory Supernova Search (LOSS; Filippenko et al. 2001)

  * •

the Las Cumbres Observatory (LCO) in $ugriz$ and $BVRI$

  * •

the optical imager (IO:O) on the robotic 2 m Liverpool Telescope (LT; Steele
2004) located at the Observatorio del Roque de los Muchachos in $griz$

  * •

the Low Resolution Imaging Spectrometer (LRIS, Oke et al. 1995) on the 10 m
Keck I telescope at Maunakea (USA) in $gVRi$

  * •

the Multi-Object Double Spectrographs MODS-1 and 2 cameras in $ugriz$ and the
near-IR LUCI camera in $JHK$ at the 8.4 m Large Binocular telescope at Mt.
Graham (USA)

  * •

1 m Nickel telescope at Lick Observatory in $BVRI$ (Li et al. 2003)

  * •

the SED Machine (Blagorodnova et al. 2018) at the Palomar 60-inch telescope at
Mount Palomar (USA) in $gri$

  * •

the 28-inch telescope at the Wise Observatory (Israel) in $gri$

  * •

the Wide Field Camera 3 in $F336W$ aboard the Hubble Space Telescope to obtain
a late-time image in August 2022

All data were reduced in a similar fashion with instrument-specific pipelines,
for example, FPipe for the SEDm data (Fremling et al. 2016), the
LOSSPhotPypeline515151https://github.com/benstahl92/LOSSPhotPypeline for the
KAIT and Lick data (Stahl et al. 2019) and IRAF. We applied aperture
photometry to extract the photometry similar to the method described in
Section A.2. In the case of KAIT and Lick data Point-spread function
photometry (Stetson 1987) using DAOPHOT was applied.

The Keck observations were performed when the host contribution was non-
negligible but smaller than 10% in all filters. Owing to the size of the host
contribution, we omit any host correction but added an error of 0.2 mag in
quadrature to those measurements. The NOT data were obtained in adverse
observing conditions. We omit to report their photometry owing to issues in
obtaining a reliable photometric calibration. The LCOGT $z$-band images suffer
from fringing, resulting in photometry with large systematic errors. We omit
to report these measurements in this paper.

##  Appendix B Spectroscopic observations

###  B.1 10 m Keck telescope

We obtained 3 epochs with LRIS between $t_{\rm max}$$-$1.4 days and $t_{\rm
max}$+562.3 days. The first two observations, acquired on 1 December 2018
($t_{\rm max}$$-$1.42 days) and 29 August 2019 ($t_{\rm max}$+231.2 days),
used the B400/3400 blue-side grism and the R400/8500 red-side grating, the
dichroic 560 and a 1$\aas@@fstack{\prime\prime}$0 wide slit. The integration
times were 300 s for each epoch. The third epoch was obtained with the
B600/4000 blue-side grism and R400/8500 red-side grating and a
1$\aas@@fstack{\prime\prime}$0 wide slit on 18 September 2020 ($t_{\rm
max}$+562.3 days). The integration time was 4935 s. All spectra were reduced
in a standard fashion with the data reduction pipeline LPipe (Perley 2019).

###  B.2 Palomar 60-inch telescope

We acquired 3 epochs of spectroscopy with the SED Machine between $t_{\rm
max}$+5.4 and $t_{\rm max}$+21.6 days. The SED Machine is a very low
resolution ($R\sim 100$) integral field unit covering the wavelength range
from 3650 to 10,000 Å. The first two epochs (9 and 12 December 2018) are of
sufficient quality and are reported to WISeREP but are not presented in the
paper owing to the availability of higher resolution and higher S/N data. The
final epoch obtained on 28 December 2018 is of very poor quality and is
reported here only for completeness. All observations were reduced using the
pipeline described by Rigault et al. (2019).

###  B.3 Palomar 200-inch telescope

We obtained one epoch of spectroscopy with the DouBle-SPectrograph (DBSP; Oke
& Gunn 1982) on 13 December 2018 ($t_{\rm max}$+8.8 days). The observations
were taken using the D-55 dichroic beam splitter, a blue grating with 600
lines per mm blazed at 4000 Å, a red grating with 316 lines per mm blazed at
7500 Å, and a 1$\aas@@fstack{\prime\prime}$5 wide slit. The data are reduced
using the python package DBSP_DRP525252https://github.com/finagle29/dbsp_drp
that is primarily based on PypeIt (Prochaska et al. 2020a, b).

###  B.4 Extended Public ESO Spectroscopic Survey of Transient Objects

Between 15 December 2018 and 20 October 2019 ($t_{\rm max}$+10.5 – $t_{\rm
max}$+275.6 days), we obtained 6 epochs of spectroscopy using EFOSC2. The
observations were performed with grisms #11, #13 and #16. Depending on weather
conditions, we used either a 1$\aas@@fstack{\prime\prime}$0 or
1$\aas@@fstack{\prime\prime}$5 wide slit. The integration times were between
1800 and 5400 s. We reduced the data in a standard fashion using the PESSTO
pipeline (Smartt et al. 2015).

###  B.5 2.56 m Nordic Optical Telescope

We acquired 3 epochs of low-resolution spectroscopy with ALFOSC between 7
January and 22 February 2019 ($t_{\rm max}$+30.8 and $t_{\rm max}$+70.3 days).
The spectra were obtained with a 1$\aas@@fstack{\prime\prime}$3 wide slit and
grism #4. For the second epoch, we used the second-order blocking filter
WG345.

The data were reduced using IRAF (epoch 2), the pipeline
foscgui535353http://sngroup.oapd.inaf.it/foscgui.html (epochs 1 and 3). The
reduction includes cosmic-ray rejection, bias corrections, flat fielding, and
wavelength calibration using HeNe arc lamps imaged immediately after the
target. The relative flux calibration was done with spectrophotometric
standard stars observed during the same night.

###  B.6 8.2 m ESO Very Large Telescope

##### X-shooter

— We obtained 7 medium-resolution spectra with the X-shooter instrument
(Vernet et al. 2011) between 10 January 2019 and 16 February 2020 ($t_{\rm
max}$+32.7 – $t_{\rm max}$+377.5 days). All observations were performed in
nodding mode and with
1$\aas@@fstack{\prime\prime}$0/0$\aas@@fstack{\prime\prime}$9/0$\aas@@fstack{\prime\prime}$9
wide slits (UVB/VIS/NIR). The first four epochs covered the full spectral
range from 3000 to 24,800 Å. For the other epochs, we used the $K$-band
blocking filter (cutting the wavelength coverage at 20,700 Å; Vernet et al.
2011) to increase the signal-to-noise ratio (S/N) in the $H$ band. The
integration times were varied between 1800 and 3600 s.

The data were reduced following Selsing et al. (2019). In brief, we first
removed cosmic-rays with the tool
astroscrappy545454https://github.com/astropy/astroscrappy, which is based on
the cosmic-ray removal algorithm by van Dokkum (2001). Afterwards, the data
were processed with the X-shooter pipeline v3.3.5 and the ESO workflow engine
ESOReflex (Goldoni et al. 2006; Modigliani et al. 2010). The UVB and VIS-arm
data were reduced in stare mode to boost the S/N. In the background limited
case, this can increase the S/N by a factor of $\sqrt{2}$ compared to the
standard nodding mode reduction. The individual rectified, wavelength- and
flux-calibrated two-dimensional spectra files were co-added using tools
developed by J.
Selsing555555https://github.com/jselsing/XSGRB_reduction_scripts. The NIR data
were reduced in nodding mode to ensure a good sky-line subtraction. In the
third step, we extracted the one-dimensional spectra of each arm in a
statistically optimal way using tools by J. Selsing. Finally, the wavelength
calibration of all spectra were corrected for barycentric motion. The spectra
of the individual arms were stitched by averaging the overlap regions.

##### FORS2

— We obtained 3 low-resolution spectra with the FORS2 spectrograph between 20
September and 1 February 2022 ($t_{\rm max}$+565.3 – $t_{\rm max}$+989.2
days). Each observation was performed with the 300V grism and a
1$\aas@@fstack{\prime\prime}$0 wide slit. The first epoch comprised of
$6\times 1100$ s (executed on 20 and 23 September), and the latter two epochs
consisted of $12\times 1200$ s each. The data were reduced with IRAF similar
to the datasets mentioned above.

###  B.7 3 m Shane telescope

We obtained a series of four spectra with the Kast double
spectrograph565656https://mthamilton.ucolick.org/techdocs/instruments/kast/
Tech%20Report%2066%20KAST%20Miller%20Stone.pdf mounted on the Shane 3 m
telescope at Lick Observatory. These spectra span the time period tmax+24.3
through tmax+89.3 days. The Kast observations utilised the $2"$ slit, 600/4310
grism, and 300/7500 grating. This instrument configuration has a combined
wavelength range of $\sim 3500$-10,500 Å. To minimise slit losses caused by
atmospheric dispersion (Filippenko 1982), the Kast spectra were acquired with
the slit oriented at or near the parallactic angle.

The Kast data were reduced following standard techniques for CCD processing
and spectrum extraction (Silverman et al. 2012) utilising IRAF routines and
custom Python and IDL codes575757https://github.com/ishivvers/TheKastShiv.
Low-order polynomial fits to comparison-lamp spectra were used to calibrate
the wavelength scale, and small adjustments derived from night-sky lines in
the target frames were applied. The spectra were flux calibrated using
observations of appropriate spectrophotometric standard stars observed on the
same night, at similar airmasses, and with an identical instrument
configuration.

The spectrum from $t_{\rm max}$+47.3 days was obtained in very poor
conditions. The quality of the spectrum was insufficient to extract a useful
spectrum.

###  B.8 8.4 m Large Binocular Telescope

We observed SN 2018ibb in the optical with the Multi-Object Double
Spectrographs MODS-1 and MODS-2 (Pogge et al. 2010) and in the near-IR with
the two LUCI (LBT Utility Camera in the Infrared; Ageorges et al. 2010;
Seifert et al. 2010) cameras. In the optical, we used MODS-1 and -2 in dual-
grating mode (grisms G400L and G670L) providing a wavelength coverage from
3200–9500 Å and a slit mask with a width of $1\aas@@fstack{\prime\prime}2$ for
each camera. The integration time was 0.8 hours for the observation on 22
October 2019 ($t_{\rm max}$+276.1 days) and on 29/30 January 2020 ($t_{\rm
max}$+361.6 days). In the NIR, we used the G200 grating and a
1$\aas@@fstack{\prime\prime}$0 wide slit. The first epoch was obtained with
the $zJspec$ bandpass filter (0.90–1.25 $\mu$m) on 19 October 2019
(integration time 0.8 hours). A second epoch was obtained on 28 January 2020
in $zJspec$ and $HKspec$ (1.47–2.35 $\mu m$). The integration of each setup
was 1 hour.

The MODS data were reduced first with modsCCDRed (Pogge 2019) version 2.04 to
remove the bias and flat-field the data using a slit-less pixel flat. After
that, we used custom IRAF scripts to extract the science spectrum using a
nearby star for tracing. The observations of the spectrophotometric standard
were combined in order to measure the trace of the dispersion along the entire
slit. This trace was used along with the wavelength calibration from arc-lamp
lines to rectify the tilt in the direction of the dispersion and the cross-
dispersion axes for the full-frame ($8192\times 3072$ pixel). The final
wavelength calibration was cross-checked with known strong auroral skylines in
the blue ([O i] $\lambda$ 5577.338) and red ([O i] $\lambda$ 6300.3) channels.
The one-dimensional spectrum was extracted from each channel using a
$1\aas@@fstack{\prime\prime}2$-wide aperture. The spectra were flux-calibrated
using the spectrophotometric standard stars. Telluric features were removed
from the red channels using the normalised spectrophotometric standard
spectrum. The data from the four channels were combined into a single spectrum
and re-binned to a common scale of $\Delta\lambda=0.85$ Å pixel-1. All data
were corrected for heliocentric motion.

The LUCI spectroscopic data were reduced at the Italian LBT Spectroscopic
Reduction Center585858http://www.iasf-milano.inaf.it/software. by means of
scripts optimised for LBT data adopting the standard procedure for long-slit
spectroscopy with bias subtraction, flat-fielding, bad-pixel correction, sky
subtraction, and cosmic-rays rejection. The wavelength calibration was
obtained from sky-lines achieving an r.m.s. of $<0.3$ Å in zJ and 0.9 Å in HK.
We flux-calibrated the spectra with telluric standard stars.

###  B.9 8.1 m Gemini-South telescope

We obtained one epoch of spectroscopy with the Gemini Multi-Object
Spectrograph (GMOS; Hook et al. 2004; Gimeno et al. 2016) at the Gemini-South
telescope at Cerro Pachon, Chile, starting on 28 January 2022 ($t_{\rm
max}$+988.1 days). We used the R150 grating with the GG455 blocking filter and
a $1^{\prime\prime}$-wide slit. We divided the observation into three sets of
$4\times 1200$ s each. Owing to technical problems, the campaign had to be
aborted after the first set was obtained.

We reduced the data using the Gemini IRAF
package595959https://www.gemini.edu/observing/phase-iii/understanding-and-
processing-data/data-processing-software version 1.14. We detect the very
faint trace of the SN 2018ibb, but due to the significantly shorter total
integration time, the quality of the spectrum was insufficient to detect
features of SN 2018ibb or its host galaxy.

##  Appendix C Bolometric light curve

The tabulated version of the bolometric light curve is shown in Table 13.

Table 13: Bolometric luminosity and blackbody properties

Phase | $\log\,L_{\rm bol}$ | $\log\,R$ | $T$
---|---|---|---
(day) | $\left(\rm erg\,s^{-1}\right)$ | (cm) | (K)
-81.4 | $43.64\pm 0.11$ | … | …
-71.4 | $43.79\pm 0.08$ | … | …
-61.4 | $43.93\pm 0.08$ | … | …
-51.4 | $43.97\pm 0.08$ | … | …
-41.4 | $44.06\pm 0.12$ | … | …
-31.4 | $44.14\pm 0.09$ | … | …
-21.4 | $44.17\pm 0.07$ | … | …
-11.4 | $44.21\pm 0.08$ | … | …
-1.4 | $44.27\pm 0.10$ | … | …
-0.4 | $44.27\pm 0.09$ | … | …
8.4 | $44.23\pm 0.04$ | $15.64\pm 0.02$ | $11869\pm 421$
18.4 | $44.19\pm 0.03$ | $15.68\pm 0.02$ | $10513\pm 306$
28.4 | $44.16\pm 0.03$ | $15.70\pm 0.01$ | $10092\pm 245$
38.4 | $44.12\pm 0.03$ | $15.70\pm 0.02$ | $9892\pm 314$
48.4 | $44.05\pm 0.03$ | $15.68\pm 0.02$ | $9567\pm 346$
58.4 | $44.03\pm 0.03$ | $15.70\pm 0.02$ | $9281\pm 357$
68.4 | $44.02\pm 0.03$ | $15.70\pm 0.02$ | $9256\pm 359$
78.4 | $44.00\pm 0.03$ | $15.68\pm 0.04$ | $9428\pm 599$
88.4 | $43.96\pm 0.03$ | $15.67\pm 0.03$ | $9089\pm 512$
98.4 | $43.90\pm 0.04$ | $15.64\pm 0.03$ | $8980\pm 475$
103.4 | $43.87\pm 0.10$ | $15.63\pm 0.04$ | $9006\pm 518$
606060The bolometric luminosity and the properties of the blackbody spectrum
are reported in 10-day bins. The blackbody radius and temperature are reported
between $t_{\rm max}$+8.4 and $t_{\rm max}$+100.3 days when multi-band
photometry from the $u$-band to the NIR are available and SN 2018ibb is during
its photospheric phase. Errors quote the statistical uncertainties. A machine-
readable table is available online.

##  Appendix D VLT/FORS2 spectrum from January 2022

Figure 37 shows the observed VLT/FORS2 spectrum from January 2022, the
unbinned spectrum light-blue and a binned version in a darker shade (bin size
30 Å). We scaled the best fit of the host galaxy SED to the brightness at the
SN explosion site (shown in red, galaxy emission lines are clipped). The shape
of the continuum of the VLT spectrum is consistent with the host galaxy SED.
The only remaining SN feature is the broad [O iii] at 5000 Å.

Figure 37:  Observed spectrum of SN 2018ibb at $t_{\rm max}$+989.2 days
(unbinned: light blue, 30 Å binning: blue; error spectrum: grey) and the
scaled galaxy SED (red). The only remaining SN feature is the broad [O iii]
$\lambda\lambda$ 4949,5007 emission line.

##  Appendix E Modelling the SN light curve with Redback

Table 61 summarises the results of fitting the multi-band light curve with the
software package Redback.

Table 14: Light-curve fits with Redback: models, parameters, priors and
marginalised posteriors

Parameter | Prior | Magnetar | Magnetar + | Magnetar + |  56Ni |  56Ni
---|---|---|---|---|---|---
|  |  |  56Ni |  56Ni |  (fixed $\kappa$’s) | (red)
|  |  |  |  (fixed $\kappa$’s) |  |  (fixed $\kappa$’s)
Fitted properties
General
ejecta mass $M_{\rm ej}$ $\left(M_{\odot}\right)$ | $\log\mathcal{U}\left(1,300\right)$ | $104^{+21}_{-15}$ | $140^{+37}_{-34}$ | $139^{+39}_{-40}$ | $100\pm 5$ | $119\pm 7$
explosion date $t_{\rm exp}$ (day) | $\mathcal{U}\left(-200,0\right)$ | $-16^{+2}_{-3}$ | $-5\pm 1$ | $-5\pm 1$ | $-68\pm 5$ | $-79\pm 7$
‘$\gamma$-ray’ opacity $\kappa_{\gamma}$ $\left(\rm cm^{2}\,g^{-1}\right)$ | $\log\mathcal{U}\left(10^{-2},10^{4}\right)$ | $0.013\pm 0.0002$ | $9.0^{+769.7}_{-8.9}$ |  |  |
optical opacity $\kappa$ $\left(\rm cm^{2}\,g^{-1}\right)$ | $\mathcal{U}\left(0.01,0.2\right)$ | $0.16^{+0.02}_{-0.03}$ | $0.11^{+0.06}_{-0.07}$ |  |  |
scaling velocity $v_{\rm scale}$ $\left({\rm km\,s}^{-1}\right)$ | $\mathcal{U}\left(1000,10000\right)$ | $5810^{+250}_{-240}$ | $6390\pm 140$ | $6390\pm 140$ | $4250^{+140}_{-150}$ | $3640^{+210}_{-200}$
white noise parameter $\sigma$ | $\log\mathcal{U}\left(10^{-3},100\right)$ | $0.25\pm 0.01$ | $0.22\pm 0.01$ | $0.21\pm 0.01$ | $0.25\pm 0.01$ | $0.23^{+0.01}_{-0.01}$
$V$-band total extinction $A_{V}$ (mag) | $\mathcal{U}\left(0,1\right)$ | $0.01\pm 0.01$ | $0.22^{+0.06}_{-0.05}$ | $0.22^{+0.06}_{-0.05}$ | $0.01^{+0.02}_{-0.01}$ | $0.52^{+0.07}_{-0.08}$
Magnetar model
magnetic field $B_{\perp}$ $\left(10^{14}\,{\rm G}\right)$ | $\log\mathcal{U}\left(0.01,20\right)$ | $0.98^{+0.05}_{-0.08}$ | $0.09^{+10.65}_{-0.07}$ | $0.06^{+11.50}_{-0.04}$ | … | …
neutron-star mass $M_{\rm NS}$ $\left(M_{\odot}\right)$ | $\mathcal{U}\left(1,2.2\right)$ | $2.1^{0.0}_{-0.1}$ | $1.6\pm 0.4$ | $1.6\pm 0.4$ | … | …
initial spin period $P_{0}$ (ms) | $\mathcal{U}\left(1,20\right)$ | $1.02^{+0.03}_{-0.01}$ | $13^{+5}_{-6}$ | $14^{+4}_{-5}$ | … | …
56Ni model
nickel fraction $f_{\rm Ni}$ | $\log\mathcal{U}\left(10^{-3},1\right)$ | … | $0.3\pm 0.1$ | $0.3\pm 0.1$ | $0.52\pm 0.02$ | $0.93^{+0.05}_{-0.09}$
Fit quality
log Bayesian evidence ($\ln\leavevmode\nobreak\ Z$) |  | $-105.4$ | $-4.3$ | $-4.8$ | $-99.0$ | $-21.5$
Number of free parameters |  | 11 | 12 | 10 | 7 | 7
Derived properties
$\gamma$-ray escape time $t_{0}$ (day) |  | $730\pm 60$ | $800\pm 100$ | $760^{+100}_{-120}$ | $980\pm 40$ | $1240^{+80}_{-70}$
nickel mass $M_{\rm Ni}$ $\left(M_{\odot}\right)$ |  | … | $48\pm 3$ | $48\pm 3$ | $52\pm 2$ | $109^{+9}_{-10}$
kinetic energy $E_{\rm kin}$ $\left(10^{51}\rm erg\right)$ |  | $59^{+14}_{-11}$ | $57^{+12}_{-14}$ | $57^{+16}_{-17}$ | $30^{+3}_{-2}$ | $16\pm 2$
rotational energy $E_{\rm rot}$ $\left(10^{51}\rm erg\right)$ |  | $35.7^{+2.0}_{-3.3}$ | $0.1^{+0.3}_{-0.1}$ | $0.1^{+0.3}_{-0.1}$ | … | …
616161The model ‘56Ni (red)’ only fitted the data in the $r$ and redder bands.
We used uniform ($\mathcal{U}$) and log uniform ($\log\mathcal{U}$) priors.
The uncertainties of the marginalised posteriors are quoted at $1\sigma$
confidence. The explosion date is measured with respect to the date of the
first detection. All marginalised posteriors are reported in linear units. The
kinetic energy of the ejecta was computed via $E_{\rm kin}=1/2\,M_{\rm
ej}\,v_{\rm scale}^{2}$ and the rotational energy of the magnetar via $E_{\rm
rot}=2\times 10^{52}\,\left(M_{\rm
NS}/1.4\,M_{\odot}\right)^{3/2}\,\left(P_{0}/1\,\rm
ms\right)^{-2}\leavevmode\nobreak\ \rm erg$.

##  Appendix F Galaxy photometry of SN 2015bn

We use science-ready coadded images from PanSTARRS DR1 and archival science-
ready images obtained with MegaCAM at the 3.58 m Canada-France-Hawaii
Telescope (CFHT). We augmente this data set with archival data from Swift/UVOT
in $w2$, $m2$ and $w1$ obtained between January 2016 and February 2017 after
the SN faded, and Subaru/Suprime-Cam (Miyazaki et al. 2002) in $B$ and $V$
bands.

The Swift/UVOT data is processed and analysed as described in Appendix A. The
Subaru data is reduced with the software package SDFRED2 (Ouchi et al. 2004).

The photometry is extracted using sufficiently large apertures to encompass
the entire host galaxy. The instrumental magnitudes of the ground-based images
is calibrated against stars from PanSTARRS. We apply known colour equations
between PS1, SDSS and Bessell-like filters to account for differences in the
filter response function as described in Section 3.1. A summary of the host
photometry is reported in Table 15.

We model the host SED with Prospector as described in Section 4.6. The
observed SED is adequately described by a galaxy model with the stellar mass
of $\log\,\leavevmode\nobreak\ M_{\star}/M_{\odot}=8.12^{+0.12}_{-0.23}$ and a
star-formation rate of $0.05^{+0.07}_{-0.01}\leavevmode\nobreak\
M_{\odot}\,\rm yr^{-1}$ (Figure 38). We use the best-fit SED to remove the
host contribution of the SN spectrum from Nicholl et al. (2018) in Section
5.4.

Table 15: Photometry of SN2015bn’s host galaxy

Telescope | Instrument | Filter | Brightness
---|---|---|---
Swift | UVOT | $w2$ | $23.47\pm 0.17$
Swift | UVOT | $m2$ | $23.26\pm 0.16$
Swift | UVOT | $u$ | $23.07\pm 0.28$
Subaru | Suprime-Cam | $B$ | $22.99\pm 0.03$
Subaru | Suprime-Cam | $V$ | $22.32\pm 0.02$
CFHT | MegaCam | $g$ | $22.51\pm 0.04$
CFHT | MegaCam | $r$ | $22.21\pm 0.04$
PanSTARRS |  | $i$ | $21.91\pm 0.12$
PanSTARRS |  | $z$ | $22.20\pm 0.28$
626262All measurements are reported in the AB system and not corrected for
reddening. Non-detections are reported at $3\sigma$ confidence.

Figure 38:  Spectral energy distribution (SED) of the host galaxy of SN 2015bn
(black data points). The solid line displays the best-fitting model of the
SED. The red squares represent the model-predicted magnitudes. The fitting
parameters are shown in the upper-left corner. The abbreviation ‘n.o.f.’
stands for the number of filters.