hyperrefSuppressing link with empty target

11institutetext:  The Oskar Klein Centre, Department of Astronomy, Stockholm
University, AlbaNova, SE-10691 Stockholm, Sweden 22institutetext: Center for
Interdisciplinary Exploration and Research in Astrophysics (CIERA),
Northwestern University, 1800 Sherman Ave., Evanston, IL 60201, USA
33institutetext: The Oskar Klein Centre, Department of Physics, Stockholm
University, Albanova University Center, 106 91 Stockholm, Sweden
44institutetext: Caltech Optical Observatories, California Institute of
Technology, Pasadena, CA 91125, USA 55institutetext: Department of Particle
Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl St, 7610001
Rehovot, Israel 66institutetext: Graduate Institute of Astronomy, National
Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan 77institutetext:
MIT-Kavli Institute for Astrophysics and Space Research, 77 Massachusetts
Ave., Cambridge, MA 02139, USA 88institutetext: Department of Physics,
Lancaster University, Lancaster LA1 4YW, UK 99institutetext: Astrophysics
Research Institute, Liverpool John Moores University, IC2, Liverpool Science
Park, 146 Brownlow Hill, Liverpool L3 5RF, UK 1010institutetext: Division of
Physics, Mathematics and Astronomy, California Institute of Technology,
Pasadena, CA 91125, USA 1111institutetext: IPAC, California Institute of
Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA

# SN 2021adxl: A luminous nearby interacting supernova in an extremely low
metallicity environment

S. J. Brennan  11 S. Schulze  2233 R. Lunnan  11 J. Sollerman  11 L. Yan  44
C. Fransson  11 I. Irani  55 J. Melinder  11 T.-W. Chen  66 K. De  77 C.
Fremling  44 Y.-L. Kim  88 D. Perley  99 P. J. Pessi  11 A. J. Drake  1010 M.
J. Graham  1010 R. R. Laher  1111 F. J. Masci  1111 J. Purdum  44 H. Rodriguez
44

SN 2021adxl is a slowly evolving, luminous, Type IIn supernova with asymmetric
emission line profiles, similar to the well-studied SN 2010jl. We present
extensive optical, near-ultraviolet, and near-infrared photometry and
spectroscopy covering $\sim$1.5 years post discovery. SN 2021adxl occurred in
an unusual environment, atop a vigorously star-forming region which is offset
from its host galaxy core. The appearance of Ly$\alpha$, O II , as well as the
compact core, would classify the host of SN 2021adxl as a Blueberry galaxy,
analogous to the higher redshift Green Pea galaxies. Using several abundance
indicators, we find a metallicity of the explosion environment of only $\sim
0.1\leavevmode\nobreak\ {\rm Z_{\odot}}$, the lowest reported metallicity for
a Type IIn SN environment. SN 2021adxl reaches a peak magnitude of
M${{}_{r}}\leavevmode\nobreak\ \approx-20.2$ mag and since discovery, SN
2021adxl has faded by only $\sim$4 magnitudes in the $r$ band with a
cumulative radiated energy of $\sim 1.5\times 10^{50}$ erg over 18 months. SN
2021adxl shows strong signs of interaction with a complex circumstellar
medium, seen by the detection of X-rays, revealed by the detection of coronal
emission lines, and through multi-component hydrogen and helium profiles. In
order to further understand this interaction, we model the H$\alpha$ profile
using a Monte-Carlo electron scattering code. The blueshifted high-velocity
component is consistent with emission from a radially thin, spherical shell
resulting in the broad emission components due to electron scattering. Using
the velocity evolution of this emitting shell, we find that the SN ejecta
collide with circumstellar material of at least $\sim$5 M⊙ assuming a steady-
state mass-loss rate of $\sim 4-6\times 10^{-3}$ M⊙ yr-1 for the first
$\sim$200 days of evolution. SN 2021adxl was last observed to be slowly
declining at $\sim 0.01\leavevmode\nobreak\ {\rm mag\leavevmode\nobreak\
d^{-1}}$, and if this trend continues, SN 2021adxl will remain observable
after its current solar conjunction. Continuing the observations of SN
2021adxl may reveal signatures of dust formation or an infrared excess,
similar to that seen for SN 2010jl.

###### Key Words.:

Supernovae: individual: SN 2021adxl– Supernovae: individual: SN 2010jl –
Stars: winds, outflows

##  1 Introduction

Massive stars ($>8$ M⊙) are expected to end their lives as core-collapse
supernovae (CCSNe) (Woosley & Weaver, 1995; Heger et al., 2003; Janka, 2012;
Crowther, 2012). All-sky photometric surveys (Bellm, 2014; Chambers et al.,
2016a; Tonry et al., 2018) are finding numerous supernovae (SNe) each night,
but the exact mechanisms by which a massive star spends its final moments is
currently unclear (see e.g., Burrows & Vartanyan, 2021). Supernovae that
display narrow Hydrogen emission lines atop a blue continuum are classified as
Type IIn SNe (Schlegel, 1990; Gal-Yam et al., 2007; Ransome et al., 2021). The
narrow emission features are suggested to arise from the excitation of a slow-
moving circum-stellar medium (CSM) near the progenitor which has been
photoionised by the ejecta-CSM interaction.

This CSM is expected to have been ejected by the progenitor in the years to
decades before core collapse (Vink et al., 2008; Ofek et al., 2014). While a
star will eject mass into the surrounding interstellar medium (ISM) throughout
its life, in the form of stellar winds with mass-loss rates of
$10^{-7}-10^{-4}$ M⊙ yr-1, studies of the progenitors of Type IIn SNe have
reported much higher mass-loss rates of $>10^{-3}$ M⊙ yr-1 (Fransson et al.,
2014; Moriya et al., 2014; Fraser, 2020; Khatami & Kasen, 2023). This likely
reflects either eruptive mass-loss events, or enhanced steady state mass loss
by the progenitor in the decades prior to core-collapse.

The physical mechanism behind such an enhanced mass-loss rate is elusive,
mainly due to the inability to directly observe these mass-loss events prior
to the supernova explosion. However, this brief period of enhanced mass loss
likely influences the photometric evolution of the supernova, including
duration and luminosity, as well as the spectroscopic appearance of
emission/absorption line profiles and their evolution (for example see
Kurfürst & Krtička, 2019; Suzuki et al., 2019, for the case of a disk/torus).
These progenitors must have mass-loss rates not typically observed during the
main sequence. It is suggested that the high mass-loss rates are similar to
those of massive ($>$ 25 M⊙) luminous blue variable (LBV) stars (e.g. Taddia
et al., 2013; Weis & Bomans, 2020) and some Type IIn SNe have been associated
with such stars (e.g. SN 2005gl; Gal-Yam et al., 2007). Alternatively, Type
IIn SNe may arise as a result of a superwind phase from a red super-giant
(RSG) star (Fransson et al., 2002; Smith et al., 2009a). Current stellar
evolution theory suggests that canonical LBVs are a short transitional phase,
between a hydrogen-rich RSG evolving towards a hydrogen-depleted Wolf-Rayet
(WR) star (Crowther, 2007; Langer, 2012; Ekström et al., 2012). The term LBV
is typically employed to define a broad phenomenology rather than an
evolutionary stage (e.g. Gal-Yam et al., 2007; Trundle et al., 2008;
Dwarkadas, 2011; Groh, 2017). Standard stellar evolution theory predicts that
single massive stars which become LBVs do so near the end of, or after, the
completion of core hydrogen burning. Thereafter they typically lose their
hydrogen-rich envelopes in the LBV phase and become WR stars, where they spend
$\sim 10^{5}$ years after which they explode as a CCSN.

To date, there is no clear understanding of how a star may undergo core-
collapse at this phase (Chevalier, 2012; Woosley, 2017). However, under
certain conditions, a progenitor may have the appearance of a LBV star shortly
before core-collapse when the progenitor is rapidly rotating (e.g. Groh et
al., 2013). In this scenario, the progenitor likely ejects the majority of its
outer envelope during the RSG stage. It should be noted that these stars
appear as LBV stars but do not necessarily have the chemical composition of
the canonical LBV (Humphreys & Davidson, 1994; Weis & Bomans, 2020). Studies
of Type IIn SNe show a diverse range of peak magnitudes, ranging from $-16$ to
$-23$ magnitudes (Nyholm et al., 2020; Ransome et al., 2021). Typically, SNe
brighter than $-21$ mag are classified as superluminous supernovae (SLSNe)
(Gal-Yam, 2012, 2019), although for hydrogen-rich transients it is not clear
if there is a defined boundary between regular SNe and SLSNe (of Type IIn) or
rather an apparent continuity.

Figure 1: P60/SEDM composite color image ($gri$) for SN 2021adxl (marked in
the inset by the magenta crosshair) from the ZTF survey, when the transient
was near peak. SN 2021adxl is situated atop strong host emission, which can be
seen in the inset.

This paper presents a large dataset for SN 2021adxl that enables us to study
the late time evolution and environment of a nearby luminous Type IIn SN. A
image of SN 2021adxl and its environment is given in Fig. 1. SN 2021adxl
111Also known as Gaia21fcd, PS22bne, ATLAS23blun, and ZTF21ackxdos. was
discovered by the Zwicky Transient Factory (ZTF; Graham et al., 2019; Bellm et
al., 2019; Dekany et al., 2020) at R.A. = 11:48:06.940, DEC. = $-$12:38:41.71
(J2000) on 2021-11-08 at a $r$-band magnitude of 14.41 mag (Fremling, 2021).
SN 2021adxl was classified by De et al. 2022 on 2022-02-02 as a Type IIn SN at
a redshift of $z=0.018$ (reported wavelengths given in rest frame unless
stated otherwise). The host of SN 2021adxl, WISEA J114806.88-123841.3,
displays a unique morphology known in the literature as a Tadpole Galaxy
(Elmegreen et al., 2012; Munoz-Tunon et al., 2014; Rosado-Belza et al., 2019),
discussed further in Sec. 3. We assume a Hubble constant $H_{0}=73$ km s-1
Mpc-1, $\Omega_{\Lambda}=0.73$ and $\Omega_{\rm M}=0.27$ (Spergel et al.,
2007). The corresponding luminosity distance $D_{L}=78.16$ Mpc and a distance
modulus of $\mu=34.47\pm 0.02$ mag are used for SN 2021adxl. We correct for
foreground extinction using ${\rm R_{V}}$=3.1, ${\rm E(B-V)}$ = 0.026 and the
extinction law given by Cardelli et al. 1989. Unless stated, we do not correct
for any potential host galaxy or circumstellar extinction, however we note
that the blue colors seen in the spectra of SN 2021adxl, as well as the host,
do not point towards significant reddening by dust. The rising part of the
light curve of SN 2021adxl was not observed, as the transient was discovered
around its peak magnitude after ending solar conjunction. The rest-frame phase
is taken with respect to the r-band discovery epoch on 2021-11-03 (MJD =
59521). Luminous SNe are intrinsically rare and given the adopted distance of
78 Mpc, SN 2021adxl is the nearest Type IIn SN in a decade, and just as SN
2010jl (49 Mpc; Smith et al., 2011; Fransson et al., 2014; Ofek et al., 2014;
Jencson et al., 2016), and due to its luminosity and slow evolution, provides
a rare opportunity to follow the evolution of such a transient for many years.

This paper is organised as follows: Section 2 provides details of the
observational dataset presented in this paper, split into photometric in Sect.
2.1 and spectroscopic data in Sect. 2.2, as well as gives an overview of the
observable properties for SN 2021adxl. SN 2021adxl occurred in a bright star
forming region, and we discuss the host environment of SN 2021adxl in Sect. 3.
A prominent feature of the optical spectra of SN 2021adxl is the broad
H$\alpha$ profile, and we explore the formation of this line in Sect. 4, as
well as any insight into the ejecta-CSM interaction in Sect. 4.1. Finally in
Sect. 5, using the results from observations, we investigate the metallicity
of the host of SN 2021adxl and, in general, interacting SNe in Sect. 5.2 as
well as discuss the possible progenitor of SN 2021adxl in Sect. 5.3.

##  2 Observations

###  2.1 Photometry

Forced photometry in $gri$ was obtained at the position of SN 2021adxl using
the ZTF forced-photometry service222https://ztfweb.ipac.caltech.edu/cgi-
bin/requestForcedPhotometry.cgi (Masci et al., 2019). Quality checks are
performed on the raw data and detections are chosen based on a signal-to-noise
ratio (S/N) of S/N $\geq$ 3\. Photometry from the ATLAS forced-photometry
server333https://fallingstar-data.com/forcedphot/ (Tonry et al., 2018; Smith
et al., 2020; Shingles et al., 2021) at the position of SN 2021adxl were
obtained in the ATLAS $c$ and $o$ filters. We compute the weighted average of
the fluxes of the observations on nightly cadence. We perform a quality cut of
3$\sigma$ in the resulting flux of each night for both filters and then
convert them to the AB magnitude system. A single epoch of photometry with the
AAlhambra Faint Object Spectrograph and Camera, (ALFOSC) on the 2.56 m Nordic
Optical Telescope (NOT) was obtained in $gr$ and three epochs from the
Liverpool Telescope (LT) with the optical imaging component of IO (Infrared-
Optical) instrument was obtained in $gri$. J-band photometry was obtained with
the Palomar Gattini-IR (PGIR) telescope shortly after SN 2021adxl was
discovered and data were reduced following De et al. 2020. Our photometric
dataset is given in Fig. 2 and photometric tables are available in the online
material.

Figure 2: Multi-band lightcurve of SN 2021adxl covering $\sim$1.6 years of its
post-peak evolution. Highlighted in grey are two distinct phases where SN
2021adxl displays two distinct decline slopes, as mentioned in the text. The
inset provides Gaia G-band data of SN 2021adxl (post-2022), as well as the
historic observations of the explosion site (pre-2022, likely dominated by
host flux). The right y-axis gives the apparent magnitude corrected for
distance. Spectral epochs are given as vertical lines at the top of the plot.

We observed the field SN 2021adxl with the Swift’s onboard X-ray telescope XRT
(Burrows et al., 2005) in photon-counting mode between 2022-02-23T16:47:35
(MJD: 59633.699) and 2022-04-16T12:19:33 (MJD: 59685.5136). We analysed all
photon-counting data with the online tools of the UK Swift team that use the
methods described by Evans et al. 2007, 2009, 2020 and the software package
HEAsoft444https://heasarc.gsfc.nasa.gov/lheasoft.

An image constructed from all XRT data reveals a fading source at RA, Decl.
(J2000) 11:48:06.72, $-$12:38:45.0 with an uncertainty of 5.3 arcsec (radius,
90% confidence), using the online tool of the UK Swift team 555
http://www.swift.ac.uk/user_objects/ (Evans et al., 2007, 2009). Using the
dynamic binning method of the swift online tools, we obtain two detections
summarised in Table 1. The number of counts is too low to robustly constrain
the X-ray spectrum. To convert the count-rate to a flux, we assume an absorbed
power-law with a photon index of 2 where the absorption components is set to
the Milky-Way value of $N({\rm H,Gal})=2.74\times 10^{20}\leavevmode\nobreak\
\rm cm^{-2}$ from (HI4PI Collaboration et al., 2016). Using these spectral
parameters, we derive count rate to flux conversion factor of $3.75\times
10^{-11}\leavevmode\nobreak\ \rm erg\,s^{-1}\,cm^{-2}/\left(ct\,s^{-1}\right)$
to infer the unabsorbed flux between 0.3 and 10 keV. The converted count rates
are summarised in Table 1, and we will further discuss the X-ray luminosity in
Sect. 2.5.

Table 1: Log of XRT observations

MJD | Phase | Count rate | $F\leavevmode\nobreak\ (0.3-10\leavevmode\nobreak\ \rm keV)$
---|---|---|---
| (days) | ($10^{-3}\leavevmode\nobreak\ {\rm s}^{-1}$) | ($10^{-12}\leavevmode\nobreak\ \rm erg\,s^{-1}\,cm^{-2}$)
$59633.9^{+26.1}_{-0.2}$ | +113 | $0.011\pm 0.003$ | $0.41\pm 0.10$
$59681.4^{+4.3}_{-13.5}$ | +161 | $0.003\pm 0.001$ | $0.12\pm 0.05$
666The flux reports the brightness after accounting for absorption in the
Milky Way.

Due to its proximity at only 78 Mpc, and the fact that SN 2021adxl is a slow
evolving luminous transient, fading by only 4.5 mags during the 1.5 year (18
month) dataset presented in this work, this SN can potentially be followed-up
for years. Figure 2 illustrates the entire multi-band observations of SN
2021adxl covering from discovery until solar conjunction in mid-2023.
Spectroscopic follow-up began in February 2021, and is discussed further in
Sect. 2.2.

Figure 3: $r/R$-band lightcurve comparison of luminous interacting SNe
including SN 2021adxl. Each transient is corrected for distance and MW
extinction. The regions marked as Phase 1 and Phase 2 correspond to the phases
in the post-peak evolution where SN 2021adxl shows unique decline rates.

The lightcurve of SN 2021adxl shows two distinct phases (referred to as Phase
1 and Phase 2) in its post-peak evolution. The peak itself is poorly sampled
and it is unclear if Phase 1 extends to the earliest detections. Approximately
100 days post discovery, SN 2021adxl plateaus onto a smooth decline of $\sim
0.6$ mag / 100 d, declining faster in the bluer bands, as is typical for Type
IIn SNe (see Fig. 10 in Nyholm et al., 2020). After the mid-2022 solar
conjunction, SN 2021adxl begins to decline at a slightly faster rate of 1 mag
/ 100 d.

The rise to peak for SN 2021adxl was not observed due to solar conjunction in
late-2021. An important aspect of Type IIn SNe is their rise time to maximum
light, as there is a strong correlation between rise time and CSM/ejecta
properties (e.g. Suzuki et al., 2016; Nyholm et al., 2020). The inset of Fig.
2 shows the Gaia G-band observations of the site of SN 2021adxl covering the
past 9 years. Prior to SN 2021adxl, there is no significant variability seen
at the explosion site. Similarity, no precursor signal is seen in forced-
photometry measurements of ZTF, and ATLAS observations.

Figure 4: Color comparison of bright Type IIn SNe including SN 2021adxl. Each
transient has been corrected for MW extinction.

A pseudo-bolometric lightcurve is constructed using
Superbol777https://github.com/mnicholl/superbol (Nicholl, 2018) from the
photometry presented in Fig. 2, and calibrated using the extinction and
distance given in Sect. 1. The resulting lightcurve is shown in Fig. 5. The
bolometric light curve can be accurately characterized by a power law decay
from $\sim$ 0 - 300 days (with respect to discovery date), given by ${\rm
L\approx 7.81\times 10^{42}\times(t/100\leavevmode\nobreak\
days)^{-1.13}\leavevmode\nobreak\ erg\leavevmode\nobreak\ s^{-1}}$ and a final
steep decay given as ${\rm L\approx 2.17\times
10^{42}\times(t/300\leavevmode\nobreak\ days)^{-3.89}\leavevmode\nobreak\
erg\leavevmode\nobreak\ s^{-1}}$ after $\sim$ 300 days. As shown in Fig. 5,
this is similar to SN 2010jl (Fransson et al., 2014), but SN 2021adxl is
around 66% less luminous.

Figure 5: Bolometric light curve for SN 2021adxl.The black dashed lines show
power law fits to the phase 1 and phase 2 light curve, which a distinct break
at $\sim$ +300 days post discovery. The blue dashed line is the similar power
law fits for SN 2010jl (Fransson et al., 2014), as well as the same fit scaled
to match SN 2021adxl by eye in light blue.

Figure 3 compares the $r/R$-band absolute light curve of SN 2021adxl with
several transients with similar photometric appearance, most notably SN 2013L
(Andrews et al., 2017; Taddia et al., 2020) and SN 2010jl (Fransson et al.,
2014; Jencson et al., 2016). Additionally, Fig. 4 gives the extinction-
corrected color evolution for each transient. Qualitatively, SN 2021adxl, SN
2013L, and SN 2010jl share the same lightcurve morphology and color evolution.
Post-peak, each of these transients light curves plateau after $\sim$ 100
days. SN 2021adxl shows a steeper decline compared to SN 2013L and SN 2010jl,
with a slope similar to that seen in SN 2016aps (Nicholl et al., 2020), albeit
$\sim$2 mags less luminous.

After $\sim$ 250 days, SN 2021adxl begins to decline faster, and the same
evolution is also clearly seen for SN 2010jl and SN 2013L, although with much
poorer cadence for the latter. A similar trend was observed also for the Type
IIn SN 2005ip, however for that SN the first plateau stage happened later, at
approximately +200 days (Stritzinger et al., 2012) and the second drop-off
much later, at +3000 days (Fox et al., 2020). For contrast, the prototype Type
IIn SN 1998S does not show any light-curve flattening, but declines rather
quickly (Schlegel, 1990; Mauerhan & Smith, 2012).

As seen from Fig. 3, there is a spread in peak magnitudes for the transients.
Under the assumption that the explosion mechanism for each transient is
similar, e.g. core-collapse (Bethe, 1990; Woosley & Weaver, 1995; Smartt,
2009) or pulsational-pair instabilities (Woosley, 2017), the heterogeneity in
brightness could be the result of different amounts of CSM and/or ejecta mass,
although this assumes the explosion energy is similar for all Type IIns. We
further discuss the ejecta and CSM properties of SN 2021adxl in Sect. 4.1

###  2.2 Spectral evolution

Figure 6: The earliest spectrum of SN 2021adxl from P60/SEDM taken on
2021-11-09. The mean flux is given in black, with 1$\sigma$ errors shaded in
grey.

Figure 6 shows the earliest spectrum of SN 2021adxl, taken around
the(apparent) peak brightness. Although the spectrum shows some undulations,
its overall appearance is consistent with a Type IIn SN, i.e a blue continuum
with narrow Hydrogen emission features, with underlying host flux seen in [O
III] [O III] $\lambda 5007$. SN 2021adxl was then classified as a Type IIn by
De et al. 2022 using the Magellan/Baade with the FIRE NIR instrument on
2022-02-01 and the classification spectrum is shown in Fig. 7.

Figure 7: Classification NIR spectrum of SN 2021adxl from the Magellan/FIRE
from 2022-02-01 (De et al., 2022). A close-up of the Pa-$\beta$ line is given
in the inset on the right, with a zoomed-in profile in the upper left. The
spectrum smoothed with a Savgol filter is given in black, with the raw data
given in grey.

Follow-up spectra were obtained using NOT+ALFOSC, the Low Resolution Imaging
Spectrometer (LRIS; Oke et al., 1995) on the 10 m Keck telescope, the Double
Spectrograph (DBSP) on the Palomar 200-inch telescope (P200), and the Spectral
Energy Distribution Machine (SEDM; Blagorodnova et al., 2018; Rigault et al.,
2019) on the Palomar 60-inch (P60) telescope. Observations using the P60 and
P200 were coordinated using the FRITZ data platform (van der Walt et al.,
2019; Coughlin et al., 2023). The spectra were reduced in a standard manner,
using LPipe (Perley, 2019), DBSP_DRP (Mandigo-Stoba et al., 2022) and PypeIt
(Prochaska et al., 2020a, b, 2020), for Keck/LRIS, P200/DBSP, and NOT/ALFOSC,
respectively, and SEDM spectra were reduced using the automated pipeline
pysedm (Rigault et al., 2019; Kim et al., 2022).

A single UV spectrum was obtained using the Cosmic Origins Spectrograph (COS)
onboard the Hubble Space Telescope (HST) on 2022-04-11 (Program ID: 16931, PI:
Yan) and is presented in Fig. 8. This spectrum is low S/N (upper panel of Fig.
8), however when a smoothing filter is applied, we recover interstellar
absorption bands (see Haser et al., 1998, and references therein), and several
emission lines at the redshift of SN 2021adxl. Most features seen in the lower
panel of Fig. 8 likely arise due to interstellar absorption within the MW,
although we make tentative detections of N iv], O iii], and He ii, as well as
Ly$\alpha$, at the redshift of SN 2021adxl.

Figure 8: HST/COS UV spectrum of SN 2021adxl taken with the G130M (FUV) and
G160M (NUV) grism on 2022-04-11 (+158d). Upper panel shows the (error)
spectrum given in (grey) black. Lower panel gives the science spectrum
smoothed using a Savitzky–Golay (Savgol) filter with window size equal to 31
with a 2nd order polynomial. We include expected wavelengths for both Milky
Way (MW) (blue) and SN 2021adxl plus its host (Green), although the detection
of some of these lines are unclear due to the low S/N of the science spectrum.

A near UV spectrum was obtained on 2022-04-11 (Program ID: 16931, PI: Yan)
using the Space Telescope Imaging Spectrograph (STIS) abroad HST and is
presented in Fig. 9. The spectrum’s appearance does show some continuum flux,
likely from SN 2021adxl, with a single broad feature at $\sim 2758\AA$. While
this may be broad/blended Mg II $\lambda\lambda 2796,2803$ (albeit offset by
$\sim$40$\AA$), it is also a possible a blend of several host/transient lines
with superimposed interstellar lines, see inset of Fig. 9. The UV appearance
of SN 2021adxl and its host is further discussed in Sect. 5.1.

Figure 9: HST/STIS with the G230LB grism on 2022-04-11 (+158d). The original
science spectrum (grey) has been smoothed (black) using a Savgol filter with
window size equal to 11 with a 2nd order polynomial. In the inset we show a
close up of the broad feature around 2759$\AA$ and include expected
wavelengths for both interstellar (blue) and SN 2021adxl (green) lines.

We obtained four medium-resolution spectra with the X-shooter instrument
(Vernet et al., 2011) between 2022-02-26 and 2023-02-26 (observing program
108.2262; PI. R. Lunnan). 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. The integration
times were varied between 2400 and 3700s. The data were reduced following
Selsing et al. 2019. In brief, we first removed cosmic-rays with the tool
astroscrappy888https://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 (in the background limited case). The
individual rectified, wavelength- and flux-calibrated two-dimensional spectra
files were co-added using tools developed by J.
Selsing999https://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. Note,
to study the host galaxy, we reduced the data in nodding mode.

Figure 10 presents four epochs of spectroscopy taken with the VLT and the
X-shooter medium resolution spectrograph. Two spectra were obtained during
Phase 1 of the lightcurve, and two during Phase 2. The spectra show
contributions from both transient light, seen by the broader emission
profiles, as well as flux from the underlying host. At the time of writing, SN
2021adxl still dominates the flux at the location, and templates are currently
unobtainable in order to remove narrow emission components from the host.A log
of the available spectra is provided in Table 2 and the epochs of spectroscopy
are indicated in Fig. 2 as the vertical lines. All wavelength reports are
given in rest-frame unless otherwise stated.

Figure 10: VLT + X-shooter for SN 2021adxl taken at four different epochs. Each spectrum has been normalised with respect to H$\alpha$ and offset for clarity. Emission lines expected from the transient (black) are denoted by the vertical lines. Telluric absorption bands arising from atmospheric O2 and H2O are denoted by the $\oplus$ symbol. We shade in regions of high noise levels from our first two spectra for visual clarity.  Table 2: Spectropscopy log for SN 2021adxl. Phase with reference to discovery date [MJD 59521] Date | MJD | Phase (d) | Telescope | Instrument | Resolution ($\lambda/\Delta\lambda$) | Exposure Time (s)
---|---|---|---|---|---|---
2021-11-09 | 59527 | +6 | P60 | SEDM IFU | 100 | 2400
2022-02-02 | 59612 | +91 | P200 | DBSP | 4000 | 300
2022-02-03 | 59613 | +92 | P60 | SEDM IFU | 100 | 2400
2022-02-04 | 59614 | +93 | P60 | SEDM IFU | 100 | 2400
2022-02-05 | 59615 | +94 | Keck | LRIS | 300-5000 | 2400
2022-02-05 | 59615 | +94 | P60 | SEDM IFU | 100 | 2400
2022-02-06 | 59616 | +95 | P60 | SEDM IFU | 100 | 2400
2022-02-07 | 59617 | +96 | P60 | SEDM IFU | 100 | 2400
2022-02-09 | 59619 | +98 | P60 | SEDM IFU | 100 | 2400
2022-02-10 | 59620 | +99 | P60 | SEDM IFU | 100 | 2400
2022-02-11 | 59621 | +100 | P60 | SEDM IFU | 100 | 2400
2022-02-14 | 59624 | +103 | NOT | ALFOSC+Grism4 | 360 | 1800
2022-02-20 | 59630 | +109 | P60 | SEDM IFU | 100 | 2400
2022-02-26 | 59636 | +115 | VLT | X-shooter | 5400/8900/5600 | 2400/2458/2400
2022-02-26 | 59636 | +115 | P60 | SEDM IFU | 100 | 2400
2022-03-02 | 59640 | +119 | P60 | SEDM IFU | 100 | 2160
2022-03-09 | 59647 | +126 | P60 | SEDM IFU | 100 | 2400
2022-03-14 | 59652 | +131 | P60 | SEDM IFU | 100 | 2400
2022-03-21 | 59659 | +138 | P60 | SEDM IFU | 100 | 2400
2022-03-21 | 59659 | +138 | P60 | SEDM IFU | 100 | 2400
2022-03-23 | 59661 | +140 | P60 | SEDM IFU | 100 | 2400
2022-03-23 | 59661 | +140 | P60 | SEDM IFU | 100 | 2400
2022-03-28 | 59666 | +145 | P60 | SEDM IFU | 100 | 2400
2022-03-30 | 59668 | +147 | NOT | ALFOSC+Grism4 | 360 | 1800
2022-04-01 | 59670 | +149 | P60 | SEDM IFU | 100 | 2400
2022-04-11 | 59680 | +159 | HST | COS/STIS | 2000-3000 | 1911
2022-04-11 | 59680 | +159 | P60 | SEDM IFU | 100 | 2400
2022-04-18 | 59687 | +166 | P60 | SEDM IFU | 100 | 2160
2022-04-26 | 59695 | +174 | NOT | ALFOSC+ Grism4 | 360 | 1800
2022-05-02 | 59701 | +180 | VLT | X-shooter | 5400/8900/5600 | 3600
2022-05-15 | 59714 | +193 | P60 | SEDM IFU | 100 | 2160
2022-05-27 | 59726 | +205 | NOT | ALFOSC+Grism4 | 360 | 1800
2022-06-12 | 59742 | +221 | P60 | SEDM IFU | 100 | 2160
2022-11-09 | 59892 | +371 | P60 | SEDM IFU | 100 | 2400
2022-11-25 | 59908 | +387 | NOT | ALFOSC+Grism4 | 360 | 1200
2022-12-08 | 59921 | +400 | P60 | SEDM IFU | 100 | 1800
2023-01-18 | 59962 | +441 | VLT | X-shooter | 5400/8900/5600 | 1800/1860/1800
2023-02-26 | 60001 | +480 | VLT | X-shooter | 5400/8900/5600 | 3600/3716/3600
2023-03-23 | 60026 | +505 | NOT | ALFOSC+Grism4 | 360 | 3300
2023-04-16 | 60050 | +529 | NOT | ALFOSC+Grism4 | 360 | 3300

###  2.3 Hydrogen spectral evolution

The most prominent and informative features in the spectra of Type IIn SNe is
the Balmer series, and specifically the H$\alpha$ profile (e.g. Roming et al.,
2012; Ransome et al., 2021; Pursiainen et al., 2022). The H$\alpha$ emission
line has been used to constrain the progenitor wind velocity (Kankare et al.,
2012; Andrews et al., 2017; Chugai, 2019), explosion geometry (Hoffman et al.,
2008; Andrews & Smith, 2018; Pursiainen et al., 2022), and can potentially
indicate late-time interaction (Silverman et al., 2013; Yan et al., 2015).

Type IIn SNe typically develop some asymmetries in their H$\alpha$ emission
line profiles in the post peak evolution (Trundle et al., 2009; Fransson et
al., 2014; Andrews et al., 2017, 2019; Moriya et al., 2020; Taddia et al.,
2020; Pursiainen et al., 2022), and there are also cases where multiple
absorption troughs are observed (Gutiérrez et al., 2017; Brennan et al.,
2022). As noticed by De et al. (2022), the NIR classification spectrum for SN
2021adxl shows strong signs of hydrogen and helium emission. The insets of
Fig. 7 give a close-up view of the Pa-$\beta$ line, which is the best isolated
hydrogen emission feature. In agreement with the initial classification, we
resolve two apparent absorption features for the Pa-$\beta$ line, one with a
trough centered around $\sim$3500 km s-1 (right inset) and the second at
$\sim$200 km s-1 (upper left inset). Although the latter has low S/N, a
consistent absorption feature is seen also in other Paschen lines.

We observe a P-Cygni profile at the approximate wavelength of Ly$\alpha$ in
Fig. 8. The emission of Ly$\alpha$ is redshifted by $\sim\leavevmode\nobreak\
100$ km s-1, with a blueshifted absorption centered at
$\sim\leavevmode\nobreak\ -100$ km s-1 (with respect to rest wavelength).
Although the appearance of Ly$\alpha$ for SN 2021adxl is in contrast to the
profile seen for SN 2010jl (Fransson et al., 2014), with SN 2021adxl showing a
P-Cygni profile, and SN 2010jl showing a purely absorption profile. This
redshifted profile for SN 2021adxl may be a result of electron scattering
(Huang & Chevalier, 2018). As this spectrum is obtained $\sim 5$ months post
discovery, the P-Cygni absorption likely originates from the pre-supernova
stellar wind, located at large distances from the ejecta-CSM interface. This
is consistent with the absorption seen earlier in Fig. 7. This would also
suggest that SN 2021adxl has a very extended, dense circumstellar environment,
which is likely correlated with the slow evolution observed in Fig. 2.
However, we can speculate that the strange Ly$\alpha$ emission profile is a
result of the host enviroment itself. We discuss this further in Sect. 3

Figure 11: Evolution of the H$\alpha$ profile from the VLT/X-shooter
observations. A pseudo-continuum has been removed from each profile, and the
spectra are normalised and offset for clarity. Phase given in grey (red)
represents spectra obtained during (after) Phase 1. A noise spike at 6852$\AA$
was manually removed from the +115 d spectrum.

Figure 11 highlights the evolution of the H$\alpha$ profile from the
VLT/X-shooter observations. The H$\alpha$ profile consists of multiple
distinct components, giving rise to an overall asymmetric appearance. A
prominent blue-shifted emission component is observed in H$\alpha$ and in
other Balmer lines. This feature is not obviously seen in other emission
features, for example in He I or Fe II, which suggests that the physical
parameters forming this component are unique to a hydrogen-rich shell of
material. This broad component does not extend to redder wavelengths, but the
rather smooth line wings are indicative of electron scattering (Huang &
Chevalier, 2018). From the earliest X-shooter spectra, a third broad component
can be resolved around rest wavelength with a superimposed narrow emission
line likely arising from slow-moving CSM, an underlying H II region, or a
combination of both.

As SN 2021adxl evolves, we see a smooth evolution of the H$\alpha$ profile,
most notably of the blueward component. The upper panel of Fig. 12 shows the
complete optical spectral dataset (excluding the low-resolution P60+SEDM
spectra). We mark the point where the flux of H$\alpha$ exceeds 10% of the
pseudo-continuum with a red cross. This component shows a smooth evolution
during Phase 1, see lower panel of Fig. 12. During Phase 2 it is unclear if
the H$\alpha$ velocity continues to follow this trend as it apparently shows a
quicker decline in velocity.

Figure 12: Same as Fig. 11 but including the complete optical spectral dataset
(excluding those from the P60/SEDM). A red cross on each profile denotes the
maximum velocity of the blue excess for each spectrum. The lower panel gives
the velocity of the bluest edge of the H$\alpha$ profile, marked by the red
cross in the upper panel. Figure 13: Comparison of the H$\alpha$ profile from
SN 2021adxl, SN 2010jl (Fransson et al., 2014), and SN 2013L (Andrews et al.,
2017; Taddia et al., 2020). Each transient shows a blue shoulder atop their
respective H$\alpha$ profile, as well as broad wings. Each profile has been
normalised and offset for clarity.

Compared to photometrically similar objects, SN 2021adxl shows qualitative
similarities to the Type IIn SNe 2010jl (Fransson et al., 2014), 2013L
(Andrews et al., 2017; Taddia et al., 2020) as well as SN 2005ip (Fox et al.,
2020). Figure 13 compares the H$\alpha$ profile of SN 2021adxl, SN 2010jl, and
SN 2013L. Each transient shows a smooth redward wing, likely a result of
electron scattering, as well as a blueward excess that evolves with time. It
should be noted that while these three transients are qualitatively similar
photometrically and spectroscopically, the photometric decline times and the
emission line amplitude and offset are different among these events, and this
may be due to differences in mass, velocity, and distribution of the emitting
material. This might also be reflected in the different lightcurves, see Fig.
3. Insights can be gained by noting the +115d spectrum of SN 2021adxl compared
to a spectrum of SN 2010jl at a similar epoch (+94d). SN 2021adxl shows flux
out to 8000 km s-1 while SN 2010jl only shows a slight blue excess out to 2000
km s-1, however this excess lines up quite well with the central wavelengths
of the H$\alpha$ profile at this time. Whereas the H$\alpha$ profile of SN
2013L at this time matches the red wing of SN 2021adxl very well, and shows a
much more dramatic flat-topped blue excess that persists for almost 4 years
post peak (Andrews et al., 2017). Comparing the photometric evolution, SN
2013L shows a rapid decline before plateauing, whereas SN 2010jl is more
similar to SN 2021adxl, albeit declining at a slower rate.

###  2.4 Helium spectral evolution

Aside from hydrogen, the spectrum of SN 2021adxl is dominated by strong Helium
emission features, such as He I $\lambda 4471$, $\lambda 5876$, $\lambda 7065$
and $\lambda 10830$, and possibly He I $\lambda 2773$ (see Fig. 9). In stark
contrast to the H$\alpha$ line, the profile of He I $\lambda 5876$ resembles a
P-Cygni profile. Figure 14 provides a cut-out around He I $\lambda 5876$,
highlighting the strange P-Cygni-like profile with an absorption feature at
$\sim$4000 km s-1.

Rather than the typical single absorption (e.g. Israelian & de Groot, 1999),
He I $\lambda 5876$ might show a second high velocity (HV) feature, extending
out to $\sim$18000 km s-1. It is unclear whether this HV absorption is indeed
associated with He I or a feature at bluer wavelength e.g. the Na I D doublet
$\lambda\lambda$5890, 5896, which is blended with the He I $\lambda 5876$
line. If this is this case, a large amount of sodium-rich material must remain
optically think more than 500 days post peak, and continue moving at very high
velocities. In comparison to SN 2010jl, this double troughed absorption
profile is not immediately obvious. In particular, the He I $\lambda 5876$
observed for SN 2010jl resembles the H$\alpha$ profile e.g. see Fig. 2.5

Figure 14: Evolution of He I $\lambda 5876$ line. Each profile is continuum-
subtracted. A double Gaussian absorption profile is fitted to the blue side of
each spectrum, and is given by the dashed black line. We do not attempt to
include the components in emission but note that the emission feature at rest
wavelength narrows with time.

An alternative suggestion is that this HV material is associated with the He I
$\lambda 5876$ line, although this feature is not clearly observed in other He
I lines, due to severe line blending and/or tellurics bands, see Fig. 10.
Seeing both absorption components would require an asymmetric explosion/CSM in
order to see both the low and high velocity component, although maintaining a
P-Cygni -like absorption profile after 500 days is also difficult to explain.

Figure 15:  Comparison of SN 2021adxl to the Type IIn SN 2010jl (Fransson et
al., 2014), as well as the transitional Type Ic-BL/IIn SN 2017ens (Chen et
al., 2018). The insets highlight the wavelength range around known coronal
emission lines. Each inset has been normalised to the peak of each respective
emission line. Interacting transients are expected to produced a significant
X-ray flux, which can be manifested in coronal line emission. Although these
observations are rare (due to the late of high resolution spectra at late
time), high-ionisation emission feature may be common in late time spectra of
interacting SNe.

###  2.5 X-ray emission from Interacting SNe

When the progenitor of SN 2021adxl exploded, a shock wave was formed due the
collision of the fast moving ejecta and the slower moving CSM. X-ray emission,
if detected, can be used to diagnose the shock wave interaction with the
circumstellar medium (Chevalier & Fransson, 2003; Dwarkadas, 2019). We
reported the detection of a decaying X-ray flux at the position of SN 2021adxl
(see Table. 1). This evolving X-ray is likely associated with SN 2021adxl and
not the underlying source itself. We find the soft X-ray flux decreases from
$\sim 3\times 10^{40}\leavevmode\nobreak\ {\rm ergs\leavevmode\nobreak\
s^{-1}}$ at +113d to $\sim 0.9\times 10^{40}\leavevmode\nobreak\ {\rm
ergs\leavevmode\nobreak\ s^{-1}}$ at +160d. This flux is roughly a factor of
10 less than SN 2010jl (Chandra et al., 2015). However, meaningful comparison
is moot due to the restricted X-ray cadence.

High ionization lines have been observed the optical spectra of nearby active
galactic nuclei (AGNs) (Oke & Sargent, 1968; Appenzeller & Oestreicher, 1988;
Lamperti et al., 2017), however they are rarely observed in SNe (Turatto et
al., 1993; Smith et al., 2009a), likely due to the lack of high to medium
resolution spectra (Gröningsson et al., 2006; Komossa et al., 2009; Smith et
al., 2009b; Pastorello et al., 2015; Fransson et al., 2014). These so-called
“coronal lines”, first observed in the solar corona, arise from collisionally
excited forbidden fine-structure transitions in highly ionized species such as
Fe VI and Fe X.

Figure 15 shows our final X-shooter spectrum taken on 2023-02-26 with insets
showing the location of several coronal emission lines. In terms of explosive
transients, coronal lines have been interpreted in terms of dissipation of the
energy of a shock wave in the circumstellar envelope. If collisional
ionization is the dominant mechanism, then the presence of these lines implies
a temperature of the emitting material of $10^{5}-10^{6}$ K (Bryans et al.,
2009) and pre-shock ionization of the circumstellar medium by X-ray emission.
For SN 2021adxl, we do not detect [Fe XIV] $\lambda 5303$ (see inset in Fig.
15), while [Fe X] $\lambda 6375$ is strong meaning the CSM temperature is
likely $\lesssim 2\times 10^{6}$ K (Jordan, 1969; Turatto et al., 1993).

##  3 Host environment of SN 2021adxl

SN 2021adxl exploded in a blue luminous host, composed of a bright star-
forming environment in the north-east, and an extended diffuse tail to the
south-west (see Fig. 16). This unique morphology is coined a “tadpole”
(Elmegreen et al., 2012). The blue compact northern region implies a high
star-formation rate (SFR) (Botticella et al., 2012; Binder & Povich, 2018),
and is a likely location for massive stars and a place where CCSNe occur.

Figure 16: Composite ($gri$) color image of the host galaxy of SN 2021adxl
obtained from the DESI Legacy Surveys (Dey et al., 2019). SN 2021adxl (not
visible in this image) exploded in the North-East part of its host, in a
bright star-forming area. The inset provides a zoom-in of the explosion site,
where the position of SN 2021adxl is marked by the crosshair.

To further understand the host environment of SN 2021adxl, the software
package Prospector version 1.1 (Johnson et al., 2021a) was used to analyse the
spectral energy distribution from the host.

We retrieved science-ready coadded images from the Galaxy Evolution Explorer
(GALEX) general release 6/7 (Martin et al., 2005), the Panoramic Survey
Telescope and Rapid Response System (Pan-STARRS, PS1) DR1 (Chambers et al.,
2016b) and images from the Wide-Field Infrared Survey Explorer (WISE; Wright
et al., 2010) processed by Lang 2014. We measured the brightness of the host
using LAMBDAR101010https://github.com/AngusWright/LAMBDAR (Lambda Adaptive
Multi-Band Deblending Algorithm in R; Wright et al., 2016) and the methods
described in Schulze et al. 2021. We also extract the photometry of the
isolated star-forming region, in which the SN occurred.Table 3 the
measurements in the different bands, and Table 4 focuses on the bright star
forming knot.

Instrument/Filter | Brightness
---|---
GALEX/FUV | $17.71\pm 0.07$
GALEX/NUV | $17.75\pm 0.03$
PS1/g | $17.26\pm 0.05$
PS1/r | $17.21\pm 0.06$
PS1/i | $17.11\pm 0.04$
PS1/z | $17.12\pm 0.34$
PS1/y | $17.19\pm 0.05$
WISE/W1 | $18.13\pm 0.10$
WISE/W2 | $18.50\pm 0.20$
Table 3: Photometric measurements of the host of SN 2021adxl. All magnitudes are in the AB system and not corrected for extinction. Instrument/Filter | Brightness
---|---
GALEX/FUV | $18.00\pm 0.08$
GALEX/NUV | $18.11\pm 0.05$
PS1/g | $17.97\pm 0.01$
PS1/r | $18.14\pm 0.01$
PS1/i | $17.93\pm 0.01$
PS1/z | $17.95\pm 0.03$
PS1/y | $18.12\pm 0.02$
Table 4: Photometry of the isolated star-forming region where SN 2021adxl
exploded. All magnitudes are in the AB system and not corrected for
extinction.

We model the observed spectral energy distribution (black data points in
Figure 18) with the software package Prospector version 1.1 (Johnson et al.,
2021b).111111Prospector uses the Flexible Stellar Population Synthesis (FSPS)
code (Conroy et al., 2009) to generate the underlying physical model and
python-fsps (Foreman-Mackey et al., 2014) to interface with FSPS in python.
The FSPS code also accounts for the contribution from the diffuse gas based on
the Cloudy models from Byler et al. (2017). We use the dynamic nested sampling
package dynesty (Speagle, 2020) to sample the posterior probability. We assume
a Chabrier IMF (Chabrier, 2003) and approximate the star formation history
(SFH) by a linearly increasing SFH at early times followed by an exponential
decline at late times [functional form $t\times\exp\left(-t/\tau\right)$,
where $t$ is the age of the SFH episode and $\tau$ is the $e$-folding
timescale]. The model is attenuated with the Calzetti et al. 2000 model. The
priors of the model parameters are set identically to those used by Schulze et
al. 2021. Figure 18 shows the observed SED (black data points) and its best
fit (grey curve). The SED is adequately described by a galaxy template with a
mass of $7.80^{+0.48}_{-0.32}\leavevmode\nobreak\ M_{\odot}$, a star-formation
rate of $0.19^{+0.07}_{-0.07}\leavevmode\nobreak\ M_{\odot}\,{\rm yr}^{-1}$.
The mass and the star-formation rate are comparable to common star-forming
galaxies of that stellar mass (grey band in Figure 17; Elbaz et al., 2007),
and they are also similar to those of the host galaxy populations of SNe IIn
and SLSNe-IIn from the PTF survey (Schulze et al., 2021) albeit in the lower
half of the mass distribution. In the same figure, we also show the location
of the star-forming knot where the SN exploded in a lighter shade. This region
has a somewhat lower specific star-formation rate (i.e., star-formation rate
normalised by galaxy mass) than of the entire galaxy but consistent within
errors.

Figure 17: The star-formation rate and stellar mass of the host galaxy of SN
2021adxl (dark red) and of the bright star-forming region where the SN
exploded (light shade) in the context of SN-IIn and SLSN-IIn host galaxies
from the PTF survey (Schulze et al., 2021). The host galaxy of SN 2021adxl
lies in the expected parameter space of SN-IIn and SLSN-IIn host galaxies but
in the lower half of the mass and SFR distributions. Its specific star-
formation rate (SFR / mass) is slightly larger than the typical star-forming
galaxies (grey band) but lower than for an average SLSN host galaxy.

(a) Flux from the host, including the flux from the bright head and the
diffuse tail.

(b) Flux from diffuse tail to the south-west.

(c) Flux from bright star forming head to the north-west.

Figure 18: Spectral energy distribution of the host galaxy from 1000 to 60,000
Å (black dots). 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. We perform measurements on the entire host flux (upper
panel), the faint tail (middle panel) and the star forming blob (lower panel)
where SN 2021adxl exploded.

We focus on the narrow emission lines and the auroral emission lines
associated with the underlying host (see review by Kewley et al., 2019, and
references therein). The final X-shooter spectrum is utilised due to its long
exposure time, and therefore higher S/N, although many of these diagnostic
lines are present in all X-shooter spectra. As seen in Fig. 26, the late time
X-shooter spectra contain numerous narrow emission lines that are typically
associated with H II regions. These narrow lines can provide insight into the
metallicity (Z), the amount of dust, the electron temperature (${\rm T_{e}}$)
and density (${\rm n_{e}}$), the age of the nebula, and the rate of star
formation. However transient flux is still clearly present in our latest
X-shooter spectrum. We extract a trace in the 2D spectrum from a region offset
from SN 2021adxl in order to minimise the contamination from SN 2021adxl,
while still detected weak lines needed for abundance measurements, given in
Fig. 19. A single narrow Gaussian emission line is fitted to each emission
feature, while simultaneously fitting a pseudo-continuum. This offset spectrum
still displays a weak offset broad component seen in H$\alpha$, so the
following measurements (i.e those that rely on hydrogen emission lines) may be
slightly overestimated due to transient contamination. Investigating the
Balmer decrement, we find H$\alpha$/H$\beta$ $\approx 2.5\pm 0.2$ and
H$\gamma$/H$\beta$ $\approx 0.47\pm 0.07$, consistent with negligible
interstellar reddening, as would be expected form the blue appearance of the
surrounding environment, see Fig. 16.

Figure 19: Spectrum extracted from a region offset from the X-shooter spectrum
taken on 2023-02-26. Emission lines used in metallicity, $T_{e}$ and $n_{e}$
measurements are marked in red. We note transient flux is still present in the
spectrum, as is seen by the broad appearance of H$\alpha$ and H$\beta$.
Figure 20: Emission-line temperature and density diagnostic plot from PyNeb
using [O III] $\lambda 4363/\lambda 5007$ as a temperature sensitive probe,
and [O II] $\lambda 3726/\lambda 3729$, and [S II] $\lambda 6731/\lambda 6716$
as density-sensitive probes.

We measure values for $T_{e}$ and $n_{e}$ using
PyNeb121212http://research.iac.es/proyecto/PyNeb/ (Luridiana et al., 2014)
using [O III] $\lambda 4363/\lambda 5007$ as a temperature sensitive probe,
and [O II] $\lambda 3726/\lambda 3729$, and [S II] $\lambda 6731/\lambda 6716$
as density-sensitive probes, see Fig. 20. Encouragingly, we find similar
results as for the abundance indicators above with $T_{e}\approx
16000\leavevmode\nobreak\ K$, $n_{e}\approx 55\leavevmode\nobreak\ $cm-3.
Using [O II] and [O III], we can constrain $T_{e}$ and $n_{e}$ well, whereas
the $n_{e}$ is not well constrained by [S II], again likely due to the low
density environment limit of 1.5 (Osterbrock & Ferland, 2006). We search for
other features to use within PyNeb, but found no other significant diagnostic
lines. These may be due to the high ionising host flux, inhibiting these
emission lines, or due to the low inferred densities. Using PyNeb, we find a
metallicity of 12 + log((O$+$ \+ O$++$/H) = 7.61 dex (0.08 $Z_{\odot}$),
taking the solar value to be ${\rm 12+\log({\rm O/H})_{\odot}=8.69}$ dex
(Asplund et al., 2021) and assuming the O ii temperature dependence from
Stasińska 1982.

To compare the explosion site with those of other SNe IIn, we also investigate
abundance indicators using the narrow H$\alpha$ and H$\beta$ components, such
as the commonly adopted N2 and O3N2 techniques (Pettini & Pagel, 2004). These
methods use the [N II]$\leavevmode\nobreak\ \lambda\leavevmode\nobreak\ 6583$
emission line which is detected in our +480d X-shooter spectrum, but has a low
$S/N\lesssim 5$, and has complex underlying flux, due to the broad H$\alpha$
profile. Although there is some scatter in the derived metallicities, all
methods strongly point towards a very low metallicity environment of
$0.05-0.2\leavevmode\nobreak\ Z_{\odot}$.

Tables 5 and 6 give the results from the various abundance methods to
determine the metallicity of the underlying environment around SN 2021adxl.
Similar results (i.e. low metallicity) are found using the transient dominated
spectrum from 2023-02-26 (e.g. Fig. 26), although to irrefutably confirm the
metallicity of the host, we require late time spectral templates once SN
2021adxl has faded.

Table 5: Measured parameters for several emission lines in the VLT/XShooter spectrum taken on 2023-02-26 (+480d). Flux has been corrected for MW extinction. Errors for each measurement are given in parentheses. Errors on the Equivalent Width (EW) are from Cayrel (1988, their Eq. 6). Note for the Hydrogen emission lines, we measure the flux from the narrow component only. | Flux [${\rm erg\leavevmode\nobreak\ s^{-1}\leavevmode\nobreak\ cm^{-2}}$] | FWHM [${\rm km\leavevmode\nobreak\ s^{-1}]}$ | EW [${\rm\AA]}$
---|---|---|---
${\rm H\leavevmode\nobreak\ \lambda 4340}$ | 3.834 (0.020) $\times 10^{-16}$ | 36.408 (0.164) | -12.790 (0.003)
${\rm H\leavevmode\nobreak\ \lambda 4861}$ | 8.351 (0.019) $\times 10^{-16}$ | 37.061 (0.156) | -19.417 (0.002)
${\rm H\leavevmode\nobreak\ \lambda 6563}$ | 2.047 (0.005) $\times 10^{-15}$ | 27.633 (0.182) | -14.439 (0.001)
${\rm[Ar\leavevmode\nobreak\ III]\leavevmode\nobreak\ \lambda 7135}$ | 2.246 (0.093) $\times 10^{-17}$ | 20.292 (0.332) | -2.494 (0.028)
${\rm[N\leavevmode\nobreak\ II]\leavevmode\nobreak\ \lambda 6583}$ | 1.495 (0.245) $\times 10^{-17}$ | 19.677 (0.626) | -0.159 (0.109)
${\rm[O\leavevmode\nobreak\ III]\leavevmode\nobreak\ \lambda 4363}$ | 6.503 (0.199) $\times 10^{-17}$ | 34.855 (0.327) | -2.242 (0.020)
${\rm[O\leavevmode\nobreak\ III]\leavevmode\nobreak\ \lambda 5007}$ | 3.079 (0.005) $\times 10^{-15}$ | 34.975 (0.202) | -139.444 (0.001)
${\rm[O\leavevmode\nobreak\ II]\leavevmode\nobreak\ \lambda 3726}$ | 4.396 (0.043) $\times 10^{-16}$ | 37.360 (0.158) | -13.415 (0.006)
${\rm[O\leavevmode\nobreak\ II]\leavevmode\nobreak\ \lambda 3729}$ | 6.253 (0.087) $\times 10^{-16}$ | 35.613 (0.209) | -19.163 (0.009)
${\rm[S\leavevmode\nobreak\ III]\leavevmode\nobreak\ \lambda 9069}$ | 5.686 (0.095) $\times 10^{-17}$ | 22.275 (0.157) | -8.518 (0.011)
${\rm[S\leavevmode\nobreak\ II]\leavevmode\nobreak\ \lambda 6716}$ | 7.301 (0.119) $\times 10^{-17}$ | 24.933 (0.558) | -6.277 (0.011)
${\rm[S\leavevmode\nobreak\ II]\leavevmode\nobreak\ \lambda 6731}$ | 5.263 (0.102) $\times 10^{-17}$ | 24.194 (0.472) | -4.766 (0.013)
Table 6: Metallicity measurements for SN 2021adxl using verious abundance indictures found in literature. | ${\rm 12+log(O/H)}$ | ${\rm Z/Z_{\odot}}$
---|---|---
PyNeb | 7.61 | 0.08
N2 | 7.76 | 0.12
O3N2 | 7.95 | 0.18
Figure 21: A 4” $\times$ 4” cutout from the HST/STIS acquisition images from
2022-04-11 (+159 d) including SN 2021adxl as well as the compact bright star
forming knot. SN 2021adxl is separated from this compact knot by roughly half
an arcsecond, or a projected distance of $\sim$0.2 kpc.

##  4 Modelling the H$\alpha$ profile

In order to understand the formation of the hydrogen emission lines, we note
two distinct features, one is the offset flat topped emission profile, which
suggests of a thin emitting shell (Jerkstrand, 2017) and the second is the
red-ward, extended wings, which are characteristic of electron scattering
(Huang & Chevalier, 2018).

The collision of the fast moving ejecta with the slower moving CSM results in
the formation of a dense shell of material. This shell is made up of swept-up,
shock-heated gas from the circumstellar medium and can efficiently cool to
relatively low temperatures compared to the high-energy, shock-heated gas
found in other parts of the supernova ejecta. This cool dense shell (CDS) of
material is bound between the outward moving forward shock and receding
reverse shock (Chugai et al., 2004; Smith, 2017a). Diffusion of radiation from
this shocked shell produces the main continuum (e.g. red line of Fig. 6) as
well as the intermediate ($\sim 10^{3}$ km s-1) components of H$\alpha$.

Under the assumption that the H$\alpha$ profile arises from photons emitted
from the CDS that undergo multiple scattering events as they travel outwards
through the surrounding media, a model of electron scattering is explored. A
Monte Carlo code was developed131313https://github.com/Astro-Sean/escatter in
order to follow photons emitted from a thin shell as they travel through a
diffuse medium. The code is based on the work by Pozdnyakov et al. 1983 and
similar codes have been employed in previous works on interacting SNe (e.g.,
Fransson et al., 2014; Taddia et al., 2020). In this model, photons are
emitted with an emissivity $\eta$, which varies with the electron density as
$\propto n_{e}^{2}$ and $n_{e}\propto r^{-2}$. Photons are followed until they
escape the scattering medium, with a probability $\propto e^{-\tau_{e}}$. We
include an occulting photosphere, meaning that any escaping photons moving
away from the observer are excluded. The optical depth, electron temperature,
and radius are degenerate in these electron scattering models (see Huang &
Chevalier, 2018, for details), and are left as restricted parameters. While
this is nonphysical for a SN explosion as these parameters evolve with time,
the models can be used to understand the formation of the H$\alpha$ profile,
as well as to further understand the main properties of the SN and, in
particular, the shock velocity ($V{\rm{}_{shock}}$) and mass-loss rate
($\dot{M}$).

Figure 22 shows the first epoch X-shooter spectrum with the best fitting
electron scattering model. The model matches the spectrum well, capturing the
blue shoulder as well as the extended red wing. However, the model fails to
capture the features around rest wavelength. This likely represents emission
from slowly moving material surrounding the SN ejecta, as well as emission
from the host.

Figure 22: H$\alpha$ profile from the VLT/X-shooter spectrum (black) with the
best fitting electron scattering model (red) for a given input spectrum
(blue). The model matches the blue and red sides of the emission line well,
but leaves an intermediate and narrow emission component at the central
wavelength.  Figure 23: Same as Fig. 22, but now including an additional
intermediate and narrow Gaussian emission component. These three components
can reproduce the entire H$\alpha$ profile well.

Similar to Fig. 22, Fig. 23 shows the best fitting model for the earliest
X-shooter spectrum, but with additional two Gaussian components, which likely
reflect emission from the host and from CSM material far away from the shock
(which we do not include in our models). As demonstrated in Fig. 12, the
H$\alpha$ profile evolves with time, and most notably, its blueward component
moves toward the central wavelength. Using a grid of electron scattering
models with an input spectrum representing a thin emitting shell, we are able
to match the best fit model to the H$\alpha$ profile for each spectrum
(excluding the SEDM spectra due to their low resolution) to better understand
how this blueward feature evolves. Figure 24 gives the results of the
evolution of the velocity needed to produce the H$\alpha$ profile at each
epoch. During Phase 1, the velocity evolution is well fitted by:

Figure 24: Input velocities of the best-fitting models needed to fit the
H$\alpha$ profile evolution. Velocities during Phase 1 are fitted with an
exponential decline, given in red. During Phase 2, the velocities are lower
and no longer follow the trend seen in Phase 1. This is likely due to the
forward shock becoming optically thin, which is also seen in the faster decay
rate in the bolometric lightcurve seen in Fig. 3. The errorbar in the upper
right denotes the velocity grid-resolution of the input spectra.

$\rm{V_{shock}}=6625\times\left[\frac{t}{100\leavevmode\nobreak\
\rm{days}}\right]^{-0.37}\leavevmode\nobreak\ \rm{km\leavevmode\nobreak\
s^{-1}}.$ (1)

At the beginning of Phase 2, the model velocities are much lower and no longer
follow Eq. 1. Models fitted in Phase 2 show more scatter in their inferred
velocity, and one should note the additional complexity of the H$\alpha$
profile at this time as shown in Fig. 12.

###  4.1 Modelling the ejecta-CSM interaction

The dominating energy input during the evolution of Type IIn SNe is due to the
interaction between the SN ejecta and the surrounding CSM (Dessart et al.,
2015). The diversity of the mass, velocities, and geometry of this interacting
material are likely responsible for the heterogeneous appearance of this
subclass (Chatzopoulos et al., 2012; Nyholm et al., 2020; Khatami & Kasen,
2023). The total luminosity generated by ejecta-CSM interaction can be high
because a radiative shock is a very efficient engine to convert kinetic energy
into radiation at optical wavelengths. The luminosity of the CSM interaction
is dependent on the velocity at which CSM enters the forward shock and on the
progenitor’s mass-loss rate ($\dot{M}$), and is typically given by:

$\rm{L_{shock}=2\pi\rho_{CSM}r_{shock}^{2}V_{shock}^{3}=\epsilon\frac{1}{2}\frac{\dot{M}}{v_{wind}}V_{shock}^{3}\leavevmode\nobreak\
{\rm erg\leavevmode\nobreak\ s^{-1}}}$ (2)

assuming a steady mass-loss rate for the CSM (Wood-Vasey et al., 2004) and a
50% conversion efficiency ($\epsilon=0.5$). It is often difficult to determine
the shock velocity from the line profiles given the uncertainty to which
extent electron scattering plays a role in determining the widths of the
intermediate lines (see discussion by Smith, 2017a). SN 2021adxl offers a
unique test to measure the degree to which electron scattering plays a role,
as discussed in Sect. 4, and therefore to gain insight into the ejecta-CSM
interaction.

Figure 25: Pseudo-bolometric lightcurve of SN 2021adxl. Best fitting model
from Eq. 2 fitting to Phase 1 photometry (spectra) is given as the blue solid
(dashed) blue line. Included in the inset is a zoom in of this phase. Both a
linear decline (dashed green) and possible shock luminosity curve (solid
orange) is fitted to Phase 2. The magenta line is the decay expected for
radioactive nickel, fitted to Phase 2, as discussed in Sect. 4.1.

Using Eqs. 1 and 2, we fit the Phase 1 portion of the lightcurve with the
mass-loss rate $\dot{M}$ as a free parameter. The CSM velocity is taken to be
$\sim$250 km s-1, as seen in the earliest spectrum, see Fig. 7, as well as the
HST/COS spectrum given, see Fig. 8. As discussed in Sect. 4, the blue
component of the H$\alpha$ profile is likely caused by line scattering from a
somewhat optically thin CDS. One interpretation of this is that photons are
emitted from the outer regions of the CDS, and while travelling radially
outwards, scatter off material swept up by the forward shock.

It is important to note that the measurements of the shock velocity as
provided in Eq. 1 are not based on the photometric evolution, but rather model
fitting of the H$\alpha$ profile. The exponent for the shock velocity
evolution in Eq. 2 is responsible for the slope, and it is encouraging that
the resulting fit in Fig. 25 matches the bolometric evolution so well.
Equations 1 and 2 fits the luminosity of Phase 1 but this does not extend to
Phase 2, where a faster, apparently linear, decline is seen. Extrapolating
both trends, we expect a break around +300 d. Following Eq. 2, a steady state
mass-loss rate of $4-6\times 10^{-3}$ M⊙ yr-1 is required to provide the
necessary luminosity if powered solely by shock interaction. Additionally, it
is possible to fit a second decline similar to Eq. 2 to Phase 2 with a quicker
velocity decline with an exponent of $\approx-1.3$, however it is difficult to
constrain this.

The use of Eq. 2 is likely an oversimplification but provides an estimate to
the the mass-loss rate from the progenitor shortly before the SN explosion. A
mass-loss rate of $10^{-3}$ M⊙ yr-1 is consistent with reported values for
other interacting SNe (Taddia et al., 2013; Ofek et al., 2014; Fransson et
al., 2014; Moriya et al., 2014, 2023) as well as the values often assumed for
Luminous Blue Variable (LBV) progenitors (Trundle et al., 2008; Dwarkadas,
2011; Groh et al., 2013; Smith, 2017b; Weis & Bomans, 2020) as discussed
further in Sect. 5.3.

As shown in Fig. 2, SN 2021adxl undergoes a steeper decline after Phase 1 and
the trend seen in Fig. 24 may be related, perhaps reflecting a change in CSM
density, distribution, or opacities. If we assume the ejecta-CSM interaction
ceases when the shock reaches the edge of the dense CSM at a time ${\rm
t_{end}}$, we can estimate the radial extent of this material (${\rm
d_{CSM}}$) using:

${\rm d_{CSM}=\int_{t_{start}}^{t_{end}}(t-t_{start})\times
V_{shock}(t)\,dt},$ (3)

where ${\rm t_{start}}$ is the time at which the SN ejecta collides with the
CSM, launching the forward shock into the dense CSM, and we use ${\rm
V_{shock}(t)}$ from Eq. 1. We do not have a constraint on the explosion date
so we can not be certain on the duration of CSM interaction.

We make an conservative estimation of ${\rm d_{CSM}}$ by assuming that ${\rm
t_{start}=100}$ days, ${\rm t_{end}=300}$ days and ${\rm t_{duration}\approx
200}$ days (although this value may be a factor of 2 or more greater), the
timeframe for which Eq. 1 is likely to be valid i.e the duration of Phase 1.
This gives an approximate size of ${\rm d_{CSM}}$ of $8\times 10^{17}$ cm
(this is more than an order of magnitude larger that that reported for SN
2010jl Dwek et al., 2021, for example ) meaning that the CSM travelling at
$\sim 250\leavevmode\nobreak\ $ km s-1 would have been ejected in the last
$\sim$1000 years. We can set a lower limit on the CSM mass in Phase 1,
assuming a constant $\dot{M}$ $=4\times 10^{-3}$ M⊙ yr-1, homologous
distribution, and spherical symmetry, as ${\rm
M_{CSM}\gtrapprox\leavevmode\nobreak\ 4.3}$ M⊙. Additionally this mass will be
substantially higher if we have overestimated ${\rm V_{wind}}$ or
underestimated ${\rm t_{start}}$ and ${\rm t_{end}}$, although this is hard to
quantify as we are uncertain how ${\rm V_{shock}}$ evolves for $t\lesssim 100$
days. This high CSM mass is similar to that measured for SN 2010jl ($\sim
1-10$ M⊙; Zhang et al., 2012; Fransson et al., 2014; Ofek et al., 2014) and is
likely a contributing reason for the slow evolution seen in both transients.

Assuming SN 2021adxl to be entirely powered by radioactive decay, Fig. 25
includes the energy deposited by a mass of explosively synthesised radioactive
material given by Jeffery 1999, and the mass of radioactive nickel needed
would be on the order of 3-10 M⊙ of material. However such a mass of nickel
would produce a much brighter transient, and would over-predict the luminosity
evolution during Phase 1. This is obviously an overestimate, and not expected
for SN 2021adxl, as the transient is likely dominated by CSM interaction even
in Phase 2. We conclude that CSM interaction likely dominates Phase 2,
although there is a different CSM mass / distribution when compared to Phase 1
meaning the progenitor has undergone several mass loss episodes in the final
moments before core-collapse.

##  5 Discussion

Figure 26:  VLT/X-shooter for SN 2021adxl taken on 2023-02-26 (+480d). The
spectrum has been fluxed calibrated to the $r$-band and each panel highlights
a wavelength range which (roughly) covers the near UV, optical, and near-IR.
Telluric bands are marked by the grey area. The bottom two panels have been
smoothed using a Savgol filter for clarity.

SN 2021adxl is an example of a nearby, bright, long-lasting interacting
supernova. This allows for a rare glimpse to the late-time evolution of the
transient and for this purpose we have obtained high-resolution spectra.
Figure 26 provides a detailed picture of the final VLT/X-shooter spectrum
taken at +480d. This highlights the weaker, narrow emission features that are
typically missed when lower-resolution spectrographs are employed. For
context, many narrow features discussed in this section are not detected in
the NOT+ALFOSC spectra. This suggest an inefficient observing strategy for
late time transient followup when only low-medium spectrum are obtained.

Figure 27: Metallicity measurements for the environment around SN 2021adxl
using the VLT/X-shooter spectrum on 2023-02-26 (+480d). Histograms are
measured metallicities for hosts of Type IIn SNe (Habergham et al., 2014;
Moriya et al., 2023), Type II SLSNe (Leloudas et al., 2015), as well as both
Type I and II SLSNe (Perley et al., 2016) using various methods, denoted in
the legend, and detailed in Sect. 3. Vertical lines mark measured values for
SN 2021adxl using different methods (see text). We also mark the reported
metallicity for SN 2010jl at $Z\approx 0.32\leavevmode\nobreak\ Z_{\odot}$
(Stoll et al., 2011) and for SN 2013L at $Z\approx 0.30\leavevmode\nobreak\
Z_{\odot}$ (Taddia et al., 2020).

###  5.1 The nature of the SN 2021adxl’s host

The Ly$\alpha$ profile seen in Fig. 8 is in contrast to the appearance of
H$\alpha$ as shown in Fig. 11. While this may be an opacity effect, or
possibly due to emission from material far away from the ejecta-CSM interface
as mentioned above, similar profiles for Ly$\alpha$ have been seen for
population of compact, star-forming galaxies at z $\sim$ 0.2 - 0.3, also known
as “green peas” (GP) galaxies (Cardamone et al., 2009). These GP galaxies have
a high ${\rm[{O\textsc{III}}]\leavevmode\nobreak\ \lambda\leavevmode\nobreak\
5007}$ / ${\rm[{O\textsc{II}}]\leavevmode\nobreak\ \lambda\leavevmode\nobreak\
3726}$ ratio, high equivalent width in O III emission lines (See Table. 5
Cardamone et al., 2009), and are typically sub- solar metallicity (Orlitová et
al., 2018; Liu et al., 2022). Our Prospector measurements also find a mass
consistent with GP ($\sim 10^{8}$ M⊙; Cardamone et al., 2009). Although due to
its proximity, the host of SN 2021adxl would be classified as a Blueberry
galaxy rather than a Green Pea (Liu et al., 2022).

As shown in Fig. 8, the Ly$\alpha$ shows a unique profile that is not clearly
seen in any other emission profile. The profile may resemble a P-Cygni
emission profile, typically observed in stellar winds and SNe, or a doubled
peaked emission profile with an absorption trough at center wavelength. We
note the similarities of the HST/STIS Ly$\alpha$ profile to that observed in
GP galaxies (e.g. Yang et al., 2016; Orlitová et al., 2018; Henry et al.,
2018). In this case, the asymmetric profile is thought to be due to resonant
scattering of Ly$\alpha$ photons off neutral hydrogen atoms, which is
dependant on the intervening material and fraction of escaping ionizing
radiation. This would also point that the broad feature in Fig. 9 would be Mg
II $\lambda\lambda$2796, 2803 (Henry et al., 2018), originating from the host
and not SN 2021adxl, although does not explain the observed wavelength offset.

###  5.2 Type IIn SNe exploding in metal poor environments

Perhaps greater insight may be gained from the local environment around SN
2021adxl. As mentioned in Sect. 3, SN 2021adxl occurred in a bright star-
forming region at the end of an elongated intensity distribution. We find that
the local environment has a very low metallicity of ${\rm
0.06\leavevmode\nobreak\ Z_{\odot}}$. This is similar to the environments for
SN 2010jl (Stoll et al., 2011) and SN 2013L (Taddia et al., 2020), as well as
for other Type IIn SNe (Habergham et al., 2014; Moriya et al., 2023).

Figure 27 compares the measured metallicities for the host of SN 2021adxl to a
sample of values for hosts of Type IIn SNe (Habergham et al., 2014; Moriya et
al., 2023), Type II SLSN (Leloudas et al., 2015), as well as both Type I and
II SLSNe (Perley et al., 2016). SN 2021adxl is at drastically lower
metallicity than the sample of Type IIn SNe (e.g. samples from Habergham et
al., 2014; Moriya et al., 2023), and also lower that for Type II SN hosts
(except perhaps for SN 2015bs; Anderson et al., 2016, 2018). The sample of
Moriya et al. 2023 does include SN 2016iaf having 12 + log(O/H) = $7.48\pm
0.81$ using the D16 method (Dopita et al., 2016), although this metallicity is
outside the reported validity range for D16.

Moriya et al. 2023 find that Type IIn SNe with a higher peak luminosity occur
in environments with lower metallicity and young stellar environment. Although
SN 2021adxl is beyond the range explored by Moriya et al. 2023, the location
and metallicity of the environment SN 2021adxl occurred in may explain its
brightness. The conjecture that more luminous IIn’s are found in less metal-
enrichment environments is in conflict with results from Leloudas et al. 2015,
Perley et al. 2016 and Schulze et al. 2018, 2021. Figure 27 is contradicting
this scenario, too. The SLSN-II sample spans the full range of metallicities
(e.g. sample from Leloudas et al., 2015, in Fig. 27). The fact that they are
super-luminous, and therefore more luminous than SN 2021adxl implies that
their environments should on average be less metal enriched than of SN
2021adxl, in contradiction with the result of Moriya et al. 2023.
Additionally,Moriya et al. 2023 find a strong inverse correlation between peak
luminosity and metallicity, which may possibly explain the brightness of SN
2021adxl although the peak magnitude would be overestimated by almost 2
magnitudes (see Fig. 1 of Moriya et al. 2023).

###  5.3 Progenitor of SN 2021adxl

A major problem in supernova science concerns identifying the progenitors of
Type IIn SNe. Progress is further hindered by the lack of clear nebular
emission line signatures (which may reveal clues to the progenitor e.g. Fraser
et al., 2013; Jerkstrand et al., 2020). We are therefore often left with
modelling the lightcurve and spectra, which in the case of interacting
transients, are dominated by the effects of interaction, obscuring the
signatures of any explosion mechanism (Arcavi et al., 2017; Woosley, 2018).

There are no progenitor detections of SN 2021adxl, so we must rely on indirect
methods. In Sect. 4.1, we concluded that SN 2021adxl can be powered by the
ejecta colliding with a previous wind with $\dot{M}\approx 10^{-3}$ M⊙ yr-1.
This mass-loss rate regime is often reserved for LBVs during their outburst
stages (Lamers, 1989; Weis, 2001; Vink, 2008; Smith, 2013), and additionally
the inferred wind velocity is also similar to that observed among Galactic
LBVs (Weis, 2001).

Such high mass-loss rates are commonly inferred for Type IIn SNe (Moriya et
al., 2014), although it is difficult to have a physical mechanism that can
lead to this amount of mass ejected (Fuller, 2017; Tsang et al., 2022). Such a
high steady state mass-loss rate is unlikely to be obtained from a single
star. Using Eq. 20 from Björklund et al. 2021, a massive hot star would
require a luminosity of $L\approx 10^{7.2}\leavevmode\nobreak\ L_{\odot}$ to
achieve a steady state mass-loss rate of $10^{-3}$ M⊙ yr-1 at a metallicity of
0.2 $Z_{\odot}$ (this is the lower validity range from Björklund et al. 2021),
which would likely exceed the Eddington luminosity of a single star. This is
similar to the luminosity expected from the 1840 Giant Eruption of Eta Carinae
(Davidson & Humphreys, 2012), where it is thought that binary interaction was
the cause of the eruption (see Smith, 2011, for a possible scenario).

The involvement of a companion in the progenitor history is an often explored
formation channel for Type IIn SNe (Chatzopoulos et al., 2012; Justham et al.,
2014; Zapartas et al., 2019), as most massive stars are expected to be born
and interact with a binary companion (Sana et al., 2012; Zapartas et al.,
2019). The environments of Type IIn do not show dramatic differences when
compared to normal Type II(Kelly & Kirshner, 2012), suggesting there is no
significant environment dependence for these two sub types, and (possibly)
they are influenced by the present (or absence) of a companion. At the time of
writing SN 2021adxl no obvious signatures of binarity (e.g. pre-explosion
variability, multi-peaked emission lines) are detected. While SN 2021adxl is a
luminous Type IIn supernova, it does not reach the often assumed -21 mag
criteria of SLSNe (Gal-Yam, 2012). However in the context of interacting SNe,
it is not clear whether SNe IIn fulfilling this criterion constitute a
separate population of events (Moriya et al., 2018; Gal-Yam, 2019; Nyholm et
al., 2020) or whether there is a continuum of possible energetics (see Fig. 1
of Moriya et al., 2023).

##  6 Conclusions

We present 1.5 years of the evolution of the luminous Type IIn SN 2021adxl,
including four epochs of spectrum from the VLT/X-shooter. SN 2021adxl, similar
to SN 2010jl, does not evolve quickly, and since discovery has only faded by
$\sim$4 magnitudes. This likely reflects that the forward shock, created from
the ejecta-CSM interaction, is moving outwards through a dense, extended CSM
environment. The effects of this environment are seen in the appearance of the
H$\alpha$ emission line, which displays a persistent blue shoulder emission
component. We find that this feature is due to electron scattering through an
increasingly diffuse medium ahead of the forward shock. Using the evolution of
this feature, we find that the post-peak evolution is dominated by the ejecta-
CSM interaction. Assuming spherical symmetry and homologous density
distribution, this translates to a shock wave moving through dense medium of
$\gtrsim$3 M⊙ extended out to at least $\sim 10^{17}$ cm. Between 250 and
350d, the properties of the CSM interaction changes. The light curve decay
increased and the broad H$\alpha$ profile narrows, likely due to a decrease in
the CSM density. This may signify a change in the evolution of the progenitor
within the last $\sim 10^{3}$ years.

Perhaps one of the most striking features of SN 2021adxl is where it occurred.
The vicinity around SN 2021adxl displayed a prominent blue appearance, and we
show that this is a low density, low metallicity environment. The peak
brightness of SN 2021adxl may be related to its metallicity, as more metal
poor progenitors may produce bright Type IIn SNe (e.g. Moriya et al., 2014).
Although this is likely not the (direct) cause of the high mass loss rates, as
Type IIn SNe occur in heterogeneous environments, with multiple formation
pathways (Ransome et al., 2022). Thanks to its proximity and (expected) slow
evolution, SN 2021adxl will be observable again in early-2024, allowing for
continued observations at +800 days.

## Acknowledgements

S. J. Brennan and R. Lunnan acknowledge support by the European Research
Council (ERC) under the European Union’s Horizon Europe research and
innovation programme (grant agreement No. 10104229 - TransPIre). S. Schulze is
partially supported by LBNL Subcontract NO. 7707915 and by the G.R.E.A.T.
research environment, funded by Vetenskapsrådet, the Swedish Research Council,
project number 2016-06012. T. W. Chen acknowledges the Yushan Young Fellow
Program by the Ministry of Education, Taiwan for the financial support. Y.-L.
Kim has received funding from the Science and Technology Facilities Council
[grant number ST/V000713/1].

Based on observations obtained with the Samuel Oschin Telescope 48-inch and
the 60-inch Telescope at the Palomar Observatory as part of the Zwicky
Transient Facility project. ZTF is supported by the National Science
Foundation under Grant No. AST-2034437 and a collaboration including Caltech,
IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm
University, the University of Maryland, Deutsches Elektronen-Synchrotron and
Humboldt University, the TANGO Consortium of Taiwan, the University of
Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National
Laboratories, IN2P3, University of Warwick, Ruhr University Bochum and
Northwestern University. Operations are conducted by COO, IPAC, and UW. SED
Machine is based upon work supported by the National Science Foundation under
Grant No. 1106171 . This work was supported by the GROWTH project funded by
the National Science Foundation under Grant No 1545949. The Oskar Klein Centre
is funded by the Swedish Research Council. The data presented here were
obtained [in part] with ALFOSC, which is provided by the instituto de
Astrofisica de Andalucia (IAA) under a joint agreement with the University of
Copenhagen and NOT. The ZTF forced-photometry service was funded under the
Heising-Simons Foundation grant #12540303 (PI: Graham).The Gordon and Betty
Moore Foundation, through both the Data-Driven Investigator Program and a
dedicated grant, provided critical funding for SkyPortal.

## Data availability

The spectroscopic data underlying this article are available in the Weizmann
Interactive Supernova Data Repository
(WISeREP141414https://wiserep.weizmann.ac.il/ Yaron & Gal-Yam, 2012). The
photometric data underlying this article are available in the online
supplementary material.

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