# Fast and Not-so-Furious: Case Study of the Fast and Faint Type IIb
SN 2021bxu

Dhvanil D. Desai1, Chris Ashall2, Benjamin J. Shappee1, Nidia Morrell3, Lluís
Galbany4,5, Christopher R. Burns6, James M. DerKacy2, Jason T. Hinkle1, Eric
Hsiao7 , Sahana Kumar7, Jing Lu7, Mark M. Phillips3, Melissa Shahbandeh8,
Maximilian D. Stritzinger9, Eddie Baron10,11, Melina C. Bersten12,13,14, Peter
J. Brown15, Thomas de Jaeger1, Nancy Elias-Rosa4, Gastón Folatelli12,13,14,
Mark E. Huber1, Paolo Mazzali16, Tomás E. Müller-Bravo4,5, Anthony L. Piro6,
Abigail Polin6, Nicholas B. Suntzeff15, Joseph P. Anderson17,18, Kenneth C.
Chambers1, Ting-Wan Chen19,20, Thomas de Boer1, Michael D. Fulton21, Hua Gao1,
Mariusz Gromadzki22, Cosimo Inserra23, Eugene A. Magnier1, Matt Nicholl21,
Fabio Ragosta24, Richard Wainscoat1, David R. Young21
1 Institute for Astronomy, University of Hawai‘i at Mānoa, 2680 Woodlawn
Drive, Honolulu, HI 96822, USA
2 Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
3 Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena,
Chile
4 Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans,
s/n, E-08193 Barcelona, Spain
5 Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capità, 2-4, Edifici
Nexus, Desp. 201, E-08034 Barcelona, Spain
6 The Observatories of the Carnegie Institution for Science, 813 Santa Barbara
St., Pasadena, CA 91101, USA
7 Department of Physics, Florida State University, 77 Chieftan Way,
Tallahassee, FL 32306, USA
8 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD
21218, USA
9 Department of Physics and Astronomy, Aarhus University, Ny Munkegade,
DK-8000 Aarhus C, Denmark
10 Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma,
440 W. Brooks, Rm 100, Norman, OK, USA
11 Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
12 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La
Plata, Paseo del Bosque S/N, B1900FWA, La Plata, Argentina
13 Instituto de Astrofísica de La Plata (IALP), CCT-CONICET-UNLP, Paseo del
Bosque S/N, B1900FWA, La Plata, Argentina
14 Kavli Institute for the Physics and Mathematics of the Universe (WPI), The
University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8583, Japan
15 Mitchell Institute for Fundamental Physics and Astronomy, Department of
Physics and Astronomy, Texas A&M University 4242 TAMU, College Station, TX,
USA
16 Astrophysics Research Institute, Liverpool John Moores University, IC2,
Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
17 European Southern Observatory, Alonso de Córdova 3107, Casilla 19,
Santiago, Chile
18 Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100,
Off. 104, Providencia, Santiago, Chile
19 Technische Universität München, TUM School of Natural Sciences, Physik-
Department, James-Franck-Straße 1, 85748 Garching, Germany
20 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Straße 1, 85748
Garching, Germany
21 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s
University Belfast, Belfast BT7 1NN, UK
22 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478
Warszawa, Poland
23 Cardiff Hub for Astrophysics Research and Technology, School of Physics &
Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF24
3AA, UK
24 Istituto Nazionale di Astrofisica, Viale del Parco Mellini 84, 00136 Rome,
Italy E-mail<EMAIL_ADDRESS>0000-0002-2164-859X 0000-0002-5221-7557
0000-0003-4631-1149 0000-0003-2535-3091 0000-0002-1296-6887
0000-0002-7566-6080 0000-0001-9668-2920 0000-0001-8367-7591
0000-0002-3900-1452 0000-0003-2734-0796 0000-0002-9301-5302
0000-0002-5571-1833 0000-0001-6272-5507 0000-0003-1059-9603
0000-0003-3939-7167 0000-0003-0227-3451 0000-0001-6965-7789
0000-0002-1066-6098 0000-0001-5486-2747 0000-0003-1916-0664
0000-0003-1015-5367 0000-0002-1650-1518 0000-0002-3968-4409
0000-0002-7965-2815 0000-0002-2555-3192 0000-0003-2132-3610
0000-0002-1341-0952 0000-0002-1229-2499

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present photometric and spectroscopic observations and analysis of SN
2021bxu (ATLAS21dov), a low-luminosity, fast-evolving Type IIb supernova (SN).
SN 2021bxu is unique, showing a large initial decline in brightness followed
by a short plateau phase. With $M_{r}=-15.93\pm 0.16\,\mathrm{mag}$ during the
plateau, it is at the lower end of the luminosity distribution of stripped-
envelope supernovae (SE-SNe) and shows a distinct $\sim$10 day plateau not
caused by H- or He-recombination. SN 2021bxu shows line velocities which are
at least $\sim 1500\,\mathrm{km\,s^{-1}}$ slower than typical SE-SNe. It is
photometrically and spectroscopically similar to Type IIb SNe during the
photospheric phases of evolution, with similarities to Ca-rich IIb SNe. We
find that the bolometric light curve is best described by a composite model of
shock interaction between the ejecta and an envelope of extended material,
combined with a typical SN IIb powered by the radioactive decay of 56Ni. The
best-fit parameters for SN 2021bxu include a 56Ni mass of
$M_{\mathrm{Ni}}=0.029^{+0.004}_{-0.005}\,\mathrm{M_{\odot}}$, an ejecta mass
of $M_{\mathrm{ej}}=0.57^{+0.04}_{-0.03}\,\mathrm{M_{\odot}}$, and an ejecta
kinetic energy of $K_{\mathrm{ej}}=9.3^{+0.7}_{-0.6}\times
10^{49}\,\mathrm{erg}$. From the fits to the properties of the extended
material of Ca-rich IIb SNe we find a trend of decreasing envelope radius with
increasing envelope mass. SN 2021bxu has $M_{\mathrm{Ni}}$ on the low end
compared to SE-SNe and Ca-rich SNe in the literature, demonstrating that SN
2021bxu-like events are rare explosions in extreme areas of parameter space.
The progenitor of SN 2021bxu is likely a low mass He star with an extended
envelope.

###### keywords:

supernovae:general – supernovae: individual: SN 2021bxu – stars: massive

††pubyear: 2023††pagerange: Fast and Not-so-Furious: Case Study of the Fast
and Faint Type IIb  SN 2021bxu–A

##  1 Introduction

Core-collapse (CC) supernovae (SNe) mark the explosive ends of the lives of
massive stars ($M\gtrsim 8\,\mathrm{M_{\odot}}$) via gravitational collapse of
their stellar cores. Some CC SNe occur from progenitors that have lost their
outer envelopes and are classified as stripped-envelope supernovae (SE-SNe;
Clocchiatti et al., 1996; Matheson et al., 2001). Optical spectral signatures
of H and He are mainly used to distinguish between the different types of SE-
SNe (Filippenko, 1997). The lack of H and Si ii $\lambda 6150$ features
defines Type Ib/c SE-SNe. Furthermore, if there is He i $\lambda 5876$
absorption present, the SN is a Type Ib and if there is weak or no He i
$\lambda 5876$ absorption, the SN is a Type Ic. In addition to these types,
SNe that show transient lines of H, making their late-time spectra appear more
like SNe Ib, are classified as Type IIb SNe.

The exact nature of progenitor scenarios is unclear but different types of SE-
SNe may be explained by various mass loss mechanisms in the progenitor star
(Filippenko et al., 1994). SNe Ic likely result from stars that have lost both
their H and He envelopes. SNe Ib from stars with less extreme stripping, that
have lost H envelope. And SNe IIb result from stars that have lost most of
their H envelope, showing weak H lines at early times (e.g., Filippenko, 1997;
Shivvers et al., 2017; Prentice & Mazzali, 2017). It is unclear if there is a
continuum between each type of SE-SN or whether this points towards multiple
mass-loss mechanisms. Some common proposed mass loss mechanisms which may be
responsible for stripping the stellar envelope(s) include: (a) outbursts in
Luminous Blue Variables (LBVs; e.g., Smith & Owocki, 2006), (b) radiation-
driven winds (e.g., Heger et al., 2003; Pauldrach et al., 2012), (c) envelope
stripping due to close binary interactions (e.g., Podsiadlowski et al., 1993;
Woosley et al., 1995; Wellstein & Langer, 1999; Wellstein et al., 2001;
Podsiadlowski et al., 2004; Benvenuto et al., 2013), (d) mass-loss in rapidly
rotating Be stars (e.g., Massa, 1975; Kogure & Hirata, 1982; Owocki, 2006), or
a combination of these.

The low-mass end of SE-SN progenitors (zero-age main-sequence mass $\approx
8-12\,\mathrm{M_{\odot}}$) is not well understood (e.g., Janka, 2012). Their
end stages of evolution depend strongly on metallicity and mass-loss history.
Some low-mass stars may experience core collapse induced by electron-capture
reactions resulting in the production of low-luminosity electron-capture (EC)
SNe (Miyaji et al., 1980; Nomoto, 1984, 1987; Hillebrandt et al., 1984; Miyaji
& Nomoto, 1987). Given a stellar initial mass function (IMF) that drops
steeply towards higher masses, we should expect a significant fraction of CC
SNe to have lower-mass progenitors (Sukhbold et al., 2016). However, due to
their expected lower luminosities and rapid photometric evolution, SNe
resulting from low-mass stars are likely more difficult to observe and follow
up, leading to an observational bias (Nomoto et al., 1982; Janka, 2012).

Recently, Calcium-rich transients have emerged as a new class of SNe showing
faster photometric evolution than normal SNe, lower luminosities, and a
nebular phase dominated by calcium emission (Perets et al., 2011; Kasliwal et
al., 2012; Shen et al., 2019; Das et al., 2022). Ca-rich transients consist of
two main sub-types: I and II. The Type I Ca-rich transients may come from the
explosion of white dwarfs (WDs) or highly-stripped CC events (Kawabata et al.,
2010; Tauris et al., 2015). They do not show hydrogen in their spectra and are
usually found in the outskirts of early-type galaxies, in old, metal-poor
environments (Perets et al., 2011). The Type II Ca-rich transients, which
possibly come from a CC event, show hydrogen features. In particular, Ca-rich
Type IIb have spectra similar to SNe IIb near peak light, but rapidly evolve
into nebular phase ($\sim$30 days after explosion) and show a [Ca ii]/[O i]
ratio of $\geq 2$ (Das et al., 2022). Contrary to the Ca-rich Type I
transients, Ca-rich Type IIb SNe are found in star-forming regions and suggest
a new class of strongly-stripped SNe (SS-SNe) which have ejecta masses less
than $\sim 1\,\mathrm{M_{\odot}}$ and stripped low-mass He stars as their
progenitors.

One of the most promising ways to study the progenitor and its outermost
layers is through early-time observations of SN explosions (e.g., Bersten et
al., 2012; Piro & Nakar, 2013; Vallely et al., 2021). Shock-breakout is an
early-time phenomenon that occurs on a timescale of a few minutes to hours
when the shock from a CC explosion of a massive star breaks through the
stellar surface (Waxman & Katz, 2017). For CC events, shock-breakout is a
promising tool for measuring the properties of the exploding star through
early-time observations of the outer layers. This manifests as X-ray and
ultraviolet (UV) brightening, followed by a post-shock breakout cooling phase
where most of the radiation is emitted in UV and optical and the envelope
expands while cooling down. The time-scale of this phenomenon depends on the
type of progenitor and the presence or absence of extended material around the
star. For example, for normal SNe Ib or Ic this lasts a few hours (e.g., Xiang
et al., 2019) and for SNe IIb with extended envelopes this is on the order of
a few days (e.g., Nomoto et al., 1993). Since the shock-breakout and cooling
depend on properties of the outermost layers of the progenitor, observing
those via early-time observations can provide measurements of the temperature,
radius, and mass of the envelope or circumstellar material (e.g., Nakar &
Piro, 2014; Piro et al., 2021).

Few SE-SNe have been observed to exhibit so-called "double-peaked" light
curves with an initial decline due to shock cooling and a second peak due to
the radioactive decay of 56Ni, which normally powers the light curve.
Generally, these objects were classified as SNe IIb. The first of this class
was the well-studied SN 1993J (Richmond et al., 1994), followed by other SE-
SNe, e.g., SN 2011dh (Arcavi et al., 2011), SN 2011fu (Morales-Garoffolo et
al., 2015), SN 2013df (Morales-Garoffolo et al., 2014), and some others as
shown in Prentice et al. (2020). The initial decline in SN 1993J was explained
as a result of a lengthened shock cooling due to the shock passing through an
extended envelope around the star instead of “breaking out” at an abrupt
surface, producing a longer initial decline. Similar to that observed in
various SNe IIb, a subset of Ca-rich transients have also been shown to have
double-peaked light curves with an initial decline of a few days that are well
fit by an extended envelope model (Das et al., 2022). For example, Ertini et
al. (2023) showed that SN 2021gno, a Ca-rich SN Ib with double-peaked light
curves, can be well modelled by a CC explosion of a highly-stripped, massive
star.

Exotic classes of SNe are being discovered by untargeted surveys such as the
All-Sky Automated Survey for SuperNovae (ASAS-SN; Shappee et al., 2014;
Kochanek et al., 2017), the Asteroid Terrestrial-impact Last Alert System
(ATLAS; Tonry et al., 2018; Smith et al., 2020) and the Zwicky Transient
Facility (ZTF; Masci et al., 2019; Bellm et al., 2019), and studied with high-
precision multi-band early follow-up by programs such as the Precision
Observations of Infant Supernova Explosions (POISE; Burns et al., 2021)
collaboration. SN 2021csp (Fraser et al., 2021), SN 2021fxy (DerKacy et al.,
2022), SN 2021gno (Ertini et al., 2023) and SN 2021aefx (Ashall et al., 2022)
are all SNe followed-up by POISE, and each of them demonstrates the power of
obtaining high-precision early-time observations by providing insight into
their sub-class of SNe.

In this work we study SN 2021bxu111https://www.wis-tns.org/object/2021bxu
(ATLAS21dov), a peculiar SE-SN discovered by ATLAS on UT 06.3 Feb 2021 (Tonry
et al., 2021) and later classified as a Type IIb SN (DerKacy, 2021). Early-
time observations from POISE show a fast declining light curve followed by a
$\sim$10 day plateau and an unusually low peak luminosity. Here we present a
detailed study of this unique event with high-precision photometry and
spectroscopy, explore a new parameter space for SNe and their progenitors, and
derive physical quantities and compare them with other known SE-SNe.

In Section 2, we provide the properties of the host galaxy. In Section 3, we
describe the photometric and spectroscopic observations of SN 2021bxu. In
Section 4, we analyze and compare the multi-band light curves and colour
curves of SN 2021bxu with SE-SNe and Ca-rich IIb SNe from literature. We also
derive and compare the pseudo-bolometric and bolometric light curves of SN
2021bxu with a sample of SE-SNe. In Section 5, we identify the observed
spectroscopic lines, compare the line velocities to a sample of SE-SNe and Ca-
rich IIb SNe, estimate the time of explosion, and compare SN 2021bxu with
similar SNe. In Section 6, we outline the models used to fit the bolometric
and pseudo-bolometric light curves, describe the fitting method and present
the best-fit results for the explosion parameters. In Section 7, we compare
and contextualize the physical parameters of SN 2021bxu with a sample of SE-
SNe and Ca-rich IIb SNe. Finally, in Section 8, we summarize our work and
present the conclusions.

##  2 Host Galaxy Properties

The host galaxy ESO 478- G 006 is classified as an Sbc galaxy with luminosity
class II-III (de Vaucouleurs et al., 1991). SN 2021bxu was discovered in one
of the spiral arms of its host galaxy.

Figure 1: Finding chart of SN 2021bxu in its host galaxy ESO 478- G 006
obtained with MUSE. The background image is a stellar-continuum subtracted
H$\alpha$ emission map obtained from the two MUSE pointings. The different
conditions of the observations are evident, the west half of the galaxy where
SN 2021bxu exploded presents a better spatial resolution ($0.72\arcsec$
seeing) than the east half ($2.01\arcsec$ seeing).

Using the Fitting and Assessment of Synthetic Templates (FAST; Kriek et al.,
2009), we fit stellar population synthesis models to the archival photometry
(see Table 7 in Appendix A) of the host galaxy ESO 478- G 006 to obtain the
global age, the total stellar mass, the star formation rate (SFR), and the
specific SFR as listed in Table 1. Our fit assumes a Cardelli et al. (1989)
extinction law with $R_{V}=3.1$, a Salpeter IMF (Salpeter, 1955), an
exponentially declining star-formation rate, the Bruzual & Charlot (2003)
stellar population models, and allows for a variable average host galaxy
extinction higher than the Galactic foreground value of
$A_{V\mathrm{,MW}}=0.045\,\mathrm{mag}$. The value of host extinction from
this fit indicates an average across the entire galaxy and is not necessarily
the value at the site of the SN.

To estimate the local host extinction and other properties, we used integral
field spectroscopy (IFS) obtained with MUSE mounted to the 8.2m Very Large
Telescope (VLT) in October 2016 as a part of the All-weather MUse Supernova
Integral-field of Nearby Galaxies (AMUSING; Galbany et al., 2016; López-Cobá
et al., 2020)) survey. Two pointings were observed covering the position of SN
2021bxu (see Figure 1). Following previous analysis of IFS data (e.g., Galbany
et al., 2014, 2018)), we extracted a $2\arcsec$ aperture spectrum from the IFS
cube centered at the SN location, and performed spectral synthesis with
STARLIGHT (Cid Fernandes et al., 2005). By subtracting the best STARLIGHT fit
from the observed spectrum, we get the gas-phase spectrum, where we fit
Gaussian profiles to the strongest emission lines to measure the main
properties of the ionized gas. In particular, the dust extinction along the
line of sight is estimated using the colour excess ($E(B-V)$) from the ratio
of the Balmer lines, assuming an intrinsic ratio
$I(\mathrm{H}\alpha)/I(\mathrm{H}\beta)=2.86$, valid for case B recombination
with $T=10{,}000\,\mathrm{K}$ and electron density $10^{2}\,\mathrm{cm^{-3}}$
(Osterbrock & Ferland, 2006), and using a Cardelli et al. (1989) extinction
law. Local properties are also listed in Table 1.

Table 1: Global and Local Properties of the Host Galaxy ESO 478- G 006 | Global (phot) | Local (spec)
---|---|---
Age [yr] | $3.2^{+5.8}_{-1.3}\times 10^{8}$ | $9^{+11}_{-8}\times 10^{8}$
$M_{*}$ [$M_{\odot}$] | $2.5^{+0.8}_{-0.3}\times 10^{10}$ | $(3.2\pm 0.2)\times 10^{6}$
SFR [$\mathrm{M_{\odot}\,yr^{-1}}$] | $30^{+4}_{-7}$ | $(7.38\pm 0.09)\times 10^{-5}$
sSFR [$\mathrm{yr^{-1}}$] | $1.1^{+0.3}_{-0.1}\times 10^{-9}$ | $2.3^{+1.2}_{-0.4}\times 10^{-11}$
$A_{V\mathrm{,host}}$ [mag] | $1.4^{+0.3}_{-0.3}$ | $0.4\pm 0.2$
$12+\log_{10}$(O/H) | $-$ | $8.71\pm 0.14$

For the purposes of magnitude corrections for SN 2021bxu, we use a null line-
of-sight host extinction at the SN site, as inferred from the absence of
strong narrow Na i D line in the SN spectra. We do not use
$A_{V\mathrm{,host}}=0.4\pm 0.2\,\mathrm{mag}$ from local spectroscopic
analysis of the host galaxy because this measurement is of the extinction from
the total line-of-sight dust column. Since we do not know if the SN exploded
in front of the dust column or behind it, using the galaxy measurement is not
optimal. However, we note that there may be up to $0.4\,\mathrm{mag}$ of host
extinction and including this in the analysis does not change the conclusions
of this study.

##  3 Data

###  3.1 Photometry

Table 2: Properties of SN 2021bxu

RA (J2000) | 02:09:16.47
---|---
$\delta$ (J2000) |  $-23$:24:45.15
$z$ a | $0.0178$
Host Galaxy | ESO 478- G 006
Host Offset b | $9.2\pm 0.6\,\mathrm{kpc}$
$E(B-V)_{\mathrm{MW}}$ c | $0.014\,\mathrm{mag}$
$E(B-V)_{\mathrm{Host}}$ | $\sim 0\,\mathrm{mag}$
$\mu$ a | $34.28\pm 0.16\,\mathrm{mag}$
$D_{\mathrm{L}}$ d | $72\pm 5\,\mathrm{Mpc}$
Last Non-Detection (MJD) | $59245.12\,\mathrm{days}$
Discovery (MJD) | $59251.28\,\mathrm{days}$
Estimated Explosion (MJD) e | $59246.3\,\pm 0.4\,\mathrm{days}$

aScolnic et al. (2018).

bProjected offset from the host-galaxy nucleus.

cSchlafly & Finkbeiner (2011).

dComputed using $\mu$ from Scolnic et al. (2018).

eSee Section 5.3.

We present high-precision multi-band photometry of SN 2021bxu obtained with
the Henrietta Swope 1.0 m telescope at Las Campanas Observatory, ASAS-SN,
ATLAS, and the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS;
Flewelling et al., 2020; Chambers et al., 2016, see Table 5 in Appendix A).

The Swope photometry, in $BVugri$ bands, was produced using custom reduction
and calibration procedures as described in Krisciunas et al. (2017) and
Phillips et al. (2019) via the POISE collaboration, which builds on the legacy
of the Carnegie Supernova Project (CSP; Phillips et al., 2019). The science
images were host-galaxy template subtracted and the nightly zero-points were
obtained on photometric nights by observing photometric standards from the
Landolt (1992) and Smith et al. (2002) catalogs. Using these zero-points, we
computed natural magnitudes of the local sequence stars (listed in Table 6 in
Appendix A) in the field, which is then used to calibrate the Swope photometry
of SN 2021bxu. The Swope photometry is ultimately in the CSP natural system.
The ATLAS photometry, in $o$ and $c$ bands, was obtained from the ATLAS Forced
Photometry server222https://fallingstar-data.com/forcedphot/ (Shingles et al.,
2021), with photometry produced as outlined in Tonry et al. (2018) and Smith
et al. (2020). The ASAS-SN $g$-band light curve was produced using subtracted
aperture photometry from ASAS-SN Sky Patrol333https://asas-sn.osu.edu/.
Finally, the Pan-STARRS (PS) observations were taken with both 1.8 m telescope
units located at the summit of Haleakala (Chambers et al., 2016), in an SDSS-
like filter system, denoted as $grizy_{\mathrm{PS}}$, and a broad
$w_{\mathrm{PS}}$ filter, which is a composite of the $gri_{\mathrm{PS}}$
filters. Pan-STARRS data are processed in real-time as described in Magnier et
al. (2020a), Magnier et al. (2020b) and Waters et al. (2020). The data are
subject to difference imaging with the Pan-STARRS1 $3\pi$ sky survey data
(Chambers et al., 2016) used as references, and photometric zero-points on the
target images were set with field stars from the Pan-STARRS1 $3\pi$ catalogue
(Flewelling et al., 2020).

Figure 2: Multi-band light curves of SN 2021bxu from Swope $BVugri$, ATLAS
$oc$, ASAS-SN $g$ and Pan-STARRS $grizy$. Non-detections in ASAS-SN $g$ and
ATLAS $o$ are shown as points with downward-facing arrows. Vertical red line-
segments at the bottom of the plot mark the epochs of spectroscopic
observations and the blue line-segment marks the discovery. The estimated time
of explosion is further explained in Section 4.3.

All light curves from Swope, ATLAS, ASAS-SN, and Pan-STARRS are shown in
Figure 2. The Pan-STARRS $w$ band has a non-detection with a $5\sigma$ upper
limit of $m_{w_{\mathrm{PS}}}>22.3\,\mathrm{mag}$ 33 days before discovery
showing no previous outbursts and an upper limit of
$m_{w_{\mathrm{PS}}}>21.7\,\mathrm{mag}$ 216 days after the last measurement
from Swope. The ATLAS $o$ band has a non-detection with a $5\sigma$ upper
limit of $m_{o}>19.6\,\mathrm{mag}$ 6.02 days before discovery and an upper
limit of $m_{o}>19.8\,\mathrm{mag}$ 101 days after the last measurement from
Swope. ASAS-SN also has a non-detection with a $5\sigma$ upper limit of
$m_{g}>17.6\,\mathrm{mag}$ 6.16 days before discovery along with the first
detection at a maximum of $m_{g}=17.17\pm 0.09\,\mathrm{mag}$ 0.2 days before
discovery. The non-detection and the first detection can help constrain the
time of explosion (see Section 5.3).

We obtained UV observations from the Ultra-Violet Optical Telescope (UVOT;
Roming et al., 2005) on the Neil Gehrels Swift Observatory (Gehrels et al.,
2004) about 20 days after the estimated explosion but did not detect the SN.
Pre-explosion imaging from 2018 and 2019 is available in the $U$ and $UVW1$
filters because of the Swift Gravitational Wave Galaxy Survey, the intent of
which is to obtain galaxy template images before the detection of transients
(Klingler et al., 2019). Using the pipeline from the Swift Optical Ultraviolet
Supernova Archive (SOUSA; Brown et al., 2014), we measure upper limits on MJD
59266.3 of $UVW1>19.1\,\mathrm{mag}$ and $U>19.1\,\mathrm{mag}$. These
magnitude limits are in the UVOT/Vega system using zero points from Breeveld
et al. (2011), the time-dependent sensitivity correction from September
2020444https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/docs/uvot/uvotcaldb_throughput_06.pdf,
and an aperture correction updated in 2022. Subsequent observations over the
next 10 days yield similar limits and are available from the SOUSA.

There are no data from ZTF or the Transiting Exoplanet Survey Satellite (TESS;
Ricker et al., 2016; Fausnaugh et al., 2021, 2022) for this object during the
time period of interest.

To convert from apparent to absolute magnitudes we need the distance modulus
and extinction corrections. The distance modulus for SN 2021bxu ($\mu=34.28\pm
0.16\,\mathrm{mag}$) is measured using precise redshift-independent distance
measurement using the Type Ia SN 2009le that exploded in the host galaxy (ESO
478- G 006; Scolnic et al. 2018). We infer no host-galaxy extinction at the
site of the SN due to a lack of narrow Na i D lines in the SN spectra
(Phillips et al., 2013). Therefore, using $\mu$ and the Galactic extinction
correction ($E(B-V)_{\mathrm{MW}}=0.014\,\mathrm{mag}$; Schlafly & Finkbeiner,
2011), we obtain the absolute magnitudes listed in Table 5. Some of the
properties of SN 2021bxu are listed in Table 2 and the photometric analysis is
further discussed in Section 4.

###  3.2 Spectroscopy

Figure 3: Spectra of SN 2021bxu from APO (green), NTT (red), NOT (orange),
Gemini (blue), and Baade (purple) from 6.74 to 30.68 days after its estimated
explosion (MJD 59246.3), all listed in Table 3. The spectrum from NTT at +20.7
days is an average of two spectra taken on the same night using the same
instrument only half-hour apart. Table 3: Spectroscopy of SN 2021bxu

Date | MJD | Phasea | Telescope | Instrument
---|---|---|---|---
(UT) | (days) | (days) |  |
08 Feb 2021 | 59253.07 | 6.7 | APO | DIS
12 Feb 2021 | 59257.02 | 10.7 | NTT | EFOSC2
14 Feb 2021 | 59259.84 | 13.5 | NOT | ALFOSC
17 Feb 2021 | 59262.22 | 15.9 | Gemini | GMOS-N
22 Feb 2021 | 59267.01 | 20.7 | NTT | EFOSC2
22 Feb 2021 | 59267.04 | 20.7 | NTT | EFOSC2
22 Feb 2021 | 59267.05 | 20.7 | Baade | IMACS
02 Mar 2021 | 59275.04 | 28.7 | Baade | IMACS
03 Mar 2021 | 59276.01 | 29.7 | NTT | EFOSC2
04 Mar 2021 | 59277.00 | 30.7 | NTT | EFOSC2

aFrom estimated explosion date (MJD 59246.3) in rest frame.

Along with the photometry, we also have a total of ten spectroscopic
observations for SN 2021bxu. These include optical spectra with spectral range
of roughly $4000-9000$ Å obtained from the Dual Imaging Spectrograph (DIS) on
the Apache Point Observatory (APO), the ESO Faint Object Spectrograph and
Camera v.2 (EFOSC2; Buzzoni et al., 1984) on the New Technology Telescope
(NTT), the Alhambra Faint Object Spectrograph and Camera (ALFOSC) on the
Nordic Optical Telescope (NOT), the Gemini Multi-Object Spectrograph (GMOS;
Hook et al., 2004) on the Gemini North telescope, and the Inamori-Magellan
Areal Camera and Spectrograph (IMACS; Dressler et al., 2011) on the Magellan
Baade telescope. The spectra from APO and Baade were reduced using standard
iraf555The Image Reduction and Analysis Facility (iraf) is distributed by the
National Optical Astronomy Observatory, which is operated by the Association
of Universities for Research in Astronomy, Inc., under cooperative agreement
with the National Science Foundation. packages with the methods as described
in Hamuy et al. (2006) and Folatelli et al. (2013). The NOT data were taken as
a part of the NUTS2 collaboration666https://nuts.sn.ie/ and reduced using the
FOSCGUI pipeline777FOSCGUI is a Python-based graphic user interface (GUI)
developed by E. Cappellaro and aimed at extracting SN spectroscopy and
photometry obtained with FOSC-like instruments. A package description can be
found at https://sngroup.oapd.inaf.it/foscgui.html. The NTT spectra were
obtained through the Public European Southern Observatory Spectroscopic Survey
of Transient Objects (PESSTO) program, and reduced using the data reduction
pipeline described in Smartt et al. (2015). The Gemini spectra were reduced
using a custom-made iraf routine. Dates and phases of all spectra are listed
in Table 3.

Figure 3 shows the spectral sequence. The original unbinned spectra are in
gray and the higher signal-to-noise ratio (SNR), binned spectra are in
colours. The unbinned spectra are resampled at a resolution of $10$ Å using
SpectRes (Carnall, 2017) to produce the binned spectra. The two NTT spectra on
UT 22 Feb 2021 are taken only one half-hour apart and, since they are on the
same telescope and instrument, we average them to produce a combined spectrum
with a higher SNR. This combined spectrum is used for all following analysis.

##  4 Photometric Analyses

###  4.1 Multi-band Light Curves

Figure 4: The gray light curves show SNe II, IIb, Ib, and Ic from the Carnegie
Supernova Project (Stritzinger et al., 2018a; Anderson et al., 2014) and Ca-
rich Type IIb SNe from Das et al. (2022). The red light curves in each subplot
are SN 2021bxu. The $g$-band panel shows the last non-detection from ASAS-SN
as the downward-pointing arrow. The epoch of maximum light for normalization
is chosen as the peak of the 56Ni component of the light curve (see Section
6).

The multi-band light curves of SN 2021bxu are presented in Figure 2, ranging
from the $u$ band to the $i$ band. There is a decrease of $\sim$1.7 mag in the
$u$ band from $\sim$17.9 to $\sim$19.5 mag in the first 3 days and thereafter
it keeps decreasing almost linearly, but with a shallower slope. However,
moving to the optical bands, a plateau starts appearing, which becomes more
prominent in the redder bands. For example, in the $g$ band, the brightness
declines by $\sim$1.5 mag in the first $\sim$5 days after discovery and it is
followed by a plateau where the brightness stays roughly constant for the next
$\sim$10 days before declining again. Looking at the $i$ band, where it gets
moderately bright again, this plateau may be interpreted as a second peak.
Using the distance modulus $\mu=34.28\pm 0.16\,\mathrm{mag}$ from Table 2, we
obtain a peak $r$-band magnitude of $M_{r}=-16.86\pm 0.16\,\mathrm{mag}$ at
the first epoch. After the initial decline, the absolute magnitude during the
plateau phase is $M_{r}=-15.93\pm 0.16\,\mathrm{mag}$ as estimated from the
56Ni peak (see Section 6).

In Figure 4, we show the light curves of SN 2021bxu in $BVugri$ bands
normalized to the bolometric 56Ni peak (discussed in Section 6) in each band
along with a sample of various types of SE-SNe (IIb, Ib, and Ic) from
Stritzinger et al. (2018a), a sample of SNe II from Anderson et al. (2014),
and a sample of Ca-rich Type IIb SNe from Das et al. (2022). The most obvious
distinction between SN 2021bxu and the rest of the sample is the presence of a
strong initial decline in brightness. Although dissimilar to most SNe II
having the typical long plateau phase, SN 2021bxu shows similarities to the
light curves of Type II-L SN 2001fa and SN 2007fz (Faran et al., 2014)
including an initial decline and rise to a second peak. However, SN 2021bxu
has much less H in the spectra than the two SNe II-L. SN 2001fa and SN 2007fz
have been proposed as a link between SNe II and SNe IIb because of the
intermediate strength H$\alpha$ P-Cygni profile between those SN Types.
Nevertheless, Pessi et al. (2019) finds no evidence of continuum between SNe
II and SNe IIb. On the other hand, the overall shape of SN 2021bxu’s later
decline matches well with the SE-SNe and especially well with the Ca-rich IIb
SNe and with SNe IIb. The Ca-rich IIb SNe show a wide range of initial
declines depending on the properties of the external layers of the progenitor
(Das et al., 2022). However, the later decline in the $g$ band for SN 2021bxu
is almost identical to that of the Ca-rich IIb and SNe IIb samples.

###  4.2 Colour Curves

With the available multi-band photometry, we produce the Milky Way reddening
corrected $B-V$, $u-g$, $g-r$, and $r-i$ colour curves for SN 2021bxu,
presented in Figure 5. The $u-g$ colour quickly reaches a reddest value of
$\sim$1.7 mag $\sim$15 days after explosion. The $B-V$ and $g-r$ colours
follow a trend similar to each other and get to their reddest value of
$\sim$1.1 mag $\sim$25 days after explosion. The $B-V$ colour shows an
unexpected change of slope around $\sim$16-19 days after explosion
corresponding to the time of the plateau phase. On the other hand, starting
off at the bluest colour, the $r-i$ colour shows a slow but steady increase
from $\sim-$0.1 mag to $\sim$0.3 mag over the $\sim$40 days after explosion.

We compare the colour curves of SN 2021bxu to templates from Stritzinger et
al. (2018b) for SE-SNe Ib, Ic, and IIb as well as SNe II. Figure 5 shows the
colour curves of SN 2021bxu together with the templates. The $B-V$ colour of
SN 2021bxu is consistent with SNe Ib until $\sim$5 days after $B$-band maximum
but it is bluer by $\sim$0.4 mag around day 12. The $u-g$ colour follows SNe
Ib up to $\sim$5 days after $g$-band maximum, but resembles SNe IIb
thereafter. On the other hand, the $g-r$ colour does not seem to match any of
the types except the first $\sim$3 days where it matches with SNe II; it is
too blue for SE-SNe and too red for SNe II after that. The $B-V$ and the $g-r$
colours peak $\sim$10 days after the templates peak, although the templates
only go up to 20 days past maximum. Finally, the $r-i$ colour matches the
trend of SNe IIb given the error bars on the data and the model. Although SN
2021bxu shows a plateau in its light curve, the colour curves do not match
those of SNe II.

Figure 5: Colour curves of SN 2021bxu using Swope photometry compared to the
Type Ib, Ic, and IIb colour curve templates from CSP SE-SNe sample. We create
a template for $B-V$ colour of a Type II SN using SN 2014G (Bose et al., 2016)
and for $g-r$ and $r-i$ colours of a Type II SN using DES15E1iuh (de Jaeger et
al., 2020). There were no $u$-band data available for Type II SNe, so we do
not have a Type II template for the $u-g$ colour. We do not plot colour curves
for Ca-rich IIb SNe due to the lack of a statistical sample or a
representative SN.

###  4.3 Bolometric Light Curves

Figure 6: Top: Bolometric light curve of SN 2021bxu from integrating the flux
in available band with blackbody extrapolation. The systematic error due to
the uncertainty in distance is shown as $L^{+13\%}_{-16\%}$. Middle: Best-fit
temperature from the blackbody fits. Bottom: Radius calculated using the best-
fit blackbody temperature and bolometric luminosity.

Our multi-band photometry is used to determine the properties of the SN such
as its luminosity, radius and photospheric temperature evolution. We construct
the bolometric light curve for SN 2021bxu by using the multi-band photometry
in the Swope $BVugri$ bands after converting magnitudes to monochromatic
fluxes, correcting for the Milky-Way extinction of
$E(B-V)_{\mathrm{MW}}=0.014\,\mathrm{mag}$, and using the distance of
$D_{\mathrm{L}}=72\pm 5\,\mathrm{Mpc}$ from Table 2. We use the magnitude
offsets and filter profiles from the CSP
webpage888https://csp.obs.carnegiescience.edu/data/filters. If a certain band
lacked observations on a given epoch, we interpolated using Gaussian Processes
with the scikit-learn (Pedregosa et al., 2011) Python library. We fit each
epoch’s spectral energy distribution (SED) with a Planck blackbody function.
The bolometric luminosity is computed by directly integrating the flux density
into the available bands and using the blackbody fits to extrapolate to the
unobserved wavelengths. The effective photospheric temperature is the best-fit
temperature from the blackbody fits, and the radius is then computed from
luminosity and temperature using the Stefan–Boltzmann law. The errors on
luminosity, temperature, and radius are derived from a Monte Carlo procedure
using the errors of the original photometry. Photometric precision is high
with errors $<1\%$ which means majority of the systematic errors come from the
uncertainty in the distance estimate and the assumption of a blackbody SED.
The uncertainty in the distance corresponds to a fractional uncertainty in
luminosity of $L^{+13\%}_{-16\%}$, which would cause the light curve to shift
up or down systematically. Figure 6 shows the bolometric luminosity,
temperature, and radius evolution.

The bolometric luminosity starts at $L_{\mathrm{bol}}\sim 3.5\times
10^{42}\,\mathrm{erg}\,\mathrm{s^{-1}}$ at discovery, drops down to
$L_{\mathrm{bol}}\sim 6.6\times 10^{41}\,\mathrm{erg}\,\mathrm{s^{-1}}$ during
the initial decline, stays flat at that value for $\sim$10 days defining a
plateau, and then declines again. The radius increases almost linearly with
time initially until $\sim$25 days past explosion, indicating that the black-
body extrapolation is reasonable and the black-body radius roughly follows the
photospheric radius. At later times, the black-body radius starts declining,
although this may lack physical meaning since the black-body approximation is
not good at these times.

The photospheric temperature drops from 13,500 K to 7000 K during the initial
photometric decline, suggesting a rapid cooling of the ejecta, and steadily
declines thereafter. Martinez et al. (2022) show that SNe II have a roughly
constant temperature evolution during their plateau at $\sim$6000 K due to
H-recombination. The temperature evolution of SN 2021bxu during the plateau is
not constant; it decreases from $\sim$7000 K to $\sim$5000 K. Combined with
the lack of H$\alpha$ emission, this suggests that H-recombination is likely
not responsible for the observed plateau in the light curve, unlike SNe II.

Figure 7: Pseudo-bolometric light curve of SN 2021bxu shown in red compared
with a sample of Type IIb SNe from Prentice et al. (2020) and Ca-rich SN
2021gno from Ertini et al. (2023) in the range $4000-10000$ Å. The time axis
for all SNe has the zero-point at the estimated time of explosion.

Pseudo-bolometric light curves are used for a direct comparison to similar
SNe. Instead of the full wavelength range, pseudo-bolometric light curves are
defined only within a finite range usually covering the wavelengths of the
observed bandpasses used. We compute the pseudo-bolometric light curve by
integrating the fluxes in the range $4000-10000$ Å and compare it with a
sample from Prentice et al. (2020) and SN 2021gno (Ertini et al., 2023), due
to their potential similarities in the light curve shape, as seen in Figure 7.
The slope of the initial decline is similar to that of SN 1993J and SN
2021gno; however, they both have a distinct second peak, whereas SN 2021bxu
shows a plateau. SN 2021gno has the lowest luminosity with rapid photometric
evolution and low explosion energy, 56Ni mass and ejecta mass, characteristic
of Ca-rich SNe (Ertini et al., 2023). We note again that SN 2021bxu is unique
due to its low peak luminosity at
$\log(L_{\mathrm{pseudo}}/\mathrm{erg\,s^{-1}})=42.0$ and a distinct plateau
phase at $\log(L_{\mathrm{pseudo}}/\mathrm{erg\,s^{-1}})\sim 41.6$ from
$\sim$10 to $\sim$20 days post-explosion. This plateau is possibly due to an
underlying secondary peak from the radioactive decay of 56Ni similar to SN
1993J and SN 2021gno (further discussed in Sections 6 and 7).

##  5 Spectroscopic Analysis

###  5.1 Line IDs

Due to the homologous expansion of the ejecta, measuring line velocities as a
function of time allows us to examine the chemical composition of the ejecta,
and understand the structure and the mixing within the explosion. We identify
the lines by comparing the spectral features to literature and cross-checking
with measured velocities (Section 5.2). There is a total of ten optical
spectra ranging from 7 to 31 days after estimated time from explosion. This
allows us to explore the velocity evolution of the absorption features.

The main absorption features are labeled in the spectra shown in Figure 3. We
identify strong absorption features from He i $\lambda 5876$ and He i $\lambda
6678$ along with weaker hydrogen Balmer series (H$\alpha$ $\lambda 6563$,
H$\beta$ $\lambda 4861$, and H$\gamma$ $\lambda 4340$). The presence of strong
helium with some hydrogen is characteristic of a Type IIb SN and confirms the
typing of SN 2021bxu as Type IIb. Absorption features from heavier elements
such as O i $\lambda 7774$, Si ii $\lambda\lambda 4130,5972,6355$, Ca ii H&K
$\lambda\lambda 3934,3969$, IR-triplet $\lambda\lambda 8498,8542,8662$, and a
forest of Fe ii lines including the strong Fe ii $\lambda 5169$, are also
present. We also identify absorption features from neutron capture elements
such as Sc ii $\lambda\lambda 5527,5698,6280$ and Ba ii $\lambda 6142$ in SN
2021bxu. Disentangling whether these are r- or s-process elements is beyond
the scope of this study. Interestingly, the neutron capture elements are
usually found in SN 1987A-like objects (Williams, 1987; Tsujimoto & Shigeyama,
2001). Sc and Ba have also been observed in the subluminous SN Ia PTF 09dav
(Sullivan et al., 2011). This demonstrates that, although SN 2021bxu is
formally classified as a SN IIb, it has some spectroscopic similarities to SNe
II and SNe Ia.

###  5.2 Line Velocities

We measure the Doppler shift of the minima of the spectral features to
determine the velocities and chemical composition of the ejecta. The
velocities for He i $\lambda\lambda 5876,6678,7065$, H$\alpha$ $\lambda 6563$,
Fe ii $\lambda 5169$, and O i $\lambda 7774$ are computed using
misfits999https://github.com/sholmbo/misfits (Holmbo, 2020), which is an
interactive tool used to measure spectral features in spectra of transients
and calculate their errors. Within misfits, we smooth the spectrum by applying
a low-pass filter to the Fourier-transformed data as described in Marion et
al. (2009) and obtain best-fit Gaussians to the absorption features with a
fixed local continuum. The best-fit mean of the Gaussian with the associated
error from Monte Carlo iterations is taken to be the absorption feature’s
observed wavelength which is then converted to a line velocity.

Figure 8 shows the velocity evolution of the selected features. The velocity
of the H$\alpha$ $\lambda 6563$ feature stays constant at $\sim
7200\,\mathrm{km\,s^{-1}}$ from $\sim$10 to $\sim$30 days after explosion,
demonstrating that at these early phases the photosphere has already reached
the bottom of the H layer. Interestingly, the feature from Si ii $\lambda
6355$ as seen in Figure 3 could have some contribution from a high velocity
H$\alpha$ $\lambda 6563$ component at $\sim
13{,}000\,\mathrm{km}\,\mathrm{s^{-1}}$. This also matches with the small
feature just bluer to H$\beta$ $\lambda 4861$. Therefore, there could be a
detached high velocity H component in the ejecta. It may be the case that this
comes from interaction with an extended envelope.

Figure 8: Evolution of line velocities of He i $\lambda 5876$, $\lambda 6678$,
$\lambda 7065$, H$\alpha$ $\lambda 6563$, Fe ii $\lambda 5169$, and O i
$\lambda 7774$ measured from the spectra using misfits. Only those epochs are
included for each line where a clear measurement was possible. Note that the
Fe ii $\lambda 5169$ velocity evolution may be uncertain because it comes from
a blend of spectral features in the forest of Fe lines. Figure 9: Evolution of
line velocities of SN 2021bxu compared to a sample of SNe IIb, Ib, Ic from Liu
et al. (2016) and Ca-rich IIb SNe from Das et al. (2022). The open markers
show a rolling median for each SN type with a bin size of five days where the
shaded regions represent the $16^{\mathrm{th}}$ and $84^{\mathrm{th}}$
percentiles of the distribution in the bin indicating the dispersion of
velocities. In the case of double-peaked light curves, the phases are relative
to the second bolometric peak. Values for SN 2021bxu are shown as filled red
circles.

The velocity of the He i $\lambda 5876$ line decreases as a function of time
as the photosphere recedes, from $\sim 7000\,\mathrm{km\,s^{-1}}$ at day $+7$
to $\sim 5700\,\mathrm{km\,s^{-1}}$ at day $+21$. At these phases the He i
$\lambda 5876$ line velocity follows the photospheric velocity, after which it
plateaus, demonstrating that the base of the He layer is at $\sim
5500\,\mathrm{km\,s^{-1}}$. The velocity of the He i $\lambda 7065$ line is
only measured in the later two spectra, thus not showing its early evolution
and making it unreliable for analysis. In SE-SNe the He lines require non-
thermal excitation. Therefore they get stronger over time as the density of
the ejecta decreases and the mean free path of the $\gamma$-rays can increase
(Lucy, 1991).

The O i $\lambda 7774$ line shows the lowest velocity at
$<4700\,\mathrm{km\,s^{-1}}$ throughout the time range. This is expected as
the progenitor star would have a layered structure prior to explosion where
heavier elements are further in, towards the center of the star. The Fe ii
$\lambda 5169$ feature shows a similar decline in velocity to that of other
lines at later times. However, it has slightly higher velocities than oxygen,
possibly caused by the fact that there could be primordial Fe-mixing
throughout the progenitor star, leading to Fe in the top layers. Due to the
Einstein coefficient values of Fe ii $\lambda 5169$, only a small abundance is
sufficient to produce the opacity needed for a strong line. This Fe ii
$\lambda 5169$ line is used to break the Arnett degeneracy in modelling the
bolometric light curve (see Section 6).

In Figure 9, we compare our measurements of line velocities with Liu et al.
(2016), which provides a spectroscopic sample of SE-SNe, and with Das et al.
(2022), which provides spectroscopic measurements for Ca-rich IIb SNe. We
compute a rolling median for each SN type with a bin size of five days where
the shaded regions represent the $16^{\mathrm{th}}$ and $84^{\mathrm{th}}$
percentiles of the distribution in the bin indicating the dispersion of
velocities. The sample of SNe IIb has velocities lower than those of SNe Ib
for most of the lines with Ca-rich IIb SNe showing velocities similar to those
of SNe IIb. All line velocities for SN 2021bxu are consistently lower than the
median of all types by at least $\sim 1500\,\mathrm{km\,s^{-1}}$, emphasizing
its uniqueness and implying that the kinetic energy of SN 2021bxu is lower
than that of typical SE-SNe.

###  5.3 Time of Explosion

Using line velocities of He i $\lambda 5876$ from the spectra (see Section
5.2) along with radii computed from the black-body fits, we trace back to the
time of explosion. Assuming a homologous expansion of ejecta, the relation is
given by $t_{\mathrm{exp}}\propto R_{\mathrm{BB}}/v_{\mathrm{ph}}$, which
becomes $t_{\mathrm{exp}}\approx R_{\mathrm{BB}}/v_{\mathrm{ph}}$ for small
initial radius compared to the post-explosion radius of the ejecta. We expect
the estimated explosion time to fall between the last non-detection (6.01 days
before discovery) and the discovery. We choose measurements from two spectra
where the He i $\lambda 5876$ velocity is still linearly declining and has not
plateaued, ensuring that these values are representative of the photosphere
and not the base of the He layer. Using the values $v_{\mathrm{ph}}=\\{6600\pm
300,6400\pm 200\\}\,\mathrm{km\,s^{-1}}$ and $R_{\mathrm{BB}}=\\{(6.1\pm
0.2)\times 10^{14},(7.6\pm 0.3)\times 10^{14}\\}\,\mathrm{cm}$, we obtain the
averaged time of explosion within the expected window, at $5.0\,\pm
0.4\,\mathrm{days}$ before discovery, on $\mathrm{MJD}\,59246.3\,\pm
0.4\,\mathrm{days}$. This is the estimated time of explosion we adopt
throughout this paper. The uncertainty on the time of explosion is purely
statistical and is appropriately propagated from the uncertainties on radius
and velocity.

###  5.4 Spectral Comparison with Similar Supernovae

Figure 10: NOT spectrum of SN 2021bxu near the 56Ni peak shown in red compared
to the NOT spectrum of Ca-rich Ib SN 2021gno (Ertini et al., 2023), the Asiago
Observatory spectrum of Type IIb SN 1993J (Barbon et al., 1995), the Keck-LRIS
spectrum of Ca-rich Type Ib SN 2016hgs (De et al., 2018), the Palomar-200inch
spectrum of Ca-rich Type IIb SN 2019pof (Das et al., 2022), the NTT spectrum
of Ca-rich Type IIb SN 2018gjx (Prentice et al., 2020), and the International
Ultraviolet Explorer (IUE) spectrum of Type II SN 1987A (Pun et al., 1995).
Major absorption features in SN 2021bxu are marked on the plot along with the
two telluric regions shown in gray bands. All phases in this figure are
relative to the 56Ni peak of each SN. Figure 11: Spectral comparison of SN
2021bxu with SN 1993J. The spectra for SN 1993J are shown in black with the
epochs labeled on the left. The spectra for SN 2021bxu are shown in red with
the epochs labeled on the right. For both objects, the epochs are relative to
the estimated time of explosion. The spectra at $+5.35$, $+10.55$, $+21.5$,
and $27.5$ days for SN 1993J are from Barbon et al. (1995), $+15.5$ days is
from Matheson et al. (2000b), and $+31.5$ days is from Matheson et al.
(2000a).

In this section, we compare the spectrum of SN 2021bxu near the peak due to
56Ni with similar SNe near their peaks. Figure 10, along with the line IDs,
shows spectra for a Type II-pec SN 1987A, a Type IIb SN 1993J, a Ca-rich Type
Ib SN 2016hgs and SN 2021gno, and Ca-rich Type IIb SNe 2018gjx, and 2019pof.
These SNe, except SN 1987A, are chosen for comparison because of their
similarities either in light curve shape or spectral features. SN 1987A is
included for its strong H lines showing recombination.

SN 2021bxu is dissimilar to SN 1987A due to the lack of strong hydrogen Balmer
features (H$\alpha$ $\lambda 6563$, H$\beta$ $\lambda 4861$, and H$\gamma$
$\lambda 4340$) and also due to the absence of the Na i D $\lambda\lambda
5890,5896$ doublet lines. However, SN 2021bxu does show features from the
neutron capture elements Sc ii $\lambda\lambda 5527,5698,6280$ and Ba ii
$\lambda 6142$ which are typically seen in SN 1987A-like objects. There are
also no signatures of strong H emission, which is usually seen in SNe IIP,
hence providing further evidence that the plateau in SN 2021bxu is unlikely to
be caused by H-recombination. SN 2021bxu is more similar to SN 1993J,
reinforcing the typing of SN 2021bxu as a Type IIb.

Comparing the spectral time-series of SN 2021bxu with SN 1993J at similar
epochs in Figure 11, we note that both SNe display similar absorption lines in
their spectra. SN 1993J shows broader features at higher velocities between
$10{,}000$ and $16{,}000\,\mathrm{km\,s^{-1}}$ (Garnavich & Ann, 1994)
compared to the velocities of SN 2021bxu that we measure in the range
$4000<v<7000\,\mathrm{km\,s^{-1}}$. This points to SN 2021bxu having lower
energies and masses than SN 1993J. Although they both show similar features,
SN 2021bxu evolves more quickly showing strong metal features from Ca ii [IR-
triplet] and O i $\lambda 7774$ by day 30. The He i $\lambda 5876$ and He i
$\lambda 6678$ features also quickly get deeper, showcasing the fast evolution
of SN 2021bxu. Moreover, SN 1993J shows weaker He i $\lambda 5876$ absorption
than SN 2021bxu but H$\alpha$ $\lambda 6563$ absorption of similar depth. Due
to their spectral similarities, it may be the case that SN 2021bxu and SN
1993J are similar objects, both with a large initial decline but with
different amount of 56Ni, energy and mass. We discuss this further in Section
7.

The Ca-rich SNe show characteristically strong Ca ii compared to O i. During
the photometric phases, SN 2021bxu shows comparably strong Ca ii absorption to
that of the Ca-rich Ib and IIb SNe. One of the defining properties of Ca-rich
transients is that they quickly transition to nebular phase marked by Ca ii
and O i emission. For instance, SN 2019ehk (Ca-rich IIb) started exhibiting
nebular features as early as 30 days after explosion. SN 2021bxu does not show
any nebular features within $\sim$30 days after explosion. Due to the lack of
late phase spectra for SN 2021bxu, we cannot directly compare it to Ca-rich
SNe using the [Ca ii]/[O i] ratio. However, the hydrogen-rich SN 2021bxu shows
dissimilarities to the Ca-rich Ib SNe (SN 2021gno, SN 2016hgs), which lack
hydrogen in their spectra, but is spectroscopically similar to IIb (SN 1993J)
and hydrogen- and Ca-rich IIb SNe (SN 2018gjx) near peak.

##  6 Modelling

Figure 12: Bolometric light curve of SN 2021bxu shown in black fit using a
two-component model: shock cooling and 56Ni decay including $\gamma$-ray
leakage. The initial decline is fit with the shock interaction with extended
envelope model from Piro et al. (2021) shown as the dotted-dashed line and
56Ni decay comes from the model from Arnett (1982) following an additional
correction for $\gamma$-ray leakage as shown by Wheeler et al. (2015) shown as
the dashed line. The best-fit model is the solid red line and the gray lines
show 100 randomly selected parameters from the Monte Carlo samples signifying
the uncertainty on the best-fit.

To understand the origin of the unique shape of SN 2021bxu’s light curve, we
analyze the explosion by fitting the bolometric and pseudo-bolometric light
curves of SN 2021bxu with SN explosion models from the literature. We do this
by using a two-component model where the first component is the initial
cooling phase, and the second component is the radioactive decay of 56Ni
including $\gamma$-ray leakage.

The analytic model we consider for the initial cooling phase is from Piro et
al. (2021, hereafter P21). P21 describes the shock interaction with the
extended material surrounding the progenitor star once it is in thermal
equilibrium and in homologous expansion phase, given a two-component density
profile with steep radial dependence in the outer region and shallower radial
dependence in the inner region. The free parameters in this model are the mass
and radius of the extended material ($M_{e}$ and $R_{e}$, respectively) with
$E_{e}$, the energy imparted by the SN shock to the extended material,
depending on the total explosion energy, ejecta mass, and mass of the extended
material. The analytic model from P21 is an improvement over previous similar
models (e.g., Piro & Nakar, 2013; Piro, 2015) as it better matches the
observations in the early shock-cooling emission and is tested against
numerical models.

Together with the P21 model, we use the analytic models from Arnett (1982,
hereafter A82) for the plateau/secondary peak from 56Ni decay, which provides
an estimate of the total ejecta mass $\left(M_{\mathrm{ej}}\right)$, 56Ni mass
$\left(M_{\mathrm{Ni}}\right)$, and total explosion energy of the SN
$\left(K_{\mathrm{ej}}\right)$. The A82 model fits for a degenerate parameter
that depends on $M_{\mathrm{ej}}$ and $K_{\mathrm{ej}}$ as
$M_{\mathrm{ej}}^{3/4}/K_{\mathrm{ej}}^{1/4}$. This degeneracy is broken by
using the photospheric velocity of the Fe ii $\lambda 5169$ line near maximum
of the 56Ni peak (Taddia et al., 2018). After interpolating between the
spectral epochs, we use $v_{\mathrm{ph,Fe}}=5200\pm
100\,\mathrm{km}\,\mathrm{s^{-1}}$ near peak. We include an additional
correction for $\gamma$-ray leakage at late times as shown by Wheeler et al.
(2015, hereafter W15) with a multiplicative factor of $1-e^{-(T_{0}/t)^{2}}$
where $T_{0}$ is the characteristic time-scale for the $\gamma$-ray leakage,
depending on $M_{\mathrm{ej}}$ and $K_{\mathrm{ej}}$ as

$T_{0}=\left(\frac{C\kappa_{\gamma}M_{\mathrm{ej}}^{2}}{K_{\mathrm{ej}}}\right)^{1/2},$
(1)

where $C$ is a dimensionless structure constant dependent on the slope of the
density profile (typically $C\sim 0.05$) and $\kappa_{\gamma}$ is the opacity
to $\gamma$-rays (fiducial value of
$\kappa_{\gamma}=0.03\,\mathrm{cm^{2}\,g^{-1}}$).

We adopt a chi-squared minimization approach to fit the bolometric and pseudo-
bolometric light curves of SN 2021bxu and obtain the best-fit parameters.
Nakar & Piro (2014) give a relation between the P21 parameters $E_{e}$ and
$M_{e}$ along with a dependency on the A82 parameters $M_{\mathrm{ej}}$ and
$K_{\mathrm{ej}}$,

$E_{e}\approx 2\times
10^{49}\left(\frac{K_{\mathrm{ej}}}{10^{51}\,\mathrm{erg}}\right)\left(\frac{M_{\mathrm{ej}}}{3\,M_{\odot}}\right)^{-0.7}\left(\frac{M_{e}}{0.01\,M_{\odot}}\right)^{0.7}\,\mathrm{erg}.$
(2)

Therefore, we eliminate $E_{e}$ from the fitting routine and later solve for
it using Eq. 2. With only $\sim$5 data points in the initial decline, the
parameters from P21 for the early light curve are difficult to constrain.
However, the model fits well the later part where A82+W15 dominates and there
are more points to fit. Changing the initial decline of the light curve barely
affects the second component assumed from 56Ni decay. The best-fit parameters
are given by the maximum-likelihood values and the uncertainties on best-fit
parameters are given by the $16^{\mathrm{th}}$ and $84^{\mathrm{th}}$
percentiles from fitting the model to Monte Carlo resamples of the light
curve. The uncertainties for the parameters derived using the best-fit values
are propagated appropriately from the uncertainties on the best-fit values.
The Monte Carlo resampling ensures that statistical uncertainties from the
photometry as well as systematic uncertainties from distance measurement are
appropriately considered.

When fitting the bolometric and pseudo-bolometric light curves, we find that
the P21+A82+W15 model can successfully describe the data. Figure 12 shows the
two components separately as well as the combined fit in red for the
bolometric light curve. The surrounding gray region is randomly drawn Monte
Carlo samples showing the uncertainty on the best-fit model. The best-fit
parameters for the bolometric and pseudo-bolometric light curve fits are
listed in Table 4. The parameters for the pseudo-bolometric light curve are
provided because they are useful in direct comparison with literature values.

Table 4: Best-fit parameters after fitting the P21+A82+W15 model to the bolometric and pseudo-bolometric light curves Parameter | Bolometric | Pseudo-bolometric
---|---|---
|  | ($4000-10000$ Å)
$M_{\mathrm{Ni}}$ $[\mathrm{M_{\odot}}]$ | $0.029^{+0.004}_{-0.005}$ | $0.016^{+0.002}_{-0.003}$
$M_{\mathrm{ej}}$ $[\mathrm{M_{\odot}}]$ | $0.57^{+0.04}_{-0.03}$ | $0.52^{+0.08}_{-0.04}$
$K_{\mathrm{ej}}$ $[10^{49}\,\mathrm{erg}]$ | $9.3^{+0.7}_{-0.6}$ | $8.5^{+1.4}_{-0.7}$
$E_{e}$ $[10^{49}\,\mathrm{erg}]$ | $2.2^{+0.2}_{-0.2}$ | $2.5^{+0.6}_{-0.3}$
$M_{e}$ $[\mathrm{M_{\odot}}]$ | $0.064^{+0.003}_{-0.003}$ | $0.081^{+0.013}_{-0.010}$
$R_{e}$ $[\mathrm{R_{\odot}}]$ | $1500^{+300}_{-300}$ | $320^{+50}_{-70}$

##  7 Discussion

In this section we attempt to put our results in context relative to other SN
explosions and models. The best-fit parameters for the bolometric light curve
are $M_{\mathrm{Ni}}=0.029^{+0.004}_{-0.005}\,\mathrm{M_{\odot}}$,
$M_{\mathrm{ej}}=0.57^{+0.04}_{-0.03}\,\mathrm{M_{\odot}}$,
$K_{\mathrm{ej}}=9.3^{+0.7}_{-0.6}\times 10^{49}\,\mathrm{erg}$,
$E_{e}=2.2^{+0.2}_{-0.2}\times 10^{49}\,\mathrm{erg}$,
$M_{e}=0.064^{+0.003}_{-0.003}\,\mathrm{M_{\odot}}$, and
$R_{e}=1500^{+300}_{-300}\,\mathrm{R_{\odot}}$ (listed in Table 4). SN 2021bxu
is a Type IIb SN with low $M_{\mathrm{Ni}}$, low luminosity, and low explosion
energy. By comparison with known classes of SNe and models explaining the
observed features of SN 2021bxu, we can infer the details of the explosion and
the progenitor system.

The fit to the P21 model shows that the extended material surrounding the
progenitor of SN 2021bxu had a large radius ($R_{e}$) and low mass ($M_{e}$).
We compare this to the best-fit masses and radii of the Ca-rich IIb sample
from Das et al. (2022), who use the same model for the initial decline. Figure
13 shows SN 2021bxu along with the sample of Ca-rich IIb SNe. We see a clear
trend of decreasing radius with increasing mass. The Ca-rich Ib SN 2021gno
also falls along this trend with $R_{e}=1100\,\mathrm{R_{\odot}}$ and
$M_{e}=0.01\,\mathrm{M_{\odot}}$. For a simple check, we show a linear best-
fit and a negative correlation using the Kendall $\tau$ test, which gives
$\tau=-0.6$ with a $p$-value of 0.01. Modelling and possible physical origins
of this correlation will be the subject of a future work.

Figure 13: Mass and radius of the extended material ($M_{e}$ and $R_{e}$,
respectively) from fitting the initial decline of SN 2021bxu compared with the
sample of Ca-rich SNe IIb (Das et al., 2022) and the Ca-rich Ib SN 2021gno
(Ertini et al., 2023). The values for Ca-rich SNe IIb (orange squares) and SN
2021bxu (red circle) are obtained from fitting the P21 model and the values
for SN 2021gno (orange triangle) are obtained using a hydrodynamic code from
Bersten et al. (2011). We note a trend of decreasing radius with increasing
mass with a linear best-fit shown as the black line and Kendall $\tau$
correlation statistic given.

Theory suggests that a plateau can arise in the light curve of a SE-SN $\sim$1
day after shock-breakout when the cooling of the photosphere slows down,
allowing the recombination of ejecta layers, primarily He (Dessart et al.,
2011). Observed in simulations by Dessart et al. (2011), this plateau is found
at $\log(L_{\mathrm{bol}}/\mathrm{erg\,s^{-1}})\sim 41$ and lasts for $\sim$10
days until the SN either re-brightens for 56Ni-rich SNe or fades away for
56Ni-poor SNe. The plateau in SN 2021bxu is observed for a time-scale
comparable to that of He-recombination, however, the timing of occurrence
differs. The plateau caused by He-recombination occurs soon after explosion,
whereas, the plateau in SN 2021bxu’s light curve is not apparent until
$\sim$10 days after explosion. Moreover, the luminosity of SN 2021bxu at the
plateau is $\log(L_{\mathrm{bol}}/\mathrm{erg\,s^{-1}})\sim 41.8$, almost an
order of magnitude higher. This suggests that the observed plateau in SN
2021bxu is likely not due to He-recombination.

SN 2021bxu shows photometric and spectroscopic similarities to SN 1993J but
with lower total mass and explosion energy. SN 1993J shows an initial decline
due to the post shock cooling through a thin H-rich envelope of extended
material and the main peak from 56Ni decay. Bersten et al. (2012) and Prentice
et al. (2016) estimate $M_{\mathrm{Ni}}$ in the range
$0.084-0.15\,\mathrm{M_{\odot}}$ and $M_{\mathrm{ej}}=3.3\,\mathrm{M_{\odot}}$
from fitting the bolometric light curve of SN 1993J. This is $\sim 3-5$ times
more 56Ni mass than in SN 2021bxu and $\sim$6 times more ejecta mass. Higher
$M_{\mathrm{Ni}}$ and $M_{\mathrm{ej}}$ lead to the main 56Ni powered light
curve to be more luminous and broader, respectively, which is seen in the
light curve of SN 1993J. The light curve of SN 1993J has been described by a
binary model with both stars having a mass of $\sim 15\,\mathrm{M_{\odot}}$
(Podsiadlowski et al., 1993; Nomoto et al., 1993). The authors conclude that
SN 1993J had a G8-K0 yellow supergiant progenitor with a binary companion. The
strong initial decline can be explained by the explosion of this progenitor
that has experienced mass-loss due to winds, or, more likely, through mass
transfer to a companion star. Given that SN 2021bxu is a similar object to SN
1993J at least in the initial decline and a two-component light curve, SN
2021bxu may be conspiring to show a plateau instead of an evident second peak
owing to the small amount of 56Ni produced and the overall lower energies.

For a direct comparison with other studies without dealing with the SED flux
extrapolation problems, we use the 56Ni mass and the ejecta mass derived from
pseudo-bolometric light curves. Figure 14 shows the best-fit parameters for SN
2021bxu from the pseudo-bolometric light curve compared to a sample of SNe
IIb, Ib, Ic from Prentice et al. (2016) and Prentice et al. (2019). In order
to obtain the pseudo-bolometric light curve in the range $4000-10000$ Å,
Prentice et al. (2016) and Prentice et al. (2019) make use of $BVRI$-bands and
fit the A82 model to find the best-fit parameters, mainly $M_{\mathrm{Ni}}$
and $M_{\mathrm{ej}}$. SN 2021bxu shows the lowest value of the entire SE-SNe
sample for $M_{\mathrm{Ni}}$ and $M_{\mathrm{ej}}$ as derived from the pseudo-
bolometric light curves.

Fully bolometric light curves are a better probe of the physical parameters
than pseudo-bolometric light curves. Figure 15 shows the best-fit parameters
from the bolometric light curve compared to a sample of SNe IIb, Ib, Ic from
Prentice et al. (2016) and Ca-rich IIb SNe from Das et al. (2022). As seen in
Figure 15, $M_{\mathrm{Ni}}$ for SN 2021bxu is also the lowest in the
bolometric sample of SE-SNe; the only lower values being that of the ultra-
stripped SNe (US-SNe). $K_{\mathrm{ej}}$ for SN 2021bxu is also on the lower
end of the comparison sample. $M_{\mathrm{ej}}$ for SN 2021bxu is closer to
more common values and falls in the category of strongly-stripped SNe (SS-
SNe). However, the best-fit parameters from a full bolometric light curve
should be interpreted cautiously because the contribution to the total
luminosity from the unobserved wavelengths is highly uncertain and treated
differently by different studies. For example, Prentice et al. (2016) assume a
10% contribution from the unobserved wavelengths after considering
$UBVRI$-bands and near-IR. In this study, we assume a blackbody SED for
extrapolation to the unobserved wavelengths after direct integration in the
observed bands.

Figure 14: Distributions of $M_{\mathrm{Ni}}$ and $M_{\mathrm{ej}}$ derived
using the pseudo-bolometric light curves in the range $4000-10000$ Å for the
sample of SNe IIb, Ib, Ic from Prentice et al. (2016) and Prentice et al.
(2019). The values for SN 2021bxu are shown in red. Figure 15: Comparison of
SN 2021bxu with a sample of SNe IIb, Ib, Ic from Taddia et al. (2018) and
Prentice et al. (2016), Ca-rich IIb SNe from Das et al. (2022), and US-SNe
from De et al. (2018) and Yao et al. (2020). The 56Ni mass
($M_{\mathrm{Ni}}$), ejecta mass ($M_{\mathrm{ej}}$), and kinetic energy
($K_{\mathrm{ej}}$) are derived using the full bolometric light curves for all
SNe. The vertical dashed lines indicate the rough boundaries dividing ultra-
stripped SNe (US-SNe), strongly-stripped SNe (SS-SNe), and stripped-envelope
SNe (SE-SNe; Das et al., 2022). The values for SN 2021bxu are shown in red in
each panel.

Observed properties like low ejecta mass, low peak luminosity, faster-than-
normal photometric and spectroscopic evolution are exhibited by the Ca-rich
class of transients. They reach nebular phase quickly within as low as $\sim$1
month after explosion (e.g., Shen et al., 2019; Nakaoka et al., 2021; Das et
al., 2022). Core collapse Ca-rich transients are characterized by the presence
of strong helium spectral features near peak and the appearance of calcium
emission soon after. However, we do not find any strong calcium emission lines
in the spectra, at least at the early epochs of $\sim$31 days after explosion.
In SN 2021bxu, the presence of hydrogen along with neutron capture elements
rules out most systems with WD progenitors.

The current popular theory suggests that Ca-rich SNe could result from low
mass He stars ($M_{\mathrm{ZAMS}}\sim 8-12\,\mathrm{M_{\odot}}$) that are
highly stripped down to less than $\sim 3\,\mathrm{M_{\odot}}$, suggesting a
new class of SS-SNe (e.g., Das et al., 2022; Ertini et al., 2023). SS-SNe form
a transition class between SE-SNe and ultra-stripped SNe (US-SNe; Tauris et
al., 2015; Das et al., 2022). As a result of their low initial masses, the
resultant ejecta masses are also low (less than $\sim 1\,\mathrm{M_{\odot}}$)
along with low 56Ni masses. Ertini et al. (2023) showed that the progenitor
scenario of the Ca-rich SN 2021gno can be modelled with a highly-stripped
massive star with an ejecta mass of $0.8\,\mathrm{M_{\odot}}$ and a 56Ni mass
of $0.024\,\mathrm{M_{\odot}}$, comparable to the values for SN 2021bxu. We
see in Figure 15 that SN 2021bxu has $M_{\mathrm{ej}}$ consistent with the
sample of Ca-rich IIb SNe and in the range demarcating SS-SNe, but
$M_{\mathrm{Ni}}$ and $K_{\mathrm{ej}}$ are on the lowest end of the
distribution.

Ca-rich IIb SNe are usually found in star-forming regions of galaxies and at
smaller separations from the center of the host galaxy compared to Type I Ca-
rich transients (Das et al., 2022). As seen in Section 2, the host galaxy ESO
478- G 006 is a star-forming galaxy with a star-formation rate of
$30^{+4}_{-7}\,\mathrm{M_{\odot}\,yr^{-1}}$, typical of the hosts of Ca-rich
IIb SNe (Das et al., 2022). The projected physical offset of SN 2021bxu from
its host is $9.2\pm 0.6\,\mathrm{kpc}$, which is again typical for Ca-rich IIb
SNe and SE-SNe (Das et al., 2022). Despite its similarities with Ca-rich IIb
SNe, the lack of nebular spectra of SN 2021bxu showing Ca ii emission makes it
difficult to conclusively judge whether SN 2021bxu is a Type IIb Ca-rich SN.
Nevertheless, SN 2021bxu is an interesting object and shows that stars can
explode with a small $M_{\mathrm{ej}}$ and $K_{\mathrm{ej}}$ with a large
radius while making small amounts of 56Ni.

##  8 Conclusion

In this study, we present observations and analysis of a unique SE-SN, SN
2021bxu. As discussed in Section 4, we show that SN 2021bxu had a large
initial decline in brightness followed by a short plateau not caused by H- or
He-recombination, which is unusual for a Type IIb SN. It is on the faint end
of the sample of SE-SNe from the literature, with the peak absolute magnitude
of $M_{r}=-16.86\pm 0.16\,\mathrm{mag}$ and the average absolute magnitude
during the plateau of $M_{r}\sim-15.9\,\mathrm{mag}$. The pseudo-bolometric
luminosity is also fainter compared to most other SE-SNe, with a peak of
$\log(L_{\mathrm{pseudo}}/\mathrm{erg\,s^{-1}})=42.0$ and a distinct plateau
phase at $\log(L_{\mathrm{pseudo}}/\mathrm{erg\,s^{-1}})\sim 41.6$. The
overall light curve shape in $gri$-bands matches most closely to that of Ca-
rich IIb SNe, most of which show an initial decline and a second peak, and to
that of SNe IIb. The initial decline in the light curve of SN 2021bxu has a
similar slope to the initial decline of SN 1993J but SN 1993J shows a distinct
second peak from 56Ni at
$\log(L_{\mathrm{pseudo}}/\mathrm{erg\,s^{-1}})=42.23$ and has a slower
evolution at late-times indicating a larger ejecta mass.

With the presence of strong helium lines and weaker hydrogen lines, SN 2021bxu
is a Type IIb SN (Section 5). We constrain the time of explosion using the He
i $\lambda 5876$ line velocity and the blackbody radius evolution and find it
to be $5.0\pm 0.4\,\mathrm{days}$ before discovery. It evolves quickly to show
absorption features from heavier metals like oxygen, calcium, silicon, iron,
and neutron capture elements like barium and scandium, which get stronger over
the 30-day spectral time-series. We note that SN 2021bxu shows spectral
similarities to Type IIb SN 1993J as well as to Ca-rich IIb SNe during
photometric phases with most of the same features observed. SN 1993J has
velocities higher by a factor of $\sim$2 compared to SN 2021bxu. We also note
similarities to SN 2021gno in terms of light curve evolution and modelling.
From a photometric and spectroscopic analysis, we conclude that SN 2021bxu is
a fast evolving SN, with a short distinct plateau phase not caused by H- or
He-recombination and with some of the lowest observed line velocities compared
to samples of Ca-rich IIb SNe and SE-SNe.

Following the modelling of the bolometric and pseudo-bolometric light curves
in Section 6, we see that the light curves of SN 2021bxu can be well modelled
by a composite model including interaction of the shock with an extended
envelope of material surrounding the progenitor and the normal radioactive
decay of 56Ni. We obtain the physical parameters for the explosion such as
$M_{\mathrm{Ni}}$, $M_{\mathrm{ej}}$, $K_{\mathrm{ej}}$, and $T_{0}$, and the
properties of the extended material $E_{e}$, $M_{e}$, and $R_{e}$ from
bolometric and pseudo-bolometric light curves (see Table 4). Ertini et al.
(2023) performed hydrodynamic modelling of SN 2021gno and similarly showed
that the initial cooling phase can be explained by extended circumstellar
material composed mainly of He, maybe with traces of H, and the second peak
can be explained by the radioactive decay of 56Ni. We note that the 56Ni mass
and kinetic energy for SN 2021bxu are on the lower end of the distribution for
Ca-rich SNe and SE-SNe. The ejecta mass falls within the range of Ca-rich SNe
and SS-SNe.

Overall we determine that SN 2021bxu likely occurred from a lower-mass
progenitor which had a large radius at the time of explosion and an extended
envelope having experienced mass-loss potentially to a companion star, similar
either to the SN 1993J scenario or to the strongly-stripped Ca-rich SNe such
as SN 2021gno. However, to fully understand and characterize explosions
similar to SN 2021bxu and to better constrain the parameters of the initial
decline in an effort to constrain the immediate surroundings of the
progenitor, further high precision multi-band observations of SNe in their
infant stages are needed. POISE promises to deliver such a data set over the
coming years. In addition to the early-time data, late-time data for such
objects in the nebular phase would also prove beneficial in discerning if they
are Ca-rich transients and allude to new classes such as SS-SNe.

## Acknowledgements

We thank Federica Chiti for helpful discussion. D.D.D. and B.J.S. acknowledge
support from NSF grant AST-1908952. C.A. and J.M.D. acknowledge support by
NASA grant JWST-GO-02114.032-A and JWST-GO-02122.032-A. E.B. acknowledges
support by NASA grant JWST-GO-02114.032-A. L.G. acknowledges financial support
from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia
Estatal de Investigación (AEI) 10.13039/501100011033, and the European Social
Fund (ESF) "Investing in your future" under the 2019 Ramón y Cajal program
RYC2019-027683-I and the PID2020-115253GA-I00 HOSTFLOWS project, from Centro
Superior de Investigaciones Científicas (CSIC) under the PIE project
20215AT016, and the program Unidad de Excelencia María de Maeztu
CEX2020-001058-M. M.D.S. and the Aarhus supernova group acknowledge support
from the Independent Research Fund Denmark (IRFD, grant number 8021-00170B)
and the Villum Fonden (28021). NUTS2 is support in part by the Instrument
Center for Danish Astrophysics (IDA). J.P.A acknowledges funding from ANID,
Millennium Science Initiative, ICN12_009. M.G. is supported by the EU Horizon
2020 research and innovation programme under grant agreement No 101004719.
T.E.M.B. acknowledges financial support from the Spanish Ministerio de Ciencia
e Innovación (MCIN), the Agencia Estatal de Investigación (AEI)
10.13039/501100011033, and the European Union Next Generation EU/PRTR funds
under the 2021 Juan de la Cierva program FJC2021-047124-I and the
PID2020-115253GA-I00 HOSTFLOWS project, from Centro Superior de
Investigaciones Científicas (CSIC) under the PIE project 20215AT016, and the
program Unidad de Excelencia María de Maeztu CEX2020-001058-M. M.N. is
supported by the European Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (grant agreement No. 948381)
and by funding from the UK Space Agency. Based on observations collected at
the European Organisation for Astronomical Research in the Southern
Hemisphere, Chile, as part of ePESSTO+ (the advanced Public ESO Spectroscopic
Survey for Transient Objects Survey). ePESSTO+ observations were obtained
under ESO program IDs 106.216C and 108.220C (PI: Inserra). Based on
observations obtained at the international Gemini Observatory, a program of
NSF’s NOIRLab, which is managed by the Association of Universities for
Research in Astronomy (AURA) under a cooperative agreement with the National
Science Foundation on behalf of the Gemini Observatory partnership: the
National Science Foundation (United States), National Research Council
(Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio
de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência,
Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space
Science Institute (Republic of Korea). This work was enabled by observations
made from the Gemini North telescope, located within the Maunakea Science
Reserve and adjacent to the summit of Maunakea. We are grateful for the
privilege of observing the Universe from a place that is unique in both its
astronomical quality and its cultural significance.

## Data Availability

The photometry presented in this paper is available in a machine-readable
format from the online journal as supplementary material. A portion is shown
in Tables 5, 6, and 7 for guidance regarding its form and content. The spectra
presented in this paper are available via the
WISeREP101010https://www.wiserep.org/ archive (Yaron & Gal-Yam, 2012).

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##  Appendix A Photometry Data Tables

This section gives the photometry for SN 2021bxu in Table 5, photometry of the
local sequence stars used to calibrate the Swope light curves in Table 6, and
photometry of the host galaxy ESO 478- G 006 in Table 7.

Table 5: Photometry of SN 2021bxu

MJD | Apparent | Absolute | Filter | Telescope
---|---|---|---|---
(days) | Magnitude | Magnitude |  |
59218.29 | $>22.29$ | $-$ | $w$ | Pan-STARRS
59245.12 | $>17.60$ | $-$ | $g$ | ASAS-SN
59245.26 | $>19.60$ | $-$ | $o$ | ATLAS
59251.06 | $17.17\pm 0.09$ | $-17.16\pm 0.19$ | $g$ | ASAS-SN
59251.29 | $17.222\pm 0.018$ | $-17.11\pm 0.16$ | $c$ | ATLAS
59252.03 | $17.462\pm 0.007$ | $-16.86\pm 0.16$ | $r$ | Swope
59252.03 | $17.599\pm 0.011$ | $-16.71\pm 0.16$ | $i$ | Swope
59252.04 | $17.315\pm 0.011$ | $-17.03\pm 0.16$ | $B$ | Swope
59252.04 | $17.371\pm 0.008$ | $-16.96\pm 0.16$ | $g$ | Swope
59252.04 | $17.372\pm 0.010$ | $-16.95\pm 0.16$ | $V$ | Swope
59252.05 | $17.29\pm 0.11$ | $-17.04\pm 0.19$ | $g$ | ASAS-SN
59253.03 | $17.689\pm 0.015$ | $-16.63\pm 0.16$ | $r$ | Swope
59253.03 | $17.615\pm 0.017$ | $-16.71\pm 0.16$ | $V$ | Swope
59253.03 | $17.672\pm 0.012$ | $-16.66\pm 0.16$ | $g$ | Swope
59253.03 | $17.838\pm 0.019$ | $-16.51\pm 0.16$ | $u$ | Swope
59253.04 | $17.657\pm 0.019$ | $-16.68\pm 0.16$ | $B$ | Swope
59253.83 | $18.03\pm 0.17$ | $-16.3\pm 0.2$ | $g$ | ASAS-SN

Note: All magnitudes are given in the AB system. This table is available in
its entirety in a machine-readable form in the online journal. A portion is
shown here for guidance regarding its form and content.

Table 6: Photometry of the local sequence stars

RA | $\delta$ | $u$ | $g$ | $r$ | $i$ | $B$ | $V$
---|---|---|---|---|---|---|---
(J2000) | (J2000) | (mag) | (mag) | (mag) | (mag) | (mag) | (mag)
$32.386726$ | $-23.520622$ | $18.00\pm 0.04$ | $15.592\pm 0.008$ | $-$ | $-$ | $16.035\pm 0.010$ | $15.115\pm 0.010$
$32.213863$ | $-23.394035$ | $16.20\pm 0.02$ | $15.412\pm 0.008$ | $15.157\pm 0.012$ | $-$ | $15.620\pm 0.012$ | $15.267\pm 0.010$
$32.385872$ | $-23.476055$ | $18.77\pm 0.07$ | $16.238\pm 0.006$ | $15.203\pm 0.012$ | $-$ | $16.697\pm 0.012$ | $15.720\pm 0.009$
$32.299397$ | $-23.491762$ | $16.92\pm 0.02$ | $15.888\pm 0.007$ | $15.498\pm 0.008$ | $15.365\pm 0.005$ | $16.152\pm 0.010$ | $15.659\pm 0.009$
$32.245022$ | $-23.485291$ | $17.00\pm 0.03$ | $15.969\pm 0.007$ | $15.594\pm 0.008$ | $15.465\pm 0.005$ | $16.223\pm 0.010$ | $15.743\pm 0.009$
$32.199989$ | $-23.418266$ | $17.87\pm 0.04$ | $16.586\pm 0.006$ | $16.047\pm 0.008$ | $15.853\pm 0.005$ | $16.905\pm 0.013$ | $16.294\pm 0.010$
$32.345909$ | $-23.366911$ | $18.69\pm 0.07$ | $17.580\pm 0.007$ | $17.130\pm 0.011$ | $17.012\pm 0.010$ | $17.87\pm 0.02$ | $17.315\pm 0.014$
$32.385616$ | $-23.374666$ | $-$ | $18.673\pm 0.017$ | $17.261\pm 0.011$ | $15.899\pm 0.006$ | $19.17\pm 0.05$ | $18.02\pm 0.02$
$32.226913$ | $-23.380148$ | $-$ | $18.568\pm 0.015$ | $17.249\pm 0.011$ | $16.204\pm 0.006$ | $19.23\pm 0.06$ | $17.91\pm 0.02$
$32.393101$ | $-23.397284$ | $18.62\pm 0.06$ | $17.688\pm 0.008$ | $17.266\pm 0.011$ | $17.132\pm 0.011$ | $17.92\pm 0.02$ | $17.442\pm 0.015$
$32.283989$ | $-23.457907$ | $-$ | $18.795\pm 0.018$ | $17.449\pm 0.012$ | $16.290\pm 0.006$ | $19.36\pm 0.06$ | $18.10\pm 0.02$
$32.393776$ | $-23.471512$ | $-$ | $19.15\pm 0.03$ | $17.874\pm 0.016$ | $17.387\pm 0.013$ | $19.56\pm 0.07$ | $18.55\pm 0.04$
$32.342522$ | $-23.359810$ | $-$ | $19.42\pm 0.03$ | $18.034\pm 0.018$ | $16.771\pm 0.008$ | $19.79\pm 0.10$ | $18.71\pm 0.04$
$32.308815$ | $-23.504641$ | $-$ | $19.31\pm 0.03$ | $18.14\pm 0.02$ | $17.499\pm 0.015$ | $19.88\pm 0.10$ | $18.65\pm 0.04$
$32.287636$ | $-23.507380$ | $-$ | $19.40\pm 0.03$ | $18.46\pm 0.03$ | $18.01\pm 0.02$ | $19.92\pm 0.10$ | $18.87\pm 0.05$
$32.300083$ | $-23.356009$ | $-$ | $19.60\pm 0.04$ | $18.39\pm 0.02$ | $17.487\pm 0.015$ | $20.48\pm 0.18$ | $19.07\pm 0.06$
$32.279221$ | $-23.437853$ | $18.95\pm 0.11$ | $18.96\pm 0.02$ | $18.51\pm 0.03$ | $18.29\pm 0.03$ | $19.15\pm 0.05$ | $18.70\pm 0.04$
$32.270084$ | $-23.510548$ | $19.74\pm 0.16$ | $18.91\pm 0.02$ | $18.66\pm 0.03$ | $18.57\pm 0.04$ | $19.09\pm 0.04$ | $18.76\pm 0.04$
$32.232883$ | $-23.362158$ | $-$ | $20.38\pm 0.09$ | $19.30\pm 0.06$ | $18.52\pm 0.04$ | $-$ | $19.87\pm 0.13$
$32.285130$ | $-23.352280$ | $-$ | $19.73\pm 0.04$ | $19.46\pm 0.07$ | $19.12\pm 0.08$ | $20.13\pm 0.13$ | $19.61\pm 0.11$
$32.226944$ | $-23.370340$ | $-$ | $19.91\pm 0.05$ | $19.42\pm 0.06$ | $19.17\pm 0.08$ | $20.20\pm 0.13$ | $19.71\pm 0.12$
$32.355373$ | $-23.451790$ | $-$ | $20.10\pm 0.07$ | $18.78\pm 0.03$ | $18.24\pm 0.03$ | $20.46\pm 0.17$ | $19.13\pm 0.07$
$32.256466$ | $-23.395746$ | $-$ | $-$ | $19.39\pm 0.06$ | $18.72\pm 0.05$ | $-$ | $-$
$32.336956$ | $-23.473841$ | $-$ | $20.22\pm 0.07$ | $19.09\pm 0.04$ | $18.50\pm 0.04$ | $-$ | $19.64\pm 0.11$
$32.302109$ | $-23.528551$ | $-$ | $20.86\pm 0.15$ | $19.75\pm 0.09$ | $18.43\pm 0.03$ | $-$ | $-$
$32.274239$ | $-23.415289$ | $-$ | $20.78\pm 0.14$ | $19.96\pm 0.13$ | $18.40\pm 0.03$ | $-$ | $-$
$32.277119$ | $-23.527414$ | $-$ | $-$ | $19.67\pm 0.08$ | $18.76\pm 0.05$ | $-$ | $-$
$32.338875$ | $-23.392778$ | $19.78\pm 0.17$ | $19.59\pm 0.04$ | $18.98\pm 0.04$ | $18.65\pm 0.04$ | $19.70\pm 0.08$ | $19.25\pm 0.07$
$32.276211$ | $-23.391331$ | $14.494\pm 0.016$ | $-$ | $-$ | $-$ | $-$ | $-$
$32.307014$ | $-23.431122$ | $16.41\pm 0.02$ | $-$ | $-$ | $-$ | $15.333\pm 0.012$ | $-$
$32.352291$ | $-23.418312$ | $15.618\pm 0.018$ | $-$ | $-$ | $-$ | $14.742\pm 0.012$ | $-$
$32.361607$ | $-23.421463$ | $15.777\pm 0.018$ | $-$ | $-$ | $-$ | $-$ | $-$

Note: All magnitudes are given in the natural Swope system. This table is
available in its entirety in a machine-readable form in the online journal.

Table 7: Photometry of the Host Galaxy ESO 478- G 006

Filter | Magnitude | Mag. Error
---|---|---
Swift UVW1 | 14.96 | 0.02
Swift U | 14.19 | 0.01
PS g | 13.05 | 0.01
PS r | 12.63 | 0.01
PS i | 12.34 | 0.01
PS z | 12.11 | 0.01
PS Y | 12.14 | 0.01
2MASS J | 11.60 | 0.02
2MASS H | 11.40 | 0.02
2MASS $\mathrm{K_{s}}$ | 11.54 | 0.03
WISE W1 | 13.01 | 0.02
WISE W2 | 13.44 | 0.02

Note: All magnitudes are given in the AB system. This table is available in
its entirety in a machine-readable form in the online journal.