# SN 2021zny: an early flux excess combined with late-time oxygen emission
suggests a double white dwarf merger event

Georgios Dimitriadis1, Kate Maguire1, Viraj R. Karambelkar2, Ryan J. Lebron3,
Chang Liu (刘畅) 4,5, Alexandra Kozyreva6, Adam A. Miller4,5, Ryan Ridden-
Harper7, Joseph P. Anderson8,9, Ting-Wan Chen6,10, Michael Coughlin11, Massimo
Della Valle12,13, Andrew Drake2, Lluís Galbany14,15, Mariusz Gromadzki16,
Steven L. Groom17, Claudia P. Gutiérrez18,19, Nada Ihanec8,16, Cosimo
Inserra20, Joel Johansson21, Tomás E. Müller-Bravo14,15, Matt Nicholl22,
Abigail Polin23,24, Ben Rusholme17, Steve Schulze21, Jesper Sollerman25,
Shubham Srivastav26, Kirsty Taggart27, Qinan Wang28, Yi Yang (杨轶) 29, David R.
Young26
1School of Physics, Trinity College Dublin, The University of Dublin, Dublin
2, Ireland
2Cahill Center for Astrophysics, California Institute of Technology, Pasadena,
CA 91125, USA
3Department of Physics and Engineering, University of Scranton, Scranton, PA
18510, USA
4Department of Physics and Astronomy, Northwestern University, 2145 Sheridan
Rd, Evanston, IL 60208, USA
5Center for Interdisciplinary Exploration and Research in Astrophysics
(CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
6Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748
Garching bei München, Germany
7School of Physical and Chemical Sciences—Te Kura Matu, University of
Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
8European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago,
Chile
9Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100,
Off. 104, Providencia, Santiago, Chile
10Technische Universität München, TUM School of Natural Sciences, Physik-
Department, James-Franck-Straße 1, 85748 Garching, Germany
11School of Physics and Astronomy, University of Minnesota, Minneapolis, MN
55455, USA
12INAF - Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131
Napoli, Italy
13Department of Physics, Ariel University, Ariel, Israel
14Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans,
s/n, E-08193 Barcelona, Spain
15Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
16Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478
Warszawa, Poland
17IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena,
CA 91125, USA
18Finnish Centre for Astronomy with ESO (FINC A), FI-20014 University of
Turku, Finland
19Tuorla Observatory, Department of Physics and Astronomy, FI-20014 University
of Turku, Finland
20Cardiff Hub for Astrophysics Research and Technology, School of Physics &
Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF24
3AA, UK
21The Oskar Klein Centre, Department of Physics, Stockholm University,
Albanova University Center, SE 106 91 Stockholm, Sweden
22Birmingham Institute for Gravitational Wave Astronomy and School of Physics
and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
23The Observatories of the Carnegie Institution for Science, 813 Santa Barbara
St., Pasadena, CA 91101, USA
24TAPIR, Walter Burke Institute for Theoretical Physics, 350-17, Caltech,
Pasadena, CA 91125, USA
25Department of Astronomy, The Oskar Klein Center, Stockholm University,
AlbaNova, 10691 Stockholm, Sweden
26Astrophysics Research Centre, School of Mathematics and Physics, Queen’s
University Belfast, Belfast BT7 1NN, UK
27Department of Astronomy and Astrophysics, University of California, Santa
Cruz, CA 95064, USA
28Physics and Astronomy Department, Johns Hopkins University, Baltimore, MD
21218, USA
29Department of Astronomy, University of California, Berkeley, CA 94720-3411,
USA E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present a photometric and spectroscopic analysis of the ultra-luminous and
slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign
starts from $\sim 5.3$ hours after explosion (making SN 2021zny one of the
earliest observed members of its class), with dense multi-wavelength coverage
from a variety of ground- and space-based telescopes, and is concluded with a
nebular spectrum $\sim 10$ months after peak brightness. SN 2021zny displayed
several characteristics of its class, such as the peak brightness
($M_{B}=-19.95$ mag), the slow decline ($\Delta m_{15}(B)=0.62$ mag), the blue
early-time colours, the low ejecta velocities and the presence of significant
unburned material above the photosphere. However, a flux excess for the first
$\sim 1.5$ days after explosion is observed in four photometric bands, making
SN 2021zny the third 03fg-like event with this distinct behavior, while its
$+313$ d spectrum shows prominent [O i] lines, a very unusual characteristic
of thermonuclear SNe. The early flux excess can be explained as the outcome of
the interaction of the ejecta with $\sim 0.04\>\mathrm{M_{\odot}}$ of H/He-
poor circumstellar material at a distance of $\sim 10^{12}$ cm, while the low
ionization state of the late-time spectrum reveals low abundances of stable
iron-peak elements. All our observations are in accordance with a progenitor
system of two carbon/oxygen white dwarfs that undergo a merger event, with the
disrupted white dwarf ejecting carbon-rich circumstellar material prior to the
primary white dwarf detonation.

###### keywords:

transients: supernovae – supernovae: individual: 2021zny

††pubyear: 2022††pagerange: SN 2021zny: an early flux excess combined with
late-time oxygen emission suggests a double white dwarf merger event–A

##  1 Introduction

The remarkable homogeneity in the properties of Type Ia supernovae (SNe Ia)
establish them as our best cosmological tools to date. Their peak brightness
is strongly correlated with their light curve’s shape (Phillips, 1993) and
colour (Riess et al., 1996), and these strong correlations allow us to
standarize them and, by measuring their relative distances, unveil the
accelerating expansion of the Universe and the discovery of dark energy (Riess
et al., 1998; Perlmutter et al., 1999). SNe Ia, in combination with other
local universe distance indicators, such as the period–luminosity relation of
Cepheid variable stars (Riess et al., 2019), the tip of the red giant branch
(TRGB; Freedman et al., 2019) and Mira variables (Huang et al., 2020) are also
able to constrain the local expansion rate (Riess et al., 2016, 2022).

While there is a theoretical and observational consensus that SNe Ia originate
from the explosive thermonuclear burning (Hoyle & Fowler, 1960) of a
degenerate carbon-oxygen white dwarf (WD) in a binary system (Whelan & Iben,
1973; Iben & Tutukov, 1984; Bloom et al., 2012), the nature of its binary
companion and the explosion mechanism itself remains unknown, maintaining an
active debate on the origins of these events (see reviews of Maoz et al.,
2014; Hoeflich, 2017; Jha et al., 2019).

Focusing on the vast majority of SNe Ia, the correlation between their maximum
luminosity and their light curve shape (usually parameterized with their
magnitude decline for the first 15 days after maximum light, $\Delta m_{15}$;
Phillips, 1993) can be explained by the nucleosynthetic yield of 56Ni, the
most abundant radioactive element the exploding WD synthesises (Colgate &
McKee, 1969), that powers the light curve. For a given ejecta mass (usually
the maximum mass a degenerate non-rotating C/O WD can sustain, the
Chandrasekhar mass, $\mathrm{M_{Ch}}$, Chandrasekhar, 1931), smaller/larger
amounts of 56Ni lead to fainter/brighter explosions with shorter/longer
timescales. This simple approach has been generally successful in explaining
the diversity in the bulk of the SN Ia population, from the subluminous 91bg-
like (Filippenko et al., 1992) to the bright 91T-like (Phillips et al., 1992),
both in their light curve properties (Kasen & Woosley, 2007) and in their
spectroscopic ones (Parrent et al., 2014).

High-cadence and/or untargeted transient surveys performed in recent years,
such as the Palomar Transient Factory (PTF; Law et al., 2009; Rau et al.,
2009), the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al.,
2014), the Distance less than 40 Mpc survey (DLT40; Tartaglia et al., 2018),
the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al., 2018),
the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS; Chambers
et al., 2016), the Young Supernova Experiment (YSE; Jones et al., 2021) and
the Zwicky Transient Facility (ZTF; Bellm et al., 2019; Graham et al., 2019a;
Masci et al., 2019; Dekany et al., 2020) have started to discover peculiar
events. While these events share many observational characteristics with SNe
Ia, they do have distinct photometric (e.g. higher or lower peak luminosities
for their decline rate) and/or spectroscopic (e.g. the presence of
hydrogen/helium) properties, challenging the canonical paradigm of the
thermonuclear scenario (see Taubenberger, 2017, for a review).

One of the most puzzling sub-types of SNe Ia is the so-called 03fg-like SNe
Ia, a rare subclass of ultra-luminous and slowly-evolving events. The
discovery of the prototype SN 2003fg (Howell et al., 2006) revealed a brighter
peak luminosity ($M_{B}=-20.09$ mag) for its decline rate ($\Delta
m_{15}(B)=0.82$ mag), and using simple analytical models (Arnett, 1982;
Jeffery, 1999) an estimate of the nickel and ejecta mass of more than the
Chandrasekhar mass was obtained. Over the next years, and as more 03fg-like
SNe Ia were discovered, an intrinsic diversity in the sub-population has been
unveiled, with some of them being less luminous and/or faster evolving, or
showing a rapid fading in the optical bands with simultaneous increase of the
near-infrared (NIR) flux. Moreover, varying spectroscopic properties were
found, such as the strengths and velocities of silicon (an element probing the
synthesized material in the ejecta) and carbon (an element probing the
unburned pristine material from the WD), or the potential presence of oxygen
in late-time spectra (see Hicken et al. 2007; Maeda et al. 2009; Scalzo et al.
2010; Taubenberger et al. 2011; Chakradhari et al. 2014; Parrent et al. 2016;
Taubenberger et al. 2019; Chen et al. 2019; Hsiao et al. 2020; Lu et al. 2021;
Dimitriadis et al. 2022 for studies on individual events and Taubenberger et
al. 2013a; Ashall et al. 2021 for sample studies).

Initial suggestions for solving the mass puzzle of 03fg-like SNe Ia were
rapidly spinning WDs as the progenitors, as the differential rotation can form
systems with super-$\mathrm{M_{Ch}}$ masses (Yoon & Langer, 2005), leading to
the adoption of the ‘super-Chandrasekhar-mass’ moniker. However, these
approaches were disputed by numerical simulations (Pfannes et al., 2010a;
Pfannes et al., 2010b; Hachinger et al., 2012; Fink et al., 2018),
particularly the nucleosynthesis and the energetics, as they produce
substantial amounts of burned material at high ejecta velocities, in contrast
with observations of (most of) 03fg-like SNe Ia. Moreover, a
super-$\mathrm{M_{Ch}}$ 56Ni explosion (needed to reproduce the enormous peak
luminosity) with low ejecta velocities, will require a huge amount of ejecta
mass, leading to strong $\gamma$-ray trapping and bright late-time bolometric
light curves, in contrast with the observations (Taubenberger et al., 2013a).
Evidently, the observed properties of 03fg-like SNe Ia, from the early rise to
the nickel decay tail, cannot consistently be explained by any 56Ni – ejecta
mass combination, which led to the introduction of alternative scenarios,
where the luminosity of the SN is not solely powered by the 56Ni decay. This
can be achieved by the interaction of the ejecta with circumstellar material
(CSM) in the close vicinity of the explosion site, that would naturally
increase the luminosity at early times and decelerate the ejecta, sustaining a
broad light curve (Hicken et al., 2007; Scalzo et al., 2010; Taubenberger et
al., 2011). The origin of this H-free CSM (as no hydrogen has ever been
observed in any 03fg-like SN Ia) is still under debate, with the disrupted
secondary C/O WD in a binary WD merger (Raskin & Kasen, 2013; Raskin et al.,
2014) or the carbon-rich envelope of an asymptotic giant branch (AGB) star at
the end of its evolution under the ‘core-degenerate’ scenario (Hoeflich &
Khokhlov, 1996; Kashi & Soker, 2011; Hsiao et al., 2020; Ashall et al., 2021)
being the primary candidates. However, the main problem with these scenarios
is that no clear signatures of this interaction have been observed, either in
spectra, as narrow emission lines, or in the light curve evolution, as a
deviation of the smooth early-time rise predicted for an explosion in a CSM-
free environment.

While the aforementioned observables have never been seen in 03fg-like SNe Ia,
various other subtypes of SNe Ia display properties that indicate a different
underlying explosion mechanism and/or progenitor system compared to normal SNe
Ia. From the spectral side, contrary to normal SNe Ia (e.g. Tucker et al.,
2020), the peculiar-Ia class of SNe Ia-CSM (Silverman et al., 2013) shows
narrow H$\alpha$ lines, including at early times, consistent with the presence
of a non-degenerate companion’s dense H-rich CSM, and occupy a similar area in
the absolute magnitude – $\Delta m_{15}$ parameter space as 03fg-like SNe Ia.
On the other hand, normal and underluminous events such as SNe 2015cp (Graham
et al., 2019b), 2016jae (Elias-Rosa et al., 2021), 2018cqj (Prieto et al.,
2020) and ASASSN-18tb (Kollmeier et al., 2019) only revealed narrow $H\alpha$
at later times, supporting a delayed ejecta-CSM interaction scenario.

The situation appears more complicated within the early photometric evolution.
Individual nearby normal SNe Ia, observed moments after explosion, such as SNe
2011fe (Nugent et al., 2011; Bloom et al., 2012), 2014J (Goobar et al., 2014)
and ASASSN-14lp (Shappee et al., 2016) show a smooth early rise, usually
parametrized as a power law, $L\propto t^{\alpha}$, where $\alpha=2$
corresponds to the canonical ‘expanding fireball’ model (Arnett, 1982; Riess
et al., 1999). Continuous, high cadence observations with transiting exoplanet
surveys, such as Kepler/K2 (Olling et al., 2015; Wang et al., 2021) and
Transiting Exoplanet Survey Satellite (Fausnaugh et al., 2021) find similar
results. Statistical sample studies (Conley et al., 2006; Hayden et al., 2010;
Ganeshalingam et al., 2011; González-Gaitán et al., 2012; Firth et al., 2015;
Papadogiannakis et al., 2019; Miller et al., 2020b) find mean values of
$1.8\leq\alpha\leq 2.4$, however, many individual SNe in the samples are
incompatible with $\alpha=2$.

Next to these well-behaved SNe Ia, some individual objects are clearly
inconsistent with a smooth rising light curve, showing early flux excesses of
various strengths, timescales and colours. Most notably, a blue and relatively
long ($\sim$2 to 5 d) ‘bump’ in the early light curves of SN 2012cg (Marion et
al., 2016), iPTF14atg (Cao et al., 2015), SN 2017cbv (Hosseinzadeh et al.,
2017), SN 2018oh (Dimitriadis et al., 2019a; Shappee et al., 2019; Li et al.,
2019) and SN 2021aefx (Hosseinzadeh et al., 2022), has been attributed to
ejecta interaction with (main sequence or subgiant) non-degenerate companions.
However, none of the above SNe Ia have shown signs of stripped material from
the donor in late-time spectral observations (e.g. see Maguire et al., 2016;
Shappee et al., 2018; Dimitriadis et al., 2019b; Tucker et al., 2019; Sand et
al., 2021), leading to alternative explanations of the early ‘bumps’, such as
the presence of 56Ni near the surface due to mixing (Piro & Nakar, 2013; Magee
et al., 2020) or the production of radioactive material in the ashes of the
helium shell under a double-detonation explosion of a sub-$\mathrm{M_{ch}}$ WD
(Polin et al., 2019). The presence of excess nucleosynthetic material in the
outermost layers of the ejecta has been proposed for the short-term ($\sim$0.5
days) and redward evolution of SN 2018aoz’s early flux excess (Ni et al.,
2022b), with Jiang et al. (2017) and De et al. (2019) favouring a double-
detonation for the longer-lasting red ‘bumps’ of MUSSES1604D and SN 2018byg,
respectively. Finally, Miller et al. (2020a) and Burke et al. (2021) identify
a long ($\sim$3.5 days) and ultraviolet (UV) bright flux excess for SN
2019yvq, for which Siebert et al. (2020), based on the strong calcium emission
at the nebular spectrum, favour a double-detonation origin, although, as
Tucker et al. (2021) note, there is no single explosion model that can
simultaneously explain its early- and late-time properties, a situation that
is encountered in almost all SNe Ia with early flux excesses (Magee et al.,
2020). Nevertheless, sample studies of SNe Ia, dedicated to identify these
early ‘bumps’, show an intrinsic rate of $18\pm 11$ per cent of early flux
excesses in SNe Ia (Deckers et al., 2022; Burke et al., 2022a; Burke et al.,
2022b), posing additional challenges on their interpretation.

Recently, a short-lived flash of optical emission was observed for two
overluminous SNe Ia, 2020hvf (Jiang et al., 2021) and 2022ilv (Srivastav et
al., 2023). For SN 2020hvf, the flux excess was observed during the high
cadence Tomo-e Gozen transient survey, using the camera’s clear filter, and
lasted for $\sim 1$ day, while for SN 2022ilv, observations in the ATLAS
o-band showed a similar early time behaviour. The authors modeled the rising
light curves, and favor interaction of the ejecta with a CSM mass of $\sim
10^{-2}-10^{-3}\>\mathrm{M_{\odot}}$ at an outer edge radius of $\sim 10^{13}$
cm. While SN 2020hvf has some notable spectral differences compared to 03fg-
like SNe Ia (relatively weak carbon lines and extremely high ejecta
velocities), these two events provide the first detection of a flux excess for
members of the 03fg-like subclass.

In this paper, we present observations of SN 2021zny, an 03fg-like SN Ia,
discovered $\sim$hours after explosion, classified two weeks before maximum
brightness and densely monitored with ground- and space-based facilities. Our
$\sim$10 months of multi-wavelength photometric and spectroscopic coverage
makes SN 2021zny one of the most well-observed 03fg-like SNe Ia, for which we
identify two striking features. Firstly, an early, short-lived flash is
observed in four photometric filters, which is consistent with a small amount
of H-free CSM interacting with the SN ejecta and secondly, the detection of
oxygen in its $+313$d late-time spectrum. We present the discovery of SN
2021zny, our observational campaign and the techniques we used for the
reduction of our data in Section 2. The analysis of its photometric and
spectroscopic properties, alongside a discussion on its distance and
extinction along the line of sight is presented in Section 3. We discuss our
findings in the context of the proposed progenitor systems of 03fg-like SNe Ia
in Section 4, and, finally, conclude in Section 5.

Throughout this paper, we will use the moniker 03fg-like SNe Ia to describe
the members of this peculiar SN Ia subclass, noting that various monikers have
been used in the literature, such as ‘super-Chandrasekhar-mass’ SNe Ia (SC SNe
Ia), 09dc-like and (carbon-rich) overluminous SNe Ia. Moreover, every phase of
a light curve is in rest-frame days. Finally, we adopt the AB magnitude system
and a Hubble constant of $H_{0}=73$ km s-1 Mpc-1.

##  2 Discovery, Observations and Data Reduction

In this Section, we present the discovery of SN 2021zny, its classification
and our photometric and spectroscopic followup observations.

###  2.1 Discovery and Classification

SN 2021zny was discovered on UT 2021 September 22.37 by the Zwicky Transient
Facility (ZTF; Bellm et al., 2019; Graham et al., 2019a; Masci et al., 2019;
Dekany et al., 2020), with the internal survey name ZTF21acdmwae, and reported
on the Transient Name Server (TNS111https://www.wis-tns.org/) on UT 2021
September 26.51 (Fremling, 2021), with a discovery magnitude of
$\textit{r}=19.33$ mag. Forced photometry on images taken by ZTF prior to
discovery revealed that the SN was also present in previous epochs, with our
first detection being on UT 2021 September 19.50 ($\mathrm{MJD}=59476.50$) at
$\textit{g}=20.35\pm 0.17$ mag (with non-detections in r and i down to $20.94$
and $20.50$ mags, respectively). Our last non-detection in both g and r-bands
was on UT 2021 September 17.4 at $>21.85$ and $21.42$ mags, respectively.

Figure 1: Pan-STARRS 5′$\times$5′ colour composite (g/r/i) image stamp of the
field of CGCG 438-018, the host of SN 2021zny. The location of the SN is
indicated with the magenta tick-marks. Standard stars in the field, used for
calibration of our photometry, are marked with red circles. The green inset
shows a zoomed-in (80″$\times$80″) region, centered on the SN location , taken
at a phase of $-3.4$ d from B-band maximum with LCO g-band.

The host of SN 2021zny is CGCG 438-018, an edge-on (star-forming) galaxy, with
the SN located at $\alpha=02^{\rm{h}}03^{\rm{m}}35^{\rm{s}}.800$,
$\delta=+15^{\rm{o}}44\arcmin 33\arcsec.36$ (J2000.0), $18.90$″ West and
$14.93$″ South of its host galaxy’s core, along its dust lane. We present a
deep pre-explosion Pan-STARRS colour composite (g/r/i) image stamp of CGCG
438-018 with the location of SN 2021zny marked with magenta tick-marks in Fig.
1, and the green inset showing a zoomed-in region of an LCO g-band image of
the supernova, taken at $-3.4$ days from B-band maximum.

SN 2021zny was classified as a young ($\sim$8 days before B-band maximum)
03fg-like SN Ia based on an optical spectrum obtained on UT 2021 September 29
by Yamanaka (2021) with the KOOLS-IFU attached to the 3.8-m Seimei telescope
at the Okayama Observatory. An additional spectrum obtained from ZTF two days
before with the Double Spectrograph (DBSP) mounted on the 5.1-m (P200) Hale
Telescope at the Palomar Observatory (Oke & Gunn, 1982) confirmed the
classification, as both spectra showed a deep absorption feature at
$\sim$6,300 Å. Such a feature can be attributed to C ii $\lambda$6580 at a
similar velocity to one of the most characteristic broad, Si ii$\lambda$6355
absorption line centered near $\sim$6,150 Å. This classification, alongside
our extremely early detection of SN 2021zny led us to initiate an extensive
follow-up campaign.

###  2.2 Observations and Data Reduction

The majority of our photometric and spectroscopic data were obtained within
the ZTF collaboration, with additional observations from various other
telescopes and instruments. In the next sections, we present the data and the
reduction techniques performed.

####  2.2.1 Photometry

SN 2021zny was observed with ZTF’s wide-field camera (in g,r and i-band
filters) mounted on the $1.2$-m Samuel Oschin (P48) Telescope, with dense
coverage from $-21.4$ to $+40.8$ d and from $+120.4$ to $+127.3$ d from peak
brightness. The images were processed with the pipeline as described in Masci
et al. (2019), which produces difference imaging, forced-photometry calibrated
light curves and post-processed, for quality filtering, with the methods
described in Yao et al. (2019).

Additional optical photometry was obtained with the IO:O camera of the
Liverpool Telescope (LT; Steele et al., 2004) in u,g,r,i and z-band filters
(PL21A09, PI: Deckers), with the Asteroid Terrestrial-impact Last Alert System
(ATLAS; Tonry et al., 2018) in the orange band, and with the Sinistro cameras
of the Las Cumbres Observatory (LCO; Brown et al., 2013) network of 1-m
telescopes through ePESSTO+ (Smartt et al., 2015) OPTICON time (2021B/001, PI:
Inserra) in u,g,r and i-band filters. The LT images were reduced with the IO:O
pipeline222https://telescope.livjm.ac.uk/TelInst/Inst/IOO/. LT images were
subtracted against Pan-STARRS (PS; Tonry et al., 2012) reference imaging, and
PSF photometry was performed and calibrated against PS photometric standards.
The ATLAS photometry was obtained from the ATLAS forced photometry server
(Shingles et al., 2021). The Sinistro images were processed with a dedicated
python/pyraf pipeline333https://github.com/LCOGT/lcogtsnpipe. UV photometric
observations were performed with the Ultraviolet Optical Telescope (UVOT;
Roming et al., 2005) on board the Neil Gehrels Swift Observatory (Gehrels et
al., 2004), with reference images taken on UT 2022 November 26. NIR
photometric (J-,H- and K- band) observations were obtained with SofI (Moorwood
et al., 1998) on the 3.58-m New Technology Telescope (NTT) through ePESSTO+
(Smartt et al., 2015) and the Wide Field Infrared Camera (WIRC; Wilson et al.,
2003) on P200. The WIRC images were reduced using a custom PYTHON package,
that uses the Swarp (Bertin et al., 2002) and Scamp (Bertin, 2006) packages.

Finally, the field of CGCG 438-018 was serendipitously observed by the
Transiting Exoplanet Survey Satellite (TESS; Ricker et al., 2015), during Year
4 of the survey, at Sector 43, from UT 2021 September 16 until October 12,
covering the rise up to $\sim$1 day before peak brightness. TESS data were
reduced and calibrated with
TESSreduce444https://github.com/CheerfulUser/TESSreduce, a dedicated pipeline
optimised for SN photometry (Ridden-Harper et al., 2021). For additional
information on the reduction steps we refer to Tinyanont et al. (2022).
Alongside reduction we also verified the validity of the TESS signal as TESS
data are subject to strong systematics around times of intense scattered
background light. In this case SN 2021zny fell on a region of TESS detector
known as a “strap" which effectively enhances the quantum efficiency of strap
pixels, and complicates the background subtraction. While the flux excess
occurs before the scattered light, which begins at $\sim$59483 MJD, we check
test apertures within the strap pixels at varying distances from the SN
2021zny aperture and find no features concurrent with the excess flux. We also
verify that the signal is present in all pixels used in the TESS aperture, and
therefore is unlikely to be a systematic of a single pixel. Our complete
photometric data set is presented in Tables 3, 4 and 5, and shown in Fig. 2.

Figure 2: Multi-wavelength (UV+uBgVroiz+JHK+TESS) light curves of SN 2021zny
in rest-frame days, with respect to B-band maximum. Downward arrows mark non-
detections at the location of the SN. The light curves are plotted with
different symbols/colours and offset, as described in the legends. Downward
arrows at the top of the figure correspond to the phases of our $-14$ d to
$124$ d spectroscopic series (Fig. 3 and Table 6), with grey (black) colours
referring to low (medium) resolution spectra, respectively. The gri+TESS light
curves during the first 4 days after $t_{\mathrm{det}}^{TESS}$ (see Section
3.3) are shown in the inset, where open grey circles correspond to the raw
TESS data.

####  2.2.2 Spectroscopy

Spectroscopic observations of SN 2021zny were initiated immediately after
discovery, resulting in a dense and wide coverage, spanning from $-13.9$ to
$+117.5$ d from B-band maximum. We obtained a total of 15 low resolution
spectra: 11 with SED Machine (SEDM; Blagorodnova et al., 2018), a fully-filled
integral field spectrograph mounted on the 1.5-m Palomar 60-inch Telescope
(P60), and 4 with the SPectrograph for the Rapid Acquisition of Transients
(SPRAT; Piascik et al., 2014) on the Liverpool Telescope (PL21A09, PI:
Deckers). We additionally obtained 17 medium resolution spectra: two with
DBSP, seven with the ESO Faint Object Spectrograph and Camera (EFOSC2; Buzzoni
et al., 1984) on NTT through ePESSTO+ (Smartt et al., 2015), one with the Low-
Resolution Imaging Spectrometer (LRIS; Oke et al., 1995) on the Keck I
telescope, one with the Low Dispersion Survey Spectrograph 3
(LDSS3)555https://www.lco.cl/technical-documentation/index-2/ on the Magellan
II telescope and two with the Alhambra Faint Object Spectrograph and Camera
(ALFOSC)666http://www.not.iac.es/instruments/alfosc at the 2.56-m Nordic
Optical Telescope (NOT) at the Observatorio del Roque de los Muchachos on La
Palma (Spain). A late-time spectrum ($313$ d from B-band maximum) was obtained
with the DEep Imaging Multi-Object Spectrograph (DEIMOS; Faber et al., 2003)
on the Keck II telescope.

The SEDM spectra were reduced with
pysedm777https://github.com/MickaelRigault/pysedm (Rigault et al., 2019; Kim
et al., 2022). All long-slit spectral observations were reduced using standard
iraf/pyraf888IRAF is distributed by the National Optical Astronomy
Observatory, which is operated by the Association of Universities for Research
in Astronomy (AURA) under a cooperative agreement with the National Science
Foundation. and IDL/python routines for bias subtractions and flat fielding of
the two-dimensional spectral images. The wavelength solution was derived using
arc lamps and additionally verified against bright night-sky emission lines,
while the final flux calibration and (whenever possible) telluric lines
removal were performed using spectro-photometric standard star spectra,
obtained on the same night. We used dedicated pipelines for the
EFOSC2999https://github.com/svalenti/pessto, the
ALFOSC101010https://github.com/jkrogager/PyNOT and the
DEIMOS111111https://pypeit.readthedocs.io/en/latest/ data. Finally,
spectrophotometry of the final flux-calibrated spectra was compared with
broad-band photometry of the same night (or interpolated values if no imaging
was performed) and scaled with a constant value if necessary.

Figure 3: The $-14$ to $+124$ d spectroscopic series of SN 2021zny. The
spectra are presented in black (grey) colours, corresponding to medium (low)
resolution spectroscopic observations. Its spectrum phase and source is
additionally labelled. The spectra have been corrected for Milky Way and host
galaxy extinction (see Section 3.1) and placed in rest-frame wavelength.
Telluric features are marked with the Earth symbol. Detailed information of
each observation is provided in Table 6.

Fig. 3 shows our $-14$ to $+124$ d spectral series, with detailed information
of each observation provided in Table 6. The $+313$ d spectrum will be
presented and analysed in Section 3.4. The complete spectroscopic data set is
available in the electronic edition.

##  3 Analysis

In this Section, we discuss SN 2021zny’s distance and extinction along the
line of sight, present its maximum light photometric and spectroscopic
properties and focus on the early rise of its light curve.

###  3.1 Distance and Extinction

NED121212https://ned.ipac.caltech.edu/ reports four redshift-independent
distances for CGCG 438-018, with a mean value of their distance moduli of
$35.19\pm 0.16$ mag. The reported redshift of the galaxy is $z=0.026602\pm
0.000064$ (Springob et al., 2005), and the cosmological distance of the host,
assuming H${}_{0}=73.0\pm 5.0$ km s-1 Mpc-1 and correcting for peculiar
motions related to the Virgo cluster and Great Attractor (Mould et al., 2000),
is estimated at $D=106.6\pm 7.5$ Mpc (distance modulus of $DM=35.14\pm 0.15$
mag). At this distance, SN 2021zny is located $12.45$ kpc from the galaxy’s
core. We will use the cosmological distance in our analysis in order to
facilitate better comparisons with other SNe.

Using the Schlafly & Finkbeiner (2011) dust maps, we recover $E(B-V)_{\rm
MW}=0.0445\pm 0.0015$ mag, which corresponds to a Milky Way extinction along
the line of sight $A(V)_{\rm MW}\simeq 0.14$ mag of visual extinction. For the
host galaxy extinction, we use our LRIS spectra to estimate the equivalent
width of the Na i D absorption line at the host’s redshift and find a value of
$0.74\pm 0.25$ Å. Using equation 9 from Poznanski et al. (2012), we infer
$E(B-V)_{\rm host}=0.10\pm 0.07$ mag, which corresponds, assuming a
Fitzpatrick (1999) reddening law with $R_{V}=3.1$, to a visual extinction
$A(V)_{\rm host}\sim 0.3$ mag. Thus, the total amount of reddening on the line
of sight is $E(B-V)_{\rm total}=0.14\pm 0.07$ mag and we adopt this value to
correct all of our photometry and spectra.

###  3.2 Photometric Properties

SN 2021zny peaked in B-band on 2021 October 11.77 UT
($\mathrm{MJD_{\textit{B},peak}}=59498.46\pm 0.5$), determined by low-order
polynomial fits to our Swift B-band light curve. The observed peak B-band
magnitude was $B=15.79\pm 0.04$ mag, and taking into account the distance to
CGCG 438-018 and the extinction on the line of sight (Section 3.1), we
estimate the B-band peak absolute magnitude to be $-19.95\pm 0.17$ mag. The
magnitude decline in B-band after 15 d was $\Delta m_{15}(B)=0.62\pm 0.09$
mag.

Figure 4: The absolute $B$-band magnitude vs. $\Delta m_{15}(B)$ of the CfA SN
Ia sample from Hicken et al. 2009 (grey crosses). The dashed line corresponds
to the Phillips relation. We overplot various well-studied 03fg-like SNe Ia as
described in the legend. Note that for SN 2022ilv we plot $g$-band
measurements.

In Fig. 4, we show the absolute $B$-band peak magnitude against $\Delta
m_{15}(B)$ for a sample of well-observed 03fg-like SNe Ia, alongside the CfA
SN Ia sample from Hicken et al. (2009). The intrinsic diversity of the 03fg-
like SN Ia population is evident in this parameter space, with next to the
usual population of high luminosity and slowly evolving SNe 2003fg (Howell et
al., 2006), 2007if (Scalzo et al., 2010), 2009dc (Taubenberger et al., 2011),
2020hvf (Jiang et al., 2021) and 2022ilv (Srivastav et al., 2023) lying the
slightly dimmer 2006gz (Hicken et al., 2007) and ASASSN-15pz (Chen et al.,
2019), the slightly dimmer but faster evolving LSQ14fmg (Hsiao et al., 2020)
and 2020esm (Dimitriadis et al., 2022), the faint and faster evolving 2012dn
(Taubenberger et al., 2019) and the faint and slow evolving ASASSN-15hy (Lu et
al., 2021). SN 2021zny lies on the extreme end of the magnitude-decline range,
with $\Delta m_{15}(B)$ values similar to the slow evolving SNe 2006gz, 2007if
and 2009dc, with its peak luminosity being $\sim 0.33$ mag brighter than what
is expected for its decline rate.

Figure 5: The UV (Swift UVW2 and UVW1-band), optical (u,g,r,i-band, various
instruments) and NIR (J,H-band, various instruments) absolute magnitude light
curves of SN 2021zny (blue circles). Comparison samples in the UV (open grey
diamonds; Milne et al., 2010) and the optical/NIR (open grey circles;
Krisciunas et al., 2017) are additionally shown, alongside the two well
observed 03fg-like SNe 2009dc (green squares; data from Taubenberger et al.,
2011; Brown et al., 2014; Friedman et al., 2015) and 2012dn (orange diamonds;
data from Taubenberger et al., 2019; Brown et al., 2014; Yamanaka et al.,
2016), and the normal SN Ia 2011fe (red upward triangles; data from Pereira et
al., 2013; Brown et al., 2012; Matheson et al., 2012).

Fig. 5 shows the light curves of SN 2021zny in absolute magnitudes. We compare
the UV, optical and NIR light curves with literature samples of low-redshift,
normal SNe Ia from Milne et al. (2010) and Krisciunas et al. (2017) in similar
filters, and with two well-observed 03fg-like SNe Ia 2009dc and 2012dn (which
additionally represent the two extremes in the 03fg-like population), and the
well-observed normal SN Ia 2011fe. While all of these comparison SNe were
photometrically observed in the UBVRI-bands, they all have excellent
spectrophotometric coverage, making it possible to estimate ugri-band light
curves, suitable for direct comparison. SN 2021zny’s gri-band light curves
have additionally been k-corrected based on SN 2009dc. Our SN 2021zny spectral
series (Fig. 3 and Table 6) consists of mainly low-resolution spectra, but it
is dense enough in time to compare the estimated k-corrections per filter and
as a function of time with the k-corrections of SNe 2009dc (Taubenberger et
al., 2011), 2012dn (Taubenberger et al., 2019) and (as a cross-check) 2011fe
(Pereira et al., 2013). The similarities of the k-corrections with the SN
2009dc ones, alongside the fact that the two SNe are close in the absolute
magnitude – decline rate parameter space, led us to use the 2009dc
k-corrections at the corresponding epochs (from $-9$ to $100$ d from maximum
light). We do not attempt to extrapolate at phases $<-9$ d, a crucial period,
due to the presence of the flux excess, however, we note that for the earliest
SN 2009dc spectrum ($-9$ days from peak), the k-corrections are
$\sim-0.05,-0.11$ and $-0.09$ for gri, respectively, which are considerably
smaller than the observed photometric uncertainties.

The light curve evolution of SN 2021zny demonstrates the usual photometric
properties associated with 03fg-like SNe Ia. Compared to the normal SN 2011fe,
SN 2021zny is substantially brighter at peak ($\sim$ 2.5, 1 and 0.7 mag in the
UV, optical and NIR, respectively), with a slower evolution in all photometric
bands. This behaviour is especially evident in the UV: our earliest Swift
observations were taken $\sim$ 10 d before B-band maximum (when SN 2021zny was
already 3.9 and 3.1 mag brighter than SN 2011fe in UVW2\- and UVW1-band,
respectively), and SN 2021zny shows a nearly flat evolution, indicating that
it peaked considerably earlier and brighter than SN 2011fe. Moreover, SN
2021zny lacks any clear evidence of the (distinctive in normal SNe Ia) strong
secondary maximum in the r,i and NIR photometric bands. Overall, the
photometric evolution of SN 2021zny strongly resembles SN 2009dc although
somewhat fainter, suggesting similar conditions in the SN ejecta with a lower
synthesized 56Ni mass.

The most striking characteristic of SN 2021zny is its light curve evolution
from $\sim-$21 to $-$18 d from maximum light (see inset of Fig. 2). All of our
photometric observations at those epochs (in the gri\- and TESS-bands) show a
prominent flux excess relative to what is expected for the majority of normal
SN Ia explosions. However, such an early flux excess exhibits a notable
resemblance to that of SN Ia 2020hvf. We will discuss in more detail this
remarkable behaviour in Section 3.3.

Figure 6: The optical ($u-g$, $g-r$, $g-i$ and $r-i$) colours of SN 2021zny
(blue circles), SN 2009dc (green squares), SN 2012dn (orange diamonds) and SN
2011fe (red upward triangles).

The optical colour evolution of SN 2021zny is presented in Fig. 6, compared
with SNe 2009dc, 2012dn and 2011fe. At early times, SN 2021zny exhibits
considerably bluer colours than normal SNe Ia (probed by SN 2011fe), similar
to SNe 2009dc and 2012dn. This discrepancy between the pre-maximum colour
curves of SN 2021zny and normal SNe Ia is particularly obvious in the bluer
photometric bands, as can be seen in $u-g$, which is a direct consequence of
the small amount of line blanketing and the weak Ca ii H&K absorption features
(Fig. 3). Finally, another characteristic of the 03fg-like SNe Ia that
separates them from normal SNe Ia is the $r-i$ colour evolution from peak up
to about +30 d. The distinctive secondary maxima in these photometric bands
and at these phases makes normal SNe Ia show a particularly sharp blue colour
evolution, while 03fg-like SNe Ia appear considerably redder, with SN 2021zny
showing an intermediate $r-i$ colour evolution.

Of particular importance is the colour evolution during our earliest
detections. At these epochs, which coincide with the flux excess detected in
gri, SN 2021zny starts extremely blue ($g-r$$\>\sim-0.5$), evolving to redder
colours. SN 2021zny is bluer than the earliest $g-r$ detection of SN 2011fe
(at $\sim$ 2 days after explosion) by 0.6 mag. At later phases, SN 2021zny
settles to the usual ‘red-blue-red’ colour evolution as seen in most
thermonuclear SNe. This colour behaviour indicates that an external power
source acts during the flux excess, with initially high temperatures (hence
the blue early colour), that gradually subsides and allows the radioactive
decay of 56Ni to dominate during the main light curve.

###  3.3 The rising light curve

As mentioned in Section 2.1, SN 2021zny was discovered on UT 2021 September
22.37 by ZTF at $\textit{r}=19.33$ mag, however, forced photometry on ZTF
images taken prior to discovery showed that the SN was also present in
previous epochs. Adopting the distance and extinction on the line of sight
from Section 3.1, and our estimate of the time of maximum from Section 3.2, SN
2021zny is first detected (on UT 2021 September 19.50) at
$\textit{g}=-15.32\pm 0.34$ mag at $-21.38$ d from maximum ($\sim 4.5$ mag
fainter than its peak brightness), with simultaneous non-detections in r\- and
i-bands. Our last simultaneous non-detections in two photometric bands were at
$-23.46$ d in g\- and r-bands, with our last (i.e. a non-detection prior to
our first g-band detection) i-band non-detection at $-26.34$ d. This indicates
that SN 2021zny exploded some time between $-23.46$ and $-21.38$ d from
maximum, with potentially exceptionally high temperatures at the time after
explosion, implied by its significant blue colours (Fig. 6). Our strong
constraint on the time of explosion is additionally corroborated by the TESS
observations, as shown on the inset of Fig. 2. We estimate the time of the
first detection in the TESS-band by following a similar approach as described
in Dimitriadis et al. (2019a): we calculate the weighted-mean of the flux on a
given time-window $x$, marking as a detection when $\mathrm{Flux}_{x}\geq
3\times\sigma_{x-1}$, and iterate this procedure by reducing the width of the
time-window. The final detection time and its uncertainty are then estimated
as the mean and standard deviation of the recorded detection times. Using this
method, we recover a time of first detection at UT 2021 September
19.28$\pm$0.16 ($\mathrm{MJD_{\textit{TESS},det}}=59476.28$), corresponding to
$-21.60\pm 0.15$ d with respect to B-band maximum.

Figure 7: Comparison of the SN 2021zny TESS light curve, normalized to peak
flux (open blue circles) with the Kepler light curves of SN 2018oh (green) and
SN 2018agk (red), with respect to each photometric band’s time of maximum. The
time of TESS first detection is shown as a blue downward arrow. Power-law fits
to the light curves, as described in the text, are additionally presented,
with the fitted parameters shown in the legend and the residuals of the fits
of each SN in the bottom panels.

In Fig. 7, we present power-law rise fits (parameterised as
$L\propto(t-t_{0})^{a}$, where $t_{0}$ is the time of first light and $a$ is
the power-law index) to the TESS-band light curve of SN 2021zny. Due to the
lack of TESS observations after peak, an estimate of the TESS-band time of
maximum is uncertain, thus we use the r-band time of maximum (UT 2021 October
10.86, $\mathrm{MJD_{\textit{r},peak}}=59497.86$) as the TESS-band time of
maximum. We compare our fits with similar fits of the Kepler-band light curves
of SN 2018oh (Dimitriadis et al., 2019a), a SN Ia showing a prominent early
flux excess, and SN 2018agk (Wang et al., 2021), a SN Ia with a smooth rising
light curve, with their residuals additionally plotted. Shaded regions
correspond to the 1-$\sigma$ uncertainty of our estimated time of first light.
We note that, while the TESS-band’s transmission curve has a similar effective
width as the Kepler-band’s ($\sim 3,800$ Å), its central wavelength is
approximately $1,500$ Å redder. Due to the gap of TESS observations from
$-14.5$ until $-10.5$ d with respect to TESS-band maximum, we restrict our
time-range fit until the flux reaches 30 per cent of maximum (for all SNe), in
contrast to the usual 40 per cent value that has been used in similar studies
(e.g. in Olling et al., 2015). We perform two separate fits for SN 2021zny:
the first is by keeping $t_{0}$ as a free parameter (shown with the solid blue
line) and the second by fixing $t_{0}$ to the TESS time of first detection
($-21.01$ d, shown with the dashed blue line). For the first fit, we find
$t_{0}=-22.06\pm 1.02$ d and $\alpha=2.61\pm 0.52$ ($\sim$1.05 days earlier
than our first detection), while by fixing $t_{0}$, we find $\alpha=2.11\pm
0.08$, with both values of the power-law index generally consistent with
estimates in the literature (e.g. see Miller et al., 2020b). However, the
residuals of both of our fits at the $-21$ to $-17.5$d phase-region resemble
the characteristic ‘S-shape’, as seen in SN 2018oh, which is attributed to the
presence of the early flux excess (although lasting significantly less), a
behaviour not seen in the smooth rise of SN 2018agk. While the significance of
this deviation is lower than SN 2018oh (due to lower fluxes), we conclude that
the post-explosion light curve evolution of SN 2021zny is in contrast with a
single power-law rise, possibly due to the presence of an additional power
source acting during a short period after the explosion.

Figure 8: Comparison of the k-corrected ZTF g-band light curve of SN 2021zny
(full blue circles) with other SNe with early time data in similar photometric
bands, as described in the legend. The light curves are normalised to their
peak magnitude and presented with respect to each photometric band’s time of
maximum. In the inset, a zoom-in of the early epochs is shown, with downward
arrows corresponding to non-detections of each SN.

Fig. 8 shows the k-corrected g-band light curve of SN 2021zny with respect to
the g-band time of maximum, which we estimate with low-order polynomial fits
to be at UT 2021 October 10.25 ($\mathrm{MJD_{\textit{g},peak}}=59497.25$). We
compare the light cure with the normal SN 2011fe (Firth et al., 2015; Pereira
et al., 2013), the slightly overluminous SN 2017cbv (Hosseinzadeh et al.,
2017), the 03fg-like SNe 2020hvf (Jiang et al., 2021), LSQ12gpw (Firth et al.,
2015) and ASASSN-15pz (Chen et al., 2019) and the Ia-peculiar SN 2019yvq
(Miller et al., 2020a), in similar photometric bands. The inset panel provides
a zoom-in of the light curves at the early epochs. We additionally plot the
ZTF SNe Ia from Yao et al. 2019 for $z<0.08$ (excluding the peculiar SNe Ia
CSM and 02cx-like) and the only nearby 03fg-like SN Ia of that sample
(ZTF18abdpvnd). K-corrections have been applied to the Yao et al. 2019 sample
and the relatively distant LSQ12gpw ($z=0.058$) and ASASSN-15pz ($z=0.015$).

The post-peak light curve evolution displays the established SN Ia diversity,
known as the width-luminosity relation (WLR; Phillips, 1993): brighter events,
such as the 91T/99aa-like SNe Ia, decline slower than fainter events, such as
the 86G/91bg-like ones. However, this diversity appears to extend at the early
light curve evolution, for which we see smooth-rise events, (SN 2011fe),
strong long-lasting ‘bumps’ (SN 2017cbv) and ‘spikes’ (SN 2019yvq), and weaker
short-lasting flux excesses (ASASSN-15pz, LSQ12gpw and SN 2020hvf).
Observationally, the early light curve of SN 2021zny resembles that of other
03fg-like SNe Ia, showing a short ($\sim$2 days) and relatively weak flux
excess as opposed to the long-lasting ($\sim$4–5 days) ones of SNe 2017cbv and
2019yvq.

For the 91T/99aa-like SN 2017cbv and similar events, such as SNe 2018oh
(Dimitriadis et al., 2019a) and 2021aefx (Hosseinzadeh et al., 2022), and the
Ia-peculiar SN 2019yvq and similar events, such as iPTF14atg (Cao et al.,
2015), that show strong long-lasting flux excesses, proposed interpretations
include the interaction of the ejecta with a non-degenerate companion, the
presence of the radioactive 56Ni in the outer layers or the production of
radioactive elements in the outer layers due to the nuclear burning in the He
shell under a sub-$\mathrm{M_{ch}}$ double-detonation scenario. On the
contrary, Jiang et al. 2021 suggest that the short ‘flash’ of SN 2020hvf is
more consistent with interaction of the SN ejecta, soon after explosion, with
a confined and dense CSM, formed at the final evolution stage of the
progenitor system.

We note that not all 03fg-like SNe Ia may show this early ‘flash’; however,
not many events of this subclass have been discovered early enough (the
earliest detection of an 03fg-like SN Ia in the Ashall et al. (2021) sample is
for SN 2015M, at $-14.1$ d). Moreover, identifying a true flux excess is not
trivial, as it requires early-time multi-wavelength observations with high
cadence, limiting the promising candidates to very nearby events, but, at the
same time, greatly reducing their potential numbers, due to the intrinsically
low rate of 03fg-like SNe Ia. Nevertheless, for the events that were
discovered early, such as SNe 2021zny, 2020hvf and 2022ilv, their early flux
excesses appear to originate from a different mechanism compared to events
such as SNe 2017cbv or 2019yvq, indicating a potential different explosion
mechanism and/or progenitor binary configuration.

###  3.4 Spectroscopic Properties

Spectral comparisons of SN 2021zny, in various phases of its evolution, with
the 03fg-like SNe Ia 2009dc, 2012dn, 2020hvf and the normal SN Ia 2011fe are
presented in Fig. 9.

Figure 9: Spectra of SN 2021zny at $-$14 (top left), $-$7 (top right), $+$3
(bottom left) and $+$124 (bottom right) days from B-band maximum are shown in
blue, and are compared with spectra at similar phases of the 03fg-like SNe
2009dc (green), 2012dn (orange), 2020hvf (pink) and the normal SN Ia 2011fe
(red), with their corresponding phases labelled. The spectra of SN 2021zny
have been deredshifted and corrected for extinction on the line of sight
according to Section 3.1, and for our comparison sample according to their
relevant studies. The $-$14 and $+$124 d spectra of SN 2021zny have been
smoothed for presentation purposes, with light/dark blue corresponding to the
raw/smoothed spectrum, respectively. All spectra are in flux density per unit
wavelength, $F_{\lambda}$. Main absorption/emission features attributed to
atomic species usually found in thermonuclear SNe in early/nebular phases are
also marked.

The earliest spectrum of SN 2021zny shows the general characteristics of the
03fg-like subclass: particularly the blue (pseudo-) continuum (described
reasonably well with a black body of $T\sim 13,900$ K), the relatively strong
absorption features of Si ii $\lambda$6355, C ii $\lambda\lambda$6580,7231 and
O i $\lambda$7774, and the extremely weak (or even absent) lines from other
intermediate-mass elements (IMEs), such as S ii, Ca ii and Mg ii. As the SN
evolves toward maximum brightness, the usual IMEs and iron-group elements seen
in thermonuclear SNe (e.g., Mg ii, Ca ii, S ii, Fe ii and Fe iii) start to
appear, but considerably weaker, compared to normal SNe Ia, in accordance with
03fg-like SNe Ia. We also note that the spectroscopic evolution of SN 2020hvf
seems to be different compared to other 03fg-like SNe Ia in our sample. For
example, SN 2020hvf displays unburned and synthesized material at remarkably
higher velocities ($\sim 10,500$ for SN 2009dc and $\sim 7,500$ km s-1 for SN
2012dn at early times) and the C ii $\lambda$6580 line appears relatively weak
while other lines appear relatively strong (e.g. Si ii $\lambda$5972). As SN
2021zny evolves towards the nebular phase, its decreasing ejecta density and
optical depth allow us to probe the inner layers of the SN, and forbidden
emission lines from iron-group elements start to appear, particularly the
forbidden lines [Fe iii], [Fe ii] and [Fe iii].

Figure 10: Spectra of SN 2021zny (blue), 2009dc (green) and 2012dn (orange)
with respect to relative line velocities of Si ii $\lambda$6355, C ii
$\lambda$6580 and C ii $\lambda$7231, as labelled. The spectral epochs for
each SN, with respect to B-band time of maximum, is shown in the legends.

A noticeable characteristic of SN 2021zny is the observed absorption line
complex at $6,000$ – $6,500$ Å. As can be seen in Fig. 9, top left, the redder
trough of the line complex (attributed to C ii $\lambda$6580) is significantly
stronger compared to the bluer one (attributed to Si ii $\lambda$6355),
indicating that at 2 weeks before maximum light, SN 2021zny had far more
unburned material above the photosphere, as compared to SNe 2009dc and 2012dn.
A close inspection to the spectra profiles reveals potentially two components
for silicon and carbon for SN 2021zny, as shown in Fig. 10, where we show the
spectra in velocity space with respect to the rest wavelengths of Si ii
$\lambda$6355, C ii $\lambda$6580 and C ii $\lambda$7231\. At $-14$ d, two
carbon components at approximately $-12,500$ and $-9,000$ km s-1 can be seen,
with potentially two silicon components at similar velocities. The
identification is more clear at $-7$d, where two silicon components are
visible at approximately $-11,000$ and $-6,000\>\mathrm{km\>s^{-1}}$, with
potentially two carbon components at similar velocities, seen in the C ii
$\lambda$7231 region. Moreover, the C ii $\lambda$6580 feature appears much
broader than what is seen in SN 2009dc (FWHM of approximately $6,000$ km s-1
compared to $4,000$ km s-1 in SN 2009dc), with the characteristic flattening
of the minimum of the P-Cygni profile, which indicates a blend of two
components. While no definite conclusion on the nature of these putative two
components can be made, it is obvious that significant unburned material at a
wide velocity range (probed by C ii $\lambda\lambda$6580, 7231) persistently
remains up to $\sim 2$ weeks after maximum.

Figure 11: Nebular time spectra of SN 2021zny (blue), 2009dc (green), 2012dn
(orange) and 2011fe (red), with the corresponding phases from B-band maximum
indicated in the legends. The spectra of SNe 2021zny and 2009dc have been
smoothed for presentation purposes, with light/dark colours corresponding to
raw/smoothed spectra. All spectra have been normalized to the baseline flux
measured around $8,000$ Å. Various spectral features of normal and 03fg-like
SNe Ia present in nebular epochs are marked. The strong and narrow emission
lines in the SN 2012dn spectrum originate from the host galaxy.

Fig. 11 shows our late-time ($313$ d from peak) spectrum of SN 2021zny,
compared with spectra of SNe 2009dc, 2012dn and 2011fe at similar phases. Our
spectrum is remarkably similar to the one of SN 2009dc, showing the
characteristic low [Fe iii] to [Fe ii] line ratio, attributed either to lower
temperatures of the ejecta or higher ejecta densities, favouring enhanced
recombination (Taubenberger et al., 2013a). An additional argument for the low
ionization state is the line complex at $7,000-7,500$ Å. In normal SNe Ia,
this feature is dominated by blends of [Fe ii] and [Ni ii], however, in SN
2021zny (and in fact in most of 03fg-like events) two sharp emission peaks are
observed.

We attempt to model this emission feature as a sum of multiple Gaussian
components for the [Fe ii] (7155, 7172, 7388 and 7453 Å) and [Ni ii] (7378,
7412 Å) blends, with the relative strengths Ai of the individual lines for
each atomic species tied as in Maguire et al. (2018), and a common velocity
shift and FWHM. We find v${}_{\mathrm{Fe}}=-735\pm 35\>\mathrm{km\>s^{-1}}$,
FWHM${}_{\mathrm{Fe}}=4,175\pm 85\>\mathrm{km\>s^{-1}}$,
v${}_{\mathrm{Ni}}=-2,710\pm 25\>\mathrm{km\>s^{-1}}$ and
FWHM${}_{\mathrm{Ni}}=3,025\pm 65\>\mathrm{km\>s^{-1}}$, with the relative
strength ratio estimated as ANi/A${}_{\mathrm{Fe}}=1.36\pm 0.03$, indicating a
significant difference at the velocity shifts of iron and nickel, with the [Ni
ii] blend being stronger than [Fe ii], as opposed to normal SNe Ia. We thus
explore the possibility of a calcium contribution, and perform a fit by adding
a double Gaussian for the [Ca ii] $\lambda\lambda$7292, 7324 doublet. Due to
the presence of the telluric A-band absorption in the spectrum at the expected
location of [Ni ii], we exclude this region from the fit and assume a common
velocity shift for [Fe ii] and [Ni ii]. For consistency, we apply the same
model to SN 2011fe and to SN 2019yvq, where its late time spectrum also shows
strong evidence on the presence of calcium (Siebert et al., 2020; Tucker et
al., 2021). We note that the spectrum of SN 2019yvq was taken at $+153$ days,
when the 77.2-day half-life cobalt radioactive decay still contributes; thus
an emission feature at $\sim 7,000$ Å, which is attributed to [Co iii], is
excluded from the fit. Our results are shown in Fig. 12 and presented in Table
1.

Figure 12: The $7,000-7,500$ Å spectral region of SN 2011fe (left), SN 2021zny (middle) and SN 2019yvq (right), with each spectrum phase from B-band maximum marked. Light grey regions correspond to spectral regions that are excluded from the fits (see text for details). Solid lines correspond to our best fits of the [Fe ii]+[Ni ii](+[Ca ii]) model, as described in the text, with dashed, dotted and dashed-dotted lines corresponding to the individual lines of each atomic species. The spectra are normalised to the inferred peak of each [Fe ii] feature ($\sim 7,150$ Å). Table 1: Best-fit parameters of the $7,000-7,500$ Å line complex measurements. Name | vFe | FWHMFe | vNi | FWHMNi | ANi/AFe | vCa | FWHMCa
---|---|---|---|---|---|---|---
| ($\mathrm{km\>s^{-1}}$) | ($\mathrm{km\>s^{-1}}$) | ($\mathrm{km\>s^{-1}}$) | ($\mathrm{km\>s^{-1}}$) |  | ($\mathrm{km\>s^{-1}}$) | ($\mathrm{km\>s^{-1}}$)
SN 2021znya | $-735\pm 35$ | $4,175\pm 85$ | $-2,710\pm 25$ | $3,025\pm 65$ | $1.36\pm 0.03$ | – | –
SN 2021znyb | $-705\pm 35$ | $4,270\pm 85$ |  $-705\>(\equiv$v${}_{\mathrm{Fe}})$ | $2,650\pm 610$ | $0.38\pm 0.05$ | $345\pm 35$ | $2,155\pm 100$
SN 2011fe | $-1,420\pm 70$ | $7,180\pm 120$ |  $-1,420\>(\equiv$v${}_{\mathrm{Fe}})$ | $8,890\pm 370$ | $0.54\pm 0.01$ | – | –
SN 2019yvq | $-1,170\pm 40$ | $9,115\pm 435$ |  $-1,170\>(\equiv$v${}_{\mathrm{Fe}})$ | $9,160\pm 1,960$ | $0.31\pm 0.02$ | $-830\pm 40$ | $5,670\pm 150$
a No calcium component.
b Assuming a common velocity offset for [Fe ii] and [Ni ii].

The most striking characteristic of the late time spectrum of SN 2021zny is
the emission feature at $\sim 6,300$ Å, seen also in SN 2012dn, marking the
second ever detection of this feature in an 03fg-like SN Ia. This feature is
routinely seen in core-collapse (and particularly stripped-envelope) SNe
(Taubenberger et al., 2009) and it has been identified as [O i]
$\lambda\lambda$6300, 6364, but has never been seen in normal SNe Ia. However,
it has been observed in the low-luminosity/slowly-evolving 02es-like SN 2010lp
(Taubenberger et al., 2013b) and iPTF14atg (Kromer et al., 2016), with the
later, interestingly, displaying an early UV flux excess (Cao et al., 2015).

We model the emission following Taubenberger et al. (2013b), and we use two
Gaussian components to account for the doublet at 6300 and 6364 Å . We adopt a
relative intensity ratio of 3:1 (appropriate for the optically thin limit at
nebular epochs) and the same relative velocity and FWHM. We find a velocity of
$1,400\pm 60\>\mathrm{km\>s^{-1}}$ and a FWHM of $2,615\pm
120\>\mathrm{km\>s^{-1}}$ (similar to [Ca ii]), with a total luminosity of
$5.65(\pm 0.21)\times 10^{36}\mathrm{erg\>s^{-1}}$. While this line
identification may be valid for low-luminosity SNe Ia (due to the low burning
efficiency), it is difficult to reconcile with the 03fg-like ones
(Taubenberger et al., 2019). If such an emission is indeed due to [O i], it
would imply the presence of substantial unburned material close the center of
the ejecta, putting stringent constraints on the explosion model.
Nevertheless, the [Ca ii]/[O i] ratio of SN 2021zny is estimated to be $\sim
2.5$, slightly higher than the one of SN 2010lp and on the low extremes of the
Ca-rich transients (Prentice et al., 2022).

Finally, several theoretical models (e.g. Lundqvist et al., 2013; Botyánszki
et al., 2018) predict that, under a single-degenerate scenario, stripped
material from the companion will be swept up and once the ejecta become
optically thin, this material will emit, producing strong and relatively
narrow (FWHM $\sim 1,000\>\mathrm{km\>s^{-1}}$) hydrogen and/or helium
emission features. Our late-time spectrum shows a narrow (FWHM$=82\pm
18\>\mathrm{km\>s^{-1}}$) H$\alpha$ emission line at the host galaxy’s
redshift, thus, we deduce that it originates from the SN host galaxy, while no
He i $\lambda\lambda$5875,6678 lines are seen. In turn, this H$\alpha$ non-
detection allows us to place an upper limit on the amount of stripped material
following the method applied in Dimitriadis et al. (2019b). As the Botyánszki
et al. (2018) synthetic spectrum was generated for an epoch of 200 d after
peak, we scale our $313$ d spectrum using our photometry to estimate the
g-band magnitude at $200$ d, since the spectral features of SNe Ia do not
change significantly between those epochs. We calculate an observed apparent
magnitude of $\textit{g}_{200}=20.48\pm 0.5$ mag, and after correcting for MW
and host galaxy extinction and adopting the distance to CGCG 438-018, we infer
an H$\alpha$ luminosity upper limit of $<2.6\times
10^{37}\>\mathrm{erg\>s^{-1}}$, which corresponds to a hydrogen mass limit of
$<6.4\times 10^{-4}\>\mathrm{M_{\odot}}$.

###  3.5 The bolometric light curve

We constructed the UVOIR pseudo-bolometric light curve for SN 2021zny from our
broadband UV/optical/NIR photometry as follows: Firstly, we correct our
photometry for Milky Way and host galaxy extinction (see Section 3.1) and
convert the magnitudes to monochromatic fluxes. We then interpolate our fluxes
with Gaussian processes at the observed epochs of the g-band photometric
measurements, and assume zero flux for the UV and NIR bands after 2021
November 24 and December 04 UT, respectively. The computed spectral energy
distribution is integrated (using the trapezoidal rule) with respect to each
photometric band’s effective wavelength, assuming zero flux at the blue end of
the UVW2-band (1,500 Å) and the red end of the K-band (24,000 Å). Finally, we
converted the integrated flux to luminosity using the distance to SN 2021zny
from Section 3.1, with the uncertainty in the luminosity dominated by the
uncertainty in the distance. We note that, in our calculations, we excluded
epochs prior to 2021 September 9 UT (when the early flux excess is detected),
as only gri and TESS-band observations were acquired, and extrapolating the UV
and NIR light curves to those phases is highly uncertain. We used the same
procedure for SNe 2009dc, 2011fe and 2012dn and construct similar pseudo-
bolometric light curves, using the published photometry, extinction estimates
and distances. Our final bolometric light curves are shown in Fig. 13 and
presented in Table 2.

Figure 13: The UVOIR (1,500 - 24,000 Å) pseudo-bolometric light curve of SN 2021zny, constructed as described in the text, is presented in blue circles. Similarly constructed light curves of SNe 2009dc, 2012dn and 2011fe are additionally shown. On the bottom panel, we show the ratio of the UV (1,500 - 3,250 Å, solid lines) and NIR (8,900 - 24,000 Å, dashed lines) to the UVOIR luminosity of each SN. Table 2: The UVOIR pseudo-bolometric light curve of SN 2021zny. Phasea | Luminosity | Luminosity error
---|---|---
(Rest-frame Days) | ($10^{43}\>\mathrm{erg\>s^{-1}}$) | ($10^{43}\>\mathrm{erg\>s^{-1}}$)
-14.78 | 1.44 | 0.20
-13.70 | 1.60 | 0.22
-10.76 | 2.22 | 0.31
-8.18 | 2.59 | 0.36
-7.20 | 2.69 | 0.37
a Relative to $B$-band maximum (MJD 59498.46).
This table is available in its entirety in machine-readable form.

A common characteristic of the UVOIR bolometric light curves of the 03fg-like
SNe Ia is the shift of the time of their peak luminosities, compared to their
B-band maximum. For the normal Type Ia SN 2011fe, the peak luminosity occurs
$\sim 0.4$ d earlier than its B-band maximum, while for SNe 2021zny, 2009dc
and 2012dn, we measure $\sim 3.2,1.4$ and $1.8$ d. This can be explained by
the significant UV contribution to the bolometric light curve at early times,
as illustrated by the bottom panel of Fig. 13. At the earliest epochs (15-10 d
before B-band maximum) of the three 03fg-like SNe Ia, the UV contribution is
$\sim$25-30 per cent while for SN 2011fe it is $\sim$6 per cent, with this
reduced UV luminosity in normal Type Ia SNe attributed to increased line
blanketing from iron peak elements, due to the increased UV opacity near the
photosphere (Mazzali, 2000). For the case of the increased UV luminosity of
03fg-like SNe Ia, possible interpretations include differences in metallicity
and/or the outer density structure of the ejecta (Mazzali et al., 2014),
although these differences cannot reproduce the observed diversity in the UV
colours of SNe Ia (Brown et al., 2015). However, an additional UV-bright power
source, apart from the radioactive decay of 56Ni, contributing substantially
at earlier times could be a natural explanation, such as the interaction of
the ejecta with surrounding CSM.

As mentioned above, we chose to exclude from our estimate of SN 2021zny’s
bolometric light curve the extremely early epochs ($-$21 to $-$18 d with
respect to B-band maximum) due to limited photometric coverage, particularly
in the UV. However, motivated by the sizable UV contribution, we attempt to
characterize this early luminosity by fitting a black body to the early gri-
band data. We recover temperatures of 29,400 , 21,000 and 13,000 K at $20.6$,
$19.6$ and $18.6$ d from B-band maximum. While undoubtedly some emission from
the 56Ni radioactive decay is present at those epochs, it is potentially
subdominant: a fit to the early Lick/Kast spectrum of SN 2011fe (Nugent et
al., 2011), taken $\sim 1.5$ d after explosion ($\sim$16.3 d from B-band
maximum) gives a temperature of $7,700$ K, which corresponds to a difference
of $\sim 17,500$ K at this epoch.

Focusing on the UVOIR bolometric light curve at peak, we can estimate the 56Ni
and ejecta masses of SN 2021zny, assuming that the near-peak luminosity
evolution of the SN is powered solely by the radioactive decay of 56Ni and
56Co. We use the analytic model of Khatami & Kasen (2019), an updated version
of the classic Arnett (1982) model, that generates results in agreement with
numerical radiation transport calculations. We use $\beta=1.6$, appropriate
for SNe Ia (see their Table 2), a peak luminosity of $2.81(\pm 0.05)\times
10^{43}\>\mathrm{erg\>s^{-1}}$ and a rise time of $18.4\pm 0.5$ d
(corresponding to the rest-frame time from the TESS-band first detection up to
the peak of the UVOIR bolometric light curve), estimated with Gaussian
processes fitting. We further assume a constant opacity of
$\kappa=0.1\>\mathrm{cm^{2}\>g^{-1}}$ (appropriate for iron-group element
dominated ejecta, see Pinto & Eastman, 2000; Piro & Nakar, 2013) and an ejecta
velocity of $v_{\mathrm{ej}}=10,000\>\mathrm{km\>s^{-1}}$, obtained from the
absorption minimum of Si ii $\lambda$6355 near maximum (see Fig. 10). We
obtain $M_{\mathrm{{}^{56}Ni}}=1.37\pm 0.02\>\mathrm{M_{\odot}}$ and
$M_{\mathrm{ej}}=1.60\pm 0.10\>\mathrm{M_{\odot}}$. Adopting the same method,
we estimate for the 03fg-like SNe 2009dc and 2012dn
$M_{\mathrm{{}^{56}Ni}}=1.79\pm 0.02\>\mathrm{M_{\odot}}$ and
$M_{\mathrm{ej}}=2.32\pm 0.22\>\mathrm{M_{\odot}}$, and
$M_{\mathrm{{}^{56}Ni}}=0.79\pm 0.01\>\mathrm{M_{\odot}}$ and
$M_{\mathrm{ej}}=1.73\pm 0.11\>\mathrm{M_{\odot}}$, respectively, while for
the normal SN 2011fe we estimate $M_{\mathrm{{}^{56}Ni}}=0.57\pm
0.01\>\mathrm{M_{\odot}}$ and $M_{\mathrm{ej}}=1.40\pm
0.02\>\mathrm{M_{\odot}}$. Our mass estimates place SN 2021zny in the 03fg-
like regime, indicating a possible super-$\mathrm{M_{Ch}}$ mass origin for its
progenitor (particularly in terms of its ejecta mass), with an enormous 56Ni
synthesised mass, which is difficult to reconcile with current explosion
models. We defer to Section 4 for a comprehensive discussion.

Finally, at later times, SN 2021zny generally shows a similar decline rate to
SN 2009dc. Our bolometric light curve coverage ends at $\sim 90$ d from B-band
maximum and the increased fading seen in other 03fg-like events (e.g. at $60$
d for SN 2012dn, $>180$ d for SN 2009dc and $>110$ d for SN 2020esm;
Dimitriadis et al., 2022) has not occurred. We also cannot assess whether it
happened at later times. For the case of SN 2012dn, this rapid change in the
decline rate was associated with a simultaneous decrease in the optical and
increase in the NIR flux (see bottom panel of Fig. 13), something we do not
see in SN 2021zny up to these epochs (although we caution that the NIR flux at
these epochs is poorly constrained and is based on extrapolation). However,
the similarity of SN 2021zny’s light curves with that of SN 2009dc
(particularly in the NIR) indicates that this, if it happened, may have
occurred at later stages of its evolution.

##  4 Discussion

In this section, we discuss the results of our analysis in the context of
proposed progenitor systems of the 03fg-like SN Ia subclass. We summarise that
a viable progenitor model for SN 2021zny must address the early flux excess,
the increased early UV luminosity, the high luminosity at peak relative to its
decline rate, the persistent and strong carbon features (a proxy of unburned
material), the absence of narrow features (particularly hydrogen) in the
spectra and the presence of oxygen, and potentially calcium, emission at
nebular epochs.

As discussed in Section 3.3, the early flux excess seen in SN 2021zny’s gri
and TESS-band light curves mostly resembles the fast early flash seen in SNe
2020hvf and 2022ilv, and appears morphologically different from other flux
excesses, such as the ones of SNe 2017cbv and 2018oh. The relatively short
timescale of the flux excess generally excludes the shock interaction between
the supernova ejecta and a non-degenerate binary companion (Kasen, 2010), as
for most favourable viewing angles the duration of the excess is 3–6 d, with
its shape appearing more like a ‘bump’ than a ‘spike’ (see Fig. 2 of Jiang et
al., 2018). Moreover, the stringent constraint on the stripped material from
the non-degenerate companion additionally disfavours this scenario (however,
see the discussion in Dimitriadis et al., 2019b). Nevertheless, we attempt a
fit of the early light curves with a model which is a combination of two
luminosity sources that power the light curve: i) a collision-powered
luminosity prescribed as in Kasen (2010) and ii) a SN-powered luminosity (i.e.
the luminosity due to the radioactive 56Ni decay in the ejecta), for which we
use a simple power-law, $L_{x}=C_{x}(t-t_{0,x})^{\alpha_{x}}$ (see also Ni et
al., 2022a, for an identical approach). We simultaneously fit the gri and
TESS-band light curves, with the free parameters of our model being the binary
separation $a_{\mathrm{sep}}$ and the parameters describing the power-law rise
$C_{g,r,i,TESS}$ and $\alpha_{g,r,i,TESS}$. The best-fitting results are shown
in Fig. 14.

Figure 14: Early ZTF g\- (top left), r\- (top right), i\- (bottom left) and
TESS-band (bottom right) absolute magnitude light curves of SN 2021zny, with
our Kasen (2010) model fits (Section 4) presented as solid lines. The
collision and power-law model components are shown as dashed and dotted lines,
respectively. The inferred parameters of our fits are labelled.

We find a seperation at the moment of the SN explosion of $1.9(\pm 7.1)\times
10^{11}\>\mathrm{cm}$ and power-law indexes for each photometric band of
$\alpha_{g}=2.30\pm 0.77$, $\alpha_{r}=2.67\pm 0.73$, $\alpha_{i}=2.68\pm
0.76$ and $\alpha_{TESS}=2.84\pm 0.60$, with the inferred power-law indexes
lying on the extreme end of their distributions (particularly for the redder
bands, Olling et al., 2015; Miller et al., 2020b). Assuming the companion
fills its Roche lobe, this separation is consistent with a low-mass main-
sequence (MS) or (marginally) a helium-star (Liu et al., 2015). The MS
companion is disfavoured due to the non-detection of $H\alpha$ at the nebular
spectrum. For the case of a helium-star companion, the absence of hydrogen and
the presence of [Ca ii] and [O i] can be naturally explained (Lundqvist et
al., 2013), as oxygen and calcium are better coolants compared to helium.
However, the velocity range of calcium and oxygen in the helium-star companion
interaction scenario is expected to be at $<1,000\>\mathrm{km\,s^{-1}}$ (Pan
et al., 2010, 2012; Liu et al., 2013), while in the case of SN 2021zny we
measure $2,340\pm 100$ and $2,615\pm 120\,\mathrm{km\>s^{-1}}$ (see Section
3.4 and Table 1). More importantly, a single-degenerate scenario with a Kasen
(2010)-like interaction, while able to partially explain the early flux
excess, is not able to reproduce most of the peak-time properties of SN
2021zny (and 03fg-like SNe Ia in general), particularly the broad light curve
in combination with the high luminosity, the observed blue colours, the low
expansion velocities and the substantial amount of unburned material (see
Section 1).

We additionally investigate the possibility of varying 56Ni distributions,
that can mimic nickel mixing to the outer layers of the ejecta, as a source of
the early flux excess by comparing the early SN 2021zny light curves with the
models of Magee et al. (2020) and Magee & Maguire (2020). As pointed out by
Magee et al. (2020), extended nickel distributions are not able to reproduce
‘bumps’ or ‘spikes’, such as the one observed in the 03fg-like LSQ12gpw (see
their table 2 and figure C.1). On the other hand, nickel shells at the outer
edge of the ejecta can potentially introduce a flux excess, with Magee &
Maguire (2020) considering the light curves of SNe 2017cbv and 2018oh. While
the parameter space investigated in that study is rather small, we produce
synthetic light curves of their models and we compared their early rise with
SN 2021zny. None of the models were able to match the observed SN 2021zny flux
excess, with the closest match being the SN 2018oh model with a
$0.01\>\mathrm{M_{\odot}}$ nickel shell and a width of
$0.06\>\mathrm{M_{\odot}}$ in mass coordinates. However, this flux excess is
still relatively long-lasting, while the rise time is significantly shorter
and the resulting peak luminosity much lower. Finally, some sub-Chandrasekhar
mass explosion models from massive C/O WDs with a thick helium-shell
undergoing a double detonation can potentially match the high luminosity and
slow decline of SN 2021zny. However, their early flux excesses are less
pronounced, their UV luminosity low and, crucially, no unburned material is
present after explosion (Polin et al., 2019).

Motivated by the resemblance of SN 2021zny to SNe 2020hvf and 2022ilv, we
attempt to fit the early light curves of SN 2021zny with a model similar to
the one presented in Jiang et al. (2021), which is a combination of two
luminosity sources that power the light curve: i) a CSM-powered luminosity
assumed to originate from a spherically-symmetric envelope and ii) a SN-
powered luminosity (i.e. the luminosity due to the radioactive 56Ni decay in
the ejecta). For the CSM-powered luminosity we use the prescription of Piro
(2015) and for the radioactive 56Ni decay, in photometric band $x$, we use a
simple power-law, $L_{x}=C_{x}(t-t_{0,x})^{\alpha_{x}}$ (see also Ni et al.,
2022a, for an identical approach). The main difference of our model from the
Jiang et al. (2021) one is the underlying SN light, for which they use a more
realistic explosion model, developed with the radiation-hydrodynamic SuperNova
Explosion Code (SNEC; Morozova et al., 2015). We assume an electron-scattering
opacity of $\kappa=0.2\>\mathrm{cm^{2}\>g^{-1}}$, appropriate for H-poor CSM
(Piro, 2015), we fix the ejecta mass to the value obtained by our bolometric
light curve estimates ($M_{\mathrm{ej}}=1.62\>\mathrm{M_{\odot}}$) and we
break the degeneracy between the kinetic energy of the ejecta,
$E_{\mathrm{ej}}$, and the ejecta velocity, $v_{\mathrm{ej}}$, by fixing the
velocity to $v_{\mathrm{ej}}=10,000\>\mathrm{km\>s^{-1}}$ (as in Section 3.5).
Finally, we assume that the time of the onset of the CSM interaction
$t_{\mathrm{0,CSM}}$ coincides with the time of first light for each
photometric band, i. e. $t_{\mathrm{0,CSM}}\equiv t_{0,x}$. We simultaneously
fit the gri and TESS-band light curves, with the free parameters of our model
being the mass $M_{\mathrm{env}}$ and radius $R_{\mathrm{env}}$, the time of
the onset of the CSM interaction $t_{\mathrm{0,CSM}}$ and the parameters
describing the power-law rise $C_{g,r,i,TESS}$ and $\alpha_{g,r,i,TESS}$. The
best-fitting results are shown in Fig. 15.

Figure 15: Early ZTF g\- (top left), r\- (top right), i\- (bottom left) and
TESS-band (bottom right) absolute magnitude light curves of SN 2021zny, with
our CSM model fits (Section 4) presented as solid lines. The CSM and power-law
model components are shown as dashed and dotted lines, respectively. The
inferred parameters of our fits are labelled.

Our final estimates for the envelope properties are $M_{\mathrm{env}}=0.003\pm
0.001\>\mathrm{M_{\odot}}$ and $R_{\mathrm{env}}=3.6(\pm 2.8)\times 10^{12}$
cm. The total mass of the CSM $M_{\mathrm{CSM}}$ is then calculated assuming a
$\rho\sim r^{-3}$ density distribution for the envelope, expected for post-
merger CSM (Piro & Morozova, 2016), and adopting a progenitor WD radius of
$3-6\times 10^{8}$ cm (Piro et al., 2010) we find $M_{\mathrm{CSM}}=0.037\pm
0.009\>\mathrm{M_{\odot}}$. The time of the onset of the CSM interaction is
$0.06\pm 0.05$ rest-frame days after $t_{\mathrm{det}}^{TESS}$ and the power-
law indexes for each photometric band are $\alpha_{g}=2.10\pm 0.25$,
$\alpha_{r}=2.30\pm 0.19$, $\alpha_{i}=2.22\pm 0.32$ and
$\alpha_{TESS}=2.24\pm 0.11$. The inferred rise times and power-law indexes
are well within estimates for overluminous SNe Ia (Olling et al., 2015; Miller
et al., 2020b). Removing the constraint of a common time of first light (but
equal for every photometric band) and the onset of the interaction, we find
$t_{\mathrm{0,CSM}}=0.08\pm 0.05$ and $t_{0}=-0.41\pm 0.98$ rest-frame days
relative to $t_{\mathrm{det}}^{TESS}$, with $\alpha_{g}=2.32\pm 0.60$,
$\alpha_{r}=2.59\pm 0.58$, $\alpha_{i}=2.43\pm 0.61$ and
$\alpha_{TESS}=2.52\pm 0.62$, placing SN 2021zny at the extreme end of the
Miller et al. (2020b) distribution, but still within limits. The subsequent
CSM parameters are $M_{\mathrm{env}}=0.003\pm 0.001\>\mathrm{M_{\odot}}$,
$R_{\mathrm{env}}=3.9(\pm 3.2)\times 10^{12}$ cm and
$M_{\mathrm{CSM}}=0.034\pm 0.010\>\mathrm{M_{\odot}}$.

Our inferred CSM properties are similar to those of SN 2020hvf and SN 2022ilv,
for which Jiang et al. (2021) and Srivastav et al. (2023) find
$M_{\mathrm{CSM}}=0.01$ and $0.001$ M⊙, respectively, with
$R_{\mathrm{\mathrm{env}}}=1\times 10^{13}$ cm. However, the epoch of SN
2021zny’s flux excess was observed in four photometric bands, including the
dense TESS-band coverage, as opposed to one band for SNe 2020hvf and 2022ilv,
allowing us to better constrain the black body temperature of the CSM-powered
luminosity component and the time of explosion. Moreover, while our model
includes several degeneracies and assumptions (particularly for $\kappa$, Mej
and vej), it reasonably fits the observed light curves, sufficiently
recovering the strength and timescale of the flux excess. Finally, we note
that, as we do not use an explosion model for the SN-powered component but
rather a power-law rise, therefore, possible effects that may alter the early
SN light curve, such as nickel mixing (e.g. see Fig. 13 and 14 of Piro &
Morozova, 2016), are not considered. Nevertheless, the three 03fg-like SNe Ia
that were discovered extremely early show an early time light curve behaviour
which is consistent with interaction of the SN ejecta with a dense shell of a
low amount of CSM relatively close to the explosion site, indicating that this
progenitor configuration might be common in the 03fg-like subclass.

A promising model that can explain the early flux excess is a merger of two
C/O WDs, where the lower mass one is disrupted during the merging process.
Hydrodynamical simulations of this binary configuration show that $\sim
10^{-3}\>\mathrm{M_{\odot}}$ hydrogen and helium-free material from the
disrupted WD can be ejected from the system, achieving escape velocities of
$\sim 2,000\>\mathrm{km\>s^{-1}}$ and resulting in material out to
$10^{13}-10^{14}$ cm (Raskin & Kasen, 2013). The interaction of the SN ejecta
with this C/O-rich CSM will result in additional UV/X-ray emission, and
additional UV photons, as the shock-heated material cools. At the same time,
the larger fraction of the disrupted WD will be quickly swept up by the
ejecta, producing the strong and broad C ii features (Raskin et al., 2014) we
observe in the early spectra of SN 2021zny. As the unburned material from the
disrupted WD forms an accretion disk around the exploding WD, strong
orientation effects are expected, with equatorial viewing angles resulting in
stronger C ii features and lower ejecta velocities due to the increased
deceleration. Several other parameters of this model can affect the observed
properties, such as the mass ratio of the two WDs (Dan et al., 2012) and
whether there is a delay between the disruption and the explosion (Raskin &
Kasen, 2013).

As already mentioned in Dimitriadis et al. (2022), while these merger models
can generally reproduce the spectroscopic properties of most 03fg-like SNe Ia,
they fail in reproducing their bolometric light curves. Raskin et al. (2014)
present bolometric light curves for three merger configurations, for varying
viewing angles, with the high total mass scenarios accurately predicting the
peak luminosity but overestimating the width of the light curve (as the total
ejecta mass is higher), while the situation is reversed for the lower total
mass ones. We note that these models do not include the potential increased
luminosity due to the interaction of the ejecta with the C/O-rich CSM.
However, Noebauer et al. (2016) consider SN 2009dc and present hydrodynamical
and radiative transfer simulations of thermonuclear explosions of
Chandrasekhar-mass WDs surrounded by relatively compact ($R_{\mathrm{env}}\sim
10^{14}\>\mathrm{cm}$) and massive ($M_{\mathrm{CSM}}\sim
0.6\mathrm{M_{\odot}}$) C/O-rich CSM, which are able to generally reproduce
the peak luminosity and the decline rate of 03fg-like events. Fig. 16 shows
the constructed optical ($3,250-8,900$ Å) bolometric light curves of SNe
2021zny (blue) and 2011fe (red), alongside the synthetic optical bolometric
light curves from Noebauer et al. (2016) of a thermonuclear explosion of
Chandrasekhar-mass WD with $1.4\>\mathrm{M_{\odot}}$ ejecta mass, producing
$1.0\>\mathrm{M_{\odot}}$ of 56Ni (dashed green line) and the same model but
embedded with $0.64\>\mathrm{M_{\odot}}$ C/O-rich material, extending to
$1.3\times 10^{14}$ cm (solid orange line). The ‘bare-ejecta’ model fails to
reproduce the width of the SN 2021zny light curve and the time of maximum.
Instead, it succeeds reproducing the general behaviour of the light curve of
SN 2011fe, but overestimates the peak luminosity, as SN 2011fe synthesized
only $0.44\>\mathrm{M_{\odot}}$ of 56Ni (Pereira et al., 2013). The
simulations involving the CSM roughly predicts the luminosity and the decline
rate of SN 2021zny at a few days after peak brightness and the overall width
of the light curve, due to the increased trapping of the $\gamma$-rays from
the additional CSM material and the increased density from the shock
compression. The mismatch during the rising part of the light curve was
addressed by increasing the content of non-carbon-oxygen material in the CSM,
enhancing the opacity and the reprocessing efficiency (see Fig. 7 and 8 from
Noebauer et al., 2016). Nevertheless several parameters of this model can also
alter the resulting light curves, with some of them already investigated by
the authors, such as the mass and extent of the CSM. Moreover, in the inset we
show the early model light curve, where a short-duration flash is seen,
created from the reprocessing and shifting to the optical regime of the strong
UV/X-ray luminosity, as a result of the shock breaking out at the edge of the
CSM. We note that the earlier bump in SN 2021zny is explained by interaction
of the SN ejecta with a more compact CSM ($R_{\mathrm{env}}\sim 10^{12}$ cm)
soon after the explosion when the WD expanded to a radius of $\sim
10^{11}\>\mathrm{cm}$ and detailed simulations of the ejecta-CSM interaction
at early times are not performed in Noebauer et al. (2016).

Figure 16: The optical ($3,250-8,900$ Å) pseudo-bolometric light curves of SNe
2021zny (blue circles) and 2011fe (red upward triangles), compared with the
bare ejecta (dashed green line) and the fiducial interaction (solid orange
line) models of Noebauer et al. (2016). In the inset, a zoom-in region of the
early epochs is shown. The early flash of the interaction model, attributed to
the high-energy radiation from the shock breaking out at the edge of the CSM
(see Fig. 5 from Noebauer et al., 2016) coincides with the observed flux
excess of SN 2021zny.

We emphasise that both our early-time simple analytical model and the model
addressing the near-peak evolution do not fully capture the physics of the
ejecta-CSM interaction. Detailed numerical simulations, accounting for both
the hydrodynamics and accurate radiative transfer calculations, are needed to
thoroughly investigate the full parameter space of this model and
simultaneously address all the phases of the phenomenon, which is not the
scope of this paper. In a subsequent study, we will use the combined sample of
SNe 2020hvf, 2021zny and 2022ilv to explore several explosion models and CSM
configurations, e.g. models with moderate 56Ni synthesized masses but
increased additional luminosity from the CSM or asymmetries and multiple
components of the CSM at more extended locations.

Our nebular observations of SN 2021zny are crucial to constrain its progenitor
system, since at these epochs the density of the ejecta is extremely low,
unveiling the immediate environment of the SN explosion and its mechanism. The
bolometric light curve up to +130 d from $B$-band maximum appears to follow a
decline rate broadly consistent with the 56Co decay, with no sign of re-
brightening (as would be expected with sustained CSM interaction) or
accelerated decline (as seen in e.g. SN 2012dn). At even later times, the
absence in our nebular spectrum of hydrogen and helium emission features,
either from stripped material from a potential non-degenerate companion or
from H/He-rich CSM at larger distances, places strong constraints on the
nature of the companion and the origin of the CSM. These observational
characteristics are difficult to explain under the ‘core-degenerate’ scenario
(Hoeflich & Khokhlov, 1996; Kashi & Soker, 2011; Hsiao et al., 2020; Ashall et
al., 2021), as it predicts a late-time UV re-brightening, originating from the
interaction of the ejecta with the AGB superwind, and narrow hydrogen/helium
emission lines from the interaction of the ejecta with the AGB’s
envelope/wind. A possible solution to this problem is that the ejecta have not
yet started to interact with the CSM, while the dense envelope prevents the
$\gamma$-rays escaping and ionising this material.

A double-degenerate merger scenario naturally explains the absence of H/He
late time emission features, since the (low-mass) CSM is not H/He-rich, as it
originates from the disrupted secondary C/O WD. Moreover, the detection of [O
i] $\lambda\lambda$6300, 6364 also favors a merger event, as this class of
models are the only ones predicting the presence of unburned oxygen at low
velocities, with the exception of turbulent pure deflagrations (Kozma et al.,
2005). However, these models predict peculiar early time spectra and low
luminosity explosions, clearly inconsistent with SN 2021zny. The presence of
oxygen at late times implies that calcium is also present (if these two
elements are located in the same cooling region), as the [Ca ii]
$\lambda\lambda$7291, 7323 lines are $\sim 100\times$ more efficient per atom
in cooling than the [O i] $\lambda\lambda$6300, 6364 lines (Fransson &
Chevalier, 1989), with the similar FWHM of the oxygen and calcium features of
SN 2021zny supporting this claim. If the red emission feature of the
$7,000-7,500$ Å line complex is indeed dominated by calcium, the remaining [Ni
ii] feature must be weak (relative to [Fe ii]), implying a small amount of
stable 58Ni synthesized in the explosion, since any nickel lines at nebular
epochs cannot originate from the 6.1 days half-life radioactive 56Ni, while
the iron lines are dominated by emission of the 56Co decay. We estimate the
ratio of Ni to Fe abundance, following equation 1 from Maguire et al. (2018),
to be $0.012\pm 0.005$, decisively inconsistent with any Chandrasekhar-mass
explosions ($0.17-0.20$) and consistent with some sub-Chandrasekhar mass
explosions ($<0.15$), although the later are strongly disfavoured from the
maximum light properties of SN 2021zny. Maguire et al. (2018) did not provide
this ratio for any merger model, thus we use the abundances of the merger
models of Kromer et al. (2013), Pakmor et al. (2010) and Pakmor et al. (2012)
(with WD masses of $0.9+0.76\>\mathrm{M_{\odot}}$,
$0.9+0.9\>\mathrm{M_{\odot}}$ and $1.1+0.9\>\mathrm{M_{\odot}}$, respectively)
obtained from the Heidelberg Supernova Model
Archive131313https://hesma.h-its.org/, and we estimate ratios of
$0.0002-0.0488$, consistent with SN 2021zny (however, see Blondin et al.,
2022, for a discussion of this approach).

While the properties of many 03fg-like SNe Ia (including SN 2021zny) are in
accordance with the merger scenario, some caveats still exist. The most
important of them is the lack of observed asymmetries in the explosion (Bulla
et al., 2016), probed by polarimetric observations (Tanaka et al., 2010;
Cikota et al., 2019, although these observations were performed after peak
brightness). To this end, early time polarimetry of 03fg-like SNe Ia will
prove extremely useful to disentangle between various proposed explosion
models. Moreover, other model parameters, such as the mass ratio of the two
WDs, the time delay between the disruption of the secondary WD during the
merger and the final detonation of the primary WD and the location of the
hotspot that causes the detonation (modifying the synthesized 56Ni mass) pose
theoretical challenges (Dan et al., 2011, 2012; Raskin & Kasen, 2013), but at
the same time could account for the diversity within the 03fg-like subclass.
Finally, a precise rate of the intrinsically rare 03fg-like SNe Ia has yet to
be calculated, and as different progenitor models involve a diverse set of
progenitor stars, a proper measurement of the rate, for which non-targeted
all-sky surveys such as ZTF are essential, is highly encouraged.

##  5 Conclusion

In this work, we presented extensive multi-wavelength photometric and
spectroscopic observations of SN 2021zny, the first 03fg-like event with a
flux excess within the first $\sim 1.5$ d after explosion, estimated at
$-21.6$ d before maximum light, and, simultaneously, prominent [O i]
$\lambda\lambda$6300, 6364 emission lines at $313$ d after peak brightness. SN
2021zny is the third member of its class, after SNe 2020hvf and 2022ilv, with
an early flash, and the first with a multi-wavelength photometric detection,
allowing us to better estimate the temperature at the flux excess epoch.
Moreover, it is the second 03fg-like SN Ia with oxygen features at late times,
a highly unusual observation within the extended thermonuclear family. Apart
from these observations, SN 2021zny displays all the usual characteristics of
the 03fg-like class, such as the high peak brightness compared to its (slow)
decline rate, the UV-bright luminosity and blue early-time UV and optical
colours (compared to normal SNe Ia), the low ejecta velocities and the
persistent carbon features in its optical spectra. From all of the above,
combined with the absence of hydrogen and helium features, the low ionization
state and the low abundance of stable iron-peak elements, inferred from the
late-time spectrum, we conclude that SN 2021zny originated from a double-
degenerate progenitor system, possibly a merger of two C/O WDs where the
explosion occurs after the secondary WD is fully disrupted, from which some
H/He-poor CSM is ejected at a close vicinity to the explosion site.

We note that it appears that every 03fg-like SN Ia that is discovered and
monitored in the first $\sim 1-3$ d from explosion shows this early flash,
indicating a common explosion mechanism and progenitor system, with their
peak-light and late-time diversities possibly explained by the variation of
other model parameters. High-quality observations from very early to very late
epochs, particularly in unexplored wavelength ranges, such as X-ray and
near/mid IR would prove crucial in order to uncover the nature of these unique
events.

## Acknowledgements

We thank the anonymous referee for helpful comments that improved the clarity
and presentation of this Paper.

GD and KM are supported by the H2020 European Research Council grant no.
758638.

AAM is partially supported by NASA grant 80NSSC22K0541.

MC acknowledges support from the National Science Foundation with grant
numbers PHY-2010970 and OAC-2117997.

LG and TEMB acknowledge financial support from the Spanish Ministerio de
Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI)
10.13039/501100011033 under the PID2020-115253GA-I00 HOSTFLOWS project, from
Centro Superior de Investigaciones Científicas (CSIC) under the PIE project
20215AT016, and by the program Unidad de Excelencia María de Maeztu
CEX2020-001058-M. LG also acknowledges MCIN, AEI and the European Social Fund
(ESF) "Investing in your future" under the 2019 Ramón y Cajal program
RYC2019-027683-I.

MG is supported by the EU Horizon 2020 research and innovation programme under
grant agreement No 101004719.

NI is partially supported by the Polish NCN DAINA grant No.
2017/27/L/ST9/03221.

MN 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 a Fellowship from the Alan Turing Institute.

SS acknowledges support from the G.R.E.A.T. research environment, funded by
Vetenskapsrådet, the Swedish Research Council, project number 2016-06012.

QW is supported in part by NASA grant 80NSSC22K0494, 80NSSC21K0242 and
80NSSC19K0112.

YY is supported by a Bengier-Winslow-Robertson Fellowship.

This work was funded by ANID, Millennium Science Initiative, ICN12_009.

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.

The ZTF forced-photometry service was funded under the Heising-Simons
Foundation grant #12540303 (PI: Graham).

The SED Machine is based upon work supported by the National Science
Foundation under Grant No. 1106171.

The Liverpool Telescope is operated on the island of La Palma by Liverpool
John Moores University in the Spanish Observatorio del Roque de los Muchachos
of the Instituto de Astrofisica de Canarias with financial support from the UK
Science and Technology Facilities Council.

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 1103.D-0328, 106.216C,
108.220C (PI: Inserra). The Las Cumbres Observatory data have been obtained
via OPTCON proposals (IDs: OPTICON 21B/001 and 22A/004). The OPTICON project
has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 730890.

Based on observations made with the Nordic Optical Telescope, owned in
collaboration by the University of Turku and Aarhus University, and operated
jointly by Aarhus University, the University of Turku and the University of
Oslo, representing Denmark, Finland and Norway, the University of Iceland and
Stockholm University at the Observatorio del Roque de los Muchachos, La Palma,
Spain, of the Instituto de Astrofisica de Canarias.

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.

Some of the data presented herein were obtained at the W. M. Keck Observatory,
which is operated as a scientific partnership among the California Institute
of Technology, the University of California and the National Aeronautics and
Space Administration. The Observatory was made possible by the generous
financial support of the W. M. Keck Foundation.

The authors wish to recognise and acknowledge the very significant cultural
role and reverence that the summit of Maunakea has always had within the
indigenous Hawaiian community. We are most fortunate to have the opportunity
to conduct observations from this mountain.

We acknowledge the use of Weizmann Interactive Supernova Data Repository
(WISeREP) maintained by the Weizmann Institute of Science computing center.

This work made use of the Heidelberg Supernova Model Archive (HESMA).

## Data Availability

The data analysed in this paper are available in the electronic edition and
via the Weizmann Interactive Supernova Data Repository (WISeREP).

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##  Appendix A Photometry and Spectroscopy

Table 3: Observed ground-based photometry of SN 2021zny. MJD | Phasea | Telescope/Instrument | Filter | Brightness | Brightness error | Brightness upper limitb
---|---|---|---|---|---|---
| (Rest-frame Days) |  |  | (AB mag) | (AB mag) | (AB mag)
59472.38 | -25.40 | P48/ZTF | r |  |  | 21.652
59472.44 | -25.34 | P48/ZTF | g |  |  | 21.735
59474.33 | -23.50 | P48/ZTF | r |  |  | 21.415
59474.40 | -23.43 | P48/ZTF | g |  |  | 21.827
59476.37 | -21.51 | P48/ZTF | i |  |  | 20.482
59476.43 | -21.45 | P48/ZTF | r |  |  | 20.940
59476.50 | -21.38 | P48/ZTF | g | 20.353 | 0.188 |
59477.29 | -20.62 | P48/ZTF | r | 20.318 | 0.238 |
59477.35 | -20.55 | P48/ZTF | g | 20.066 | 0.190 |
59478.28 | -19.65 | P48/ZTF | r |  |  | 20.325
59478.38 | -19.56 | P48/ZTF | g | 20.146 | 0.336 |
59479.32 | -18.63 | P48/ZTF | i | 19.415 | 0.244 |
59479.37 | -18.59 | P48/ZTF | r | 19.299 | 0.116 |
59483.28 | -14.78 | P48/ZTF | g | 17.207 | 0.040 |
59483.32 | -14.74 | P48/ZTF | i | 17.364 | 0.052 |
59484.34 | -13.75 | P48/ZTF | r | 16.941 | 0.032 |
59484.39 | -13.70 | P48/ZTF | g | 16.903 | 0.032 |
59485.44 | -12.68 | ATLAS | o | 16.880 | 0.020 |
59486.30 | -11.84 | P48/ZTF | r | 16.682 | 0.031 |
59487.31 | -10.86 | P48/ZTF | i | 16.736 | 0.036 |
59487.34 | -10.83 | P48/ZTF | r | 16.449 | 0.029 |
59487.41 | -10.76 | P48/ZTF | g | 16.352 | 0.029 |
59487.52 | -10.65 | ATLAS | o | 16.570 | 0.010 |
59488.35 | -9.85 | P48/ZTF | r | 16.327 | 0.029 |
59489.58 | -8.65 | ATLAS | o | 16.370 | 0.010 |
59490.05 | -8.19 | LT/IO:O | u | 16.230 | 0.060 |
59490.06 | -8.18 | LT/IO:O | z | 16.770 | 0.040 |
59490.06 | -8.18 | LT/IO:O | i | 16.580 | 0.040 |
59490.06 | -8.18 | LT/IO:O | g | 16.060 | 0.050 |
59490.06 | -8.18 | LT/IO:O | r | 16.190 | 0.030 |
59491.00 | -7.26 | NTT/SOFI | H | 17.734 | 0.090 |
59491.00 | -7.26 | NTT/SOFI | J | 17.009 | 0.050 |
59491.07 | -7.20 | LT/IO:O | g | 15.970 | 0.050 |
59491.07 | -7.20 | LT/IO:O | u | 16.160 | 0.030 |
59491.08 | -7.19 | LT/IO:O | i | 16.510 | 0.040 |
59491.08 | -7.19 | LT/IO:O | r | 16.100 | 0.040 |
59491.08 | -7.19 | LT/IO:O | z | 16.680 | 0.050 |
59491.24 | -7.03 | P48/ZTF | r | 16.041 | 0.029 |
59491.34 | -6.93 | P48/ZTF | i | 16.442 | 0.033 |
59491.52 | -6.76 | ATLAS | o | 16.210 | 0.010 |
59493.42 | -4.91 | ATLAS | o | 16.150 | 0.010 |
59494.93 | -3.44 | LCO/Sinistro | u | 16.162 | 0.024 |
59494.93 | -3.43 | LCO/Sinistro | g | 15.783 | 0.011 |
59494.94 | -3.43 | LCO/Sinistro | r | 15.966 | 0.010 |
59494.94 | -3.43 | LCO/Sinistro | i | 16.377 | 0.012 |
a Relative to $B$-band maximum (MJD 59498.46)
b 3$\sigma$ upper limit
This table is available in its entirety in machine-readable form.
Table 4: Observed Swift photometry of SN 2021zny. MJD | Phasea | Filter | Brightness | Brightness error
---|---|---|---|---
| (Rest-frame Days) |  | (AB mag) | (AB mag)
59487.83 | -10.35 | V | 16.285 | 0.083
59487.84 | -10.34 | UVM2 | 18.713 | 0.054
59487.86 | -10.33 | UVW1 | 17.845 | 0.046
59487.86 | -10.32 | U | 16.498 | 0.029
59487.86 | -10.32 | B | 16.291 | 0.034
59487.86 | -10.32 | UVW2 | 19.127 | 0.063
a Relative to $B$-band maximum (MJD 59498.46)
This table is available in its entirety in machine-readable form.
Table 5: Observed TESS 6h averaged photometry of SN 2021zny. MJD | Phasea | Brightness | Brightness error | Brightness upper limitb
---|---|---|---|---
| (Rest-frame Days) | (AB mag) | (AB mag) | (AB mag)
59473.71 | -24.10 |  |  | 19.831
59473.87 | -23.94 |  |  | 20.252
59474.12 | -23.70 |  |  | 20.114
59474.37 | -23.45 |  |  | 20.187
59474.62 | -23.21 |  |  | 20.218
59474.88 | -22.96 |  |  | 20.200
59475.12 | -22.72 |  |  | 20.132
59475.37 | -22.48 |  |  | 20.449
59475.62 | -22.24 |  |  | 20.399
59475.87 | -21.99 |  |  | 20.438
59476.12 | -21.75 |  |  | 20.188
59476.37 | -21.51 |  |  | 20.202
59476.62 | -21.26 |  |  | 20.315
59476.87 | -21.02 | 20.089 | 0.350 |
59477.12 | -20.78 | 19.638 | 0.202 |
59477.37 | -20.53 |  |  | 20.276
59477.61 | -20.30 | 20.164 | 0.361 |
59477.87 | -20.05 |  |  | 20.699
59478.12 | -19.80 | 20.341 | 0.362 |
59478.37 | -19.56 | 20.223 | 0.318 |
59478.62 | -19.32 |  |  | 20.249
59478.87 | -19.07 | 19.865 | 0.250 |
59479.12 | -18.83 | 19.737 | 0.198 |
59479.37 | -18.59 | 19.283 | 0.182 |
59479.62 | -18.34 | 19.003 | 0.107 |
59479.87 | -18.10 | 18.861 | 0.093 |
a Relative to $B$-band maximum (MJD 59498.46)
This table is available in its entirety in machine-readable form.
Table 6: Observing log of the optical spectra of SN 2021zny. Obs Date | Phasea | Telescope/Instrument | Slit Width | Grism/Grating
---|---|---|---|---
(UT) | (Rest-frame Days) |  |  |
2021-09-27 | -13.90 | P60/SEDM | – | –
2021-09-27 | -13.82 | P200/DBSP |  |
2021-09-27 | -13.69 | P60/SEDM | – | –
2021-09-30 | -10.84 | P60/SEDM | – | –
2021-10-03 | -8.03 | NTT/EFOSC | 1$\aas@@fstack{\prime\prime}$0 | Gr#11+Gr#16
2021-10-03 | -8.00 | LT/SPRAT | 1$\aas@@fstack{\prime\prime}$8 | 600 lines/mm, Red
2021-10-04 | -7.08 | P60/SEDM | – | –
2021-10-04 | -6.65 | Keck/LRIS | 1$\aas@@fstack{\prime\prime}$0 | 400/3400+400/8500
2021-10-06 | -5.11 | NTT/EFOSC | 1$\aas@@fstack{\prime\prime}$0 | Gr#11+Gr#16
2021-10-09 | -1.45 | LT/SPRAT | 1$\aas@@fstack{\prime\prime}$8 | 600 lines/mm, Red
2021-10-14 | 2.68 | NTT/EFOSC | 1$\aas@@fstack{\prime\prime}$0 | Gr#11+Gr#16
2021-10-25 | 14.15 | LT/SPRAT | 1$\aas@@fstack{\prime\prime}$8 | 600 lines/mm, Red
2021-10-26 | 14.32 | NTT/EFOSC | 1$\aas@@fstack{\prime\prime}$0 | Gr#11+Gr#16
2021-10-30 | 18.29 | P60/SEDM | – | –
2021-10-30 | 19.25 | Magellan/LDSS3 | 1$\aas@@fstack{\prime\prime}$0 | VPH-ALL
2021-11-04 | 23.09 | NTT/EFOSC | 1$\aas@@fstack{\prime\prime}$0 | Gr#11+Gr#16
2021-11-26 | 44.48 | NTT/EFOSC | 1$\aas@@fstack{\prime\prime}$0 | Gr#11+Gr#16
2021-11-30 | 48.32 | P60/SEDM | – | –
2021-12-01 | 49.21 | P200/DBSP |  |
2021-12-08 | 56.85 | LT/SPRAT | 1$\aas@@fstack{\prime\prime}$8 | 600 lines/mm, Red
2021-12-22 | 70.62 | NOT/ALFOSC | 1$\aas@@fstack{\prime\prime}$3 | grism #4
2022-01-03 | 81.57 | P60/SEDM | – | –
2022-01-04 | 82.43 | NTT/EFOSC | 1$\aas@@fstack{\prime\prime}$0 | Gr#11+Gr#16
2022-01-07 | 85.41 | P60/SEDM | – | –
2022-01-12 | 90.30 | P60/SEDM | – | –
2022-01-21 | 99.11 | P60/SEDM | – | –
2022-01-26 | 103.88 | P60/SEDM | – | –
2022-02-15 | 124.02 | NOT/ALFOSC | 1$\aas@@fstack{\prime\prime}$0 | grism #4
2022-08-29 | 312.59 | Keck/DEIMOS | – | LVMslitC
a Relative to $B$-band maximum (MJD 59498.46)