MNRAS
521,
1162–1183
(2023)
https://doi.org/10.1093/mnras/stad536
Advance
Access
publication
2023
February
27
SN
2021zny:
an
early
flux
excess
combined
with
late-time
oxygen
emission
suggests
a
double
white
dwarf
merger
event
Georgios
Dimitriadis
,
1
‹
Kate
Maguire
,
1
Viraj
R.
Karambelkar,
2
Ryan
J.
Lebron,
3
Chang
Liu
(
)
,
4
,
5
Alexandra
Kozyre
v
a,
6
Adam
A.
Miller
,
4
,
5
Ryan
Ridden-Harper
,
7
Joseph
P.
Anderson,
8
,
9
Ting-Wan
Chen,
6
,
10
Michael
Coughlin
,
11
Massimo
Della
Valle
,
12
,
13
Andrew
Drake,
2
Llu
́
ıs
Galbany,
14
,
15
Mariusz
Gromadzki,
16
Steven
L.
Groom,
17
Claudia
P.
Guti
́
errez
,
18
,
19
Nada
Ihanec,
8
,
16
Cosimo
Inserra
,
20
Joel
Johansson,
21
To m
́
as
E.
M
̈
uller
-Bra
v
o,
14
,
15
Matt
Nicholl,
22
Abigail
Polin,
23
,
24
Ben
Rusholme
,
17
Steve
Schulze,
21
Jesper
Sollerman
,
25
Shubham
Sri
v
astav,
26
Kirsty
Ta g ga r t
,
27
Qinan
Wa n g
,
28
Yi
Ya n g
(
)
29
and
David
R.
Young
26
Affiliations
are
listed
at
the
end
of
the
paper
Accepted
2023
February
15.
Received
2023
February
13;
in
original
form
2023
January
10
A
B
S
T
R
A
C
T
We
present
a
photometric
and
spectroscopic
analysis
of
the
ultraluminous
and
slowly
evolving
03fg-like
Type
Ia
SN
2021zny.
Our
observational
campaign
starts
from
∼
5.3
h
after
explosion
(making
SN
2021zny
one
of
the
earliest
observed
members
of
its
class),
with
dense
multiwavelength
coverage
from
a
variety
of
ground-
and
space-based
telescopes,
and
is
concluded
with
a
nebular
spectrum
∼
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
(
m
15
(
B
)
=
0.62
mag),
the
blue
early-time
colours,
the
low
ejecta
velocities,
and
the
presence
of
significant
unburned
material
abo
v
e
the
photosphere.
Ho
we
v
er,
a
flux
e
xcess
for
the
first
∼
1.5
d
after
explosion
is
observed
in
four
photometric
bands,
making
SN
2021zny
the
third
03fg-like
event
with
this
distinct
behaviour,
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
∼
0
.
04
M
of
H/He-poor
circumstellar
material
at
a
distance
of
∼
10
12
cm,
while
the
low
ionization
state
of
the
late-time
spectrum
re
veals
lo
w
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.
K
ey
words:
transients:
supernov
ae
– supernov
ae:
indi
vidual:
2021zny.
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,
Press
&
Kirshner
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
disco
v
ery
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
e
xplosiv
e
thermonuclear
burning
(Hoyle
&
Fowler
1960
)
of
a
degenerate
carbon–oxygen
white
dwarf
(WD)
in
a
binary
system
(Whelan
&
Iben
1973
;
Iben
&
Tutukov
1984
;
Bloom
E-mail:
dimitrig@tcd.ie
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,
Mannucci
&
Nelemans
2014
;
Hoeflich
2017
;
Jha,
Maguire
&
Sulli
v
an
2019
).
Focusing
on
the
vast
majority
of
SNe
Ia,
the
correlation
between
their
maximum
luminosity
and
their
light-curve
shape
(usually
parametrized
with
their
magnitude
decline
for
the
first
15
d
after
maximum
light,
m
15
;
Phillips
1993
)
can
be
explained
by
the
nucle-
osynthetic
yield
of
56
Ni,
the
most
abundant
radioactive
element
the
exploding
WD
synthesizes
(Colgate
&
McKee
1969
),
that
powers
the
light
curv
e.
F
or
a
giv
en
ejecta
mass
(usually
the
maximum
mass
a
de-
generate
non-rotating
C/O
WD
can
sustain,
the
Chandrasekhar
mass,
M
Ch
,
Chandrasekhar
1931
),
smaller/larger
amounts
of
56
Ni
lead
to
fainter/brighter
explosions
with
shorter/longer
time-scales.
This
sim-
ple
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,
Friesen
&
Parthasarathy
2014
).
High-cadence
and/or
untargeted
transient
surv
e
ys
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
© 2023
The
Author(s)
Published
by
Oxford
University
Press
on
behalf
of
Royal
Astronomical
Society
Downloaded from https://academic.oup.com/mnras/article/521/1/1162/7059222 by California Institute of Technology user on 19 July 2023
SN
2021zny
1163
MNRAS
521,
1162–1183
(2023)
Supernovae
(ASAS-SN;
Shappee
et
al.
2014
),
the
Distance
less
than
40
Mpc
surv
e
y
(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
ev
ents
share
man
y
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
re
vie
w).
One
of
the
most
puzzling
subtypes
of
SNe
Ia
is
the
so-called
03fg-
like
SNe
Ia,
a
rare
subclass
of
ultraluminous
and
slowly
evolving
ev
ents.
The
disco
v
ery
of
the
prototype
SN
2003fg
(Howell
et
al.
2006
)
revealed
a
brighter
peak
luminosity
(
M
B
=
−
20.09
mag)
for
its
decline
rate
(
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
subpopulation
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.
Moreo
v
er,
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
,
2019
;
Chakradhari
et
al.
2014
;
Parrent
et
al.
2016
;
Chen
et
al.
2019
;
Hsiao
et
al.
2020
;
Lu
et
al.
2021
;
Dimitriadis
et
al.
2022
for
studies
on
indi
vidual
e
vents
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-
M
Ch
masses
(Yoon
&
Langer
2005
),
leading
to
the
adoption
of
the
‘super
-Chandrasekhar
-mass’
moniker.
Ho
we
ver,
these
approaches
were
disputed
by
numerical
simulations
(Pfannes
et
al.
2010a
;
Pfannes,
Niemeyer
&
Schmidt
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.
Moreo
v
er,
a
super-M
Ch
56
Ni
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
56
Ni
– ejecta
mass
combination,
which
led
to
the
introduction
of
alternative
scenarios,
where
the
luminosity
of
the
SN
is
not
solely
powered
by
the
56
Ni
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.
Ho
we
ver,
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
)
sho
ws
narro
w
H
α
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
–
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
α
at
later
times,
supporting
a
delayed
ejecta–CSM
interaction
scenario.
The
situation
appears
more
complicated
within
the
early
pho-
tometric
e
volution.
Indi
vidual
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
∝
t
α
,
where
α
=
2
corresponds
to
the
canonical
‘expanding
fireball’
model
(Arnett
1982
;
Riess
et
al.
1999
).
Continuous,
high
cadence
observations
with
transiting
e
xoplanet
surv
e
ys,
such
as
Kepler/K2
(Olling
et
al.
2015
;
Wa n g
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,
Li
&
Filippenko
2011
;
Gonz
́
alez-Gait
́
an
et
al.
2012
;
Firth
et
al.
2015
;
Papadogiannakis
et
al.
2019
;
Miller
et
al.
2020b
)
find
mean
values
of
1.8
≤
α
≤
2.4;
ho
we
v
er,
man
y
individual
SNe
in
the
samples
are
incompatible
with
α
=
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,
time-scales,
and
colours.
Most
notably,
a
blue
and
relatively
long
(
∼
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
;
Li
et
al.
2019
;
Shappee
et
al.
2019
)
and
SN
2021aefx
(Hosseinzadeh
et
al.
2022
),
has
been
attributed
to
ejecta
interaction
with
(main
sequence
or
subgiant)
non-degenerate
companions.
Ho
we
ver,
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
,
Shappee
&
W
isniewski
2019
;
Sand
et
al.
2021
),
leading
to
alternativ
e
e
xplanations
of
the
early
‘bumps’,
such
as
the
presence
of
56
Ni
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-M
ch
WD
(Polin,
Nugent
&
Kasen
2019
).
The
presence
of
excess
nucleosynthetic
material
in
the
outermost
layers
of
the
ejecta
has
been
proposed
for
the
short-term
(
∼
0.5
d)
and
redward
evolution
of
SN
2018aoz’s
early
flux
excess
(Ni
et
al.
2022b
),
with
Jiang
et
al.
(
2017
)
and
De
et
al.
(
2019
)
fa
v
ouring
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
(
∼
3.5
d)
and
ultraviolet
(UV)
bright
flux
excess
Downloaded from https://academic.oup.com/mnras/article/521/1/1162/7059222 by California Institute of Technology user on 19 July 2023
1164
G.
Dimitriadis
et
al.
MNRAS
521,
1162–1183
(2023)
for
SN
2019yvq,
for
which
Siebert
et
al.
(
2020
),
based
on
the
strong
calcium
emission
at
the
neb
ular
spectrum,
fa
v
our
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
±
11
per
cent
of
early
flux
excesses
in
SNe
Ia
(Deckers
et
al.
2022
;
Burke
et
al.
2022a
,
b
),
posing
additional
challenges
on
their
interpretation.
Recently,
a
short-lived
flash
of
optical
emission
was
observed
for
two
o
v
erluminous
SNe
Ia,
2020hvf
(Jiang
et
al.
2021
)
and
2022ilv
(Sri
v
astav
et
al.
2023
).
F
or
SN
2020hvf,
the
flux
e
xcess
was
observ
ed
during
the
high
cadence
Tomo-e
Gozen
transient
surv
e
y,
using
the
camera’s
clear
filter,
and
lasted
for
∼
1
d,
while
for
SN
2022ilv,
obser-
vations
in
the
ATLAS
o
band
showed
a
similar
early
time
behaviour.
The
authors
modelled
the
rising
light
curves,
and
fa
v
our
interaction
of
the
ejecta
with
a
CSM
mass
of
∼
10
−
2
–10
−
3
M
at
an
outer
edge
radius
of
∼
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,
disco
v
ered
∼
hours
after
explosion,
classified
two
weeks
before
maximum
brightness
and
densely
monitored
with
ground-
and
space-based
facilities.
Our
∼
10
months
of
multi-wavelength
photometric
and
spectroscopic
co
v
erage
makes
SN
2021zn
y
one
of
the
most
well-observed
03fg-like
SNe
Ia,
for
which
we
identify
two
striking
features.
First,
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
+
313d
late-time
spectrum.
We
present
the
disco
v
ery
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
Sec-
tion
3
.
We
discuss
our
findings
in
the
context
of
the
proposed
progen-
itor
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)
o
v
erluminous
SNe
Ia.
Moreo
v
er,
ev
ery
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
disco
v
ery
of
SN
2021zn
y,
its
classifica-
tion
and
our
photometric
and
spectroscopic
follow-up
observations.
2.1
Disco
v
ery
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
surv
e
y
name
ZTF21acdmwae,
and
reported
on
the
Transient
Name
Figure
1.
Pan-STARRS
5
arcmin
×
5
arcmin
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
arcsec
×
80
arcsec)
region,
centred
on
the
SN
location,
taken
at
a
phase
of
−
3.4
d
from
B
-band
maximum
with
LCO
g
band.
Server
(TNS
1
)
on
UT
2021
September
26.51
(Fremling
2021
),
with
a
disco
v
ery
magnitude
of
r
=
19.33
mag.
F
orced
photometry
on
images
taken
by
ZTF
prior
to
disco
v
ery
rev
ealed
that
the
SN
was
also
present
in
previous
epochs,
with
our
first
detection
being
on
UT
2021
September
19.50
(MJD
=
59476.50)
at
g
=
20.35
±
0.17
mag
(with
non-detections
in
r
and
i
down
to
20.94
and
20.50
mag,
respectively).
Our
last
non-detection
in
both
g
and
r
bands
was
on
UT
2021
September
17.4
at
>
21.85
and
21.42
mag,
respectively.
The
host
of
SN
2021zny
is
CGCG
438-018,
an
edge-on
(star-
forming)
galaxy,
with
the
SN
located
at
α
=
02
h
03
m
35
.
s
800,
δ
=
+
15
◦
44
33
.
36
(J2000.0),
18.90
arcsec
west
and
14.93
arcsec
south
of
its
host
galaxy’s
core,
along
its
dust
lane.
We
present
a
deep
pre-e
xplosion
P
an-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
d
from
B
-band
maximum.
SN
2021zny
was
classified
as
a
young
(
∼
8
d
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
2
d
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
∼
6300
Å.
Such
a
feature
can
be
attributed
to
C
II
λ
6580
at
a
similar
velocity
to
one
of
the
most
characteristic
broad,
Si
II
λ
6355
absorption
line
centered
near
∼
6150
Å.
This
classification,
alongside
our
extremely
early
detection
of
SN
2021zny
led
us
to
initiate
an
e
xtensiv
e
follow-up
campaign.
1
https://
www.wis-tns.org/
Downloaded from https://academic.oup.com/mnras/article/521/1/1162/7059222 by California Institute of Technology user on 19 July 2023