# Resolving the explosion of supernova 2023ixf in Messier 101 within its
complex circumstellar environment

E. A. Zimmerman  1,29∗ I. Irani  1,29∗ P. Chen  1 A. Gal-Yam  1 S. Schulze  2
D. A. Perley  3 J. Sollerman  4 A. V. Filippenko  5 T. Shenar  6 O. Yaron  1
S. Shahaf  1 R. J. Bruch  7,1 E. O. Ofek  1 A. De Cia  8,9 T. G. Brink  5 Y.
Yang  5 S. S. Vasylyev  5 S. Ben Ami  1 M. Aubert  10 A. Badash  1 J. S. Bloom
5 P. J. Brown  11 K. De  12,13 G. Dimitriadis  14 C. Fransson  4 C. Fremling
15,16 K. Hinds  3 A. Horesh  17 J. P. Johansson  2 M. M. Kasliwal  16 S. R.
Kulkarni  16 D. Kushnir  1 C. Martin  18 M. Matuzewski  18 R. C. McGurk  19 A.
A. Miller  20,21 J. Morag  1 J. D. Neil  16 P. E. Nugent  22,5 R. S. Post  23
N. Z. Prusinski  18 Y. Qin  16 A. Raichoor  22,5 R. Riddle  15 M. Rowe  11 B.
Rusholme  24 I. Sfaradi  17 K. M. Sjoberg  25,26 M. Soumagnac  27,22 R. D.
Stein  16 N. L. Strotjohann  1 J. H. Terwel  14,26 T. Wasserman  1 J. Wise  3
A. Wold  24 L. Yan  15 K. Zhang  5,28

###### Abstract

Observing a supernova explosion shortly after it occurs can reveal important
information about the physics of stellar explosions and the nature of the
progenitor stars of supernovae (SNe)[1]. When a star with a well-defined edge
explodes in vacuum, the first photons to escape from its surface appear as a
brief shock-breakout flare. The duration of this flare can extend to at most a
few hours[1, 2] even for nonspherical breakouts from supergiant stars[3, 4],
after which the explosion ejecta should expand and cool. Alternatively, for
stars exploding within a distribution of sufficiently dense optically thick
circumstellar material, the first photons escape from the material beyond the
stellar edge, and the duration of the initial flare can extend to several
days, during which the escaping emission indicates photospheric heating[5].
The difficulty in detecting SN explosions promptly after the event has so far
limited data regarding supergiant stellar explosions mostly to serendipitous
observations[2, 6] that, owing to the lack of ultraviolet (UV) data, were
unable to determine whether the early emission is heating or cooling, and
hence the nature of the early explosion event. Here, we report observations of
SN 2023ixf in the nearby galaxy Messier 101, covering the early days of the
event. Using UV spectroscopy from the Hubble Space Telescope (HST) as well as
a comprehensive set of additional multiwavelength observations, we trace the
photometric and spectroscopic evolution of the event and are able to
temporally resolve the emergence and evolution of the SN emission. We derive a
reliable bolometric light curve of the event and show that it indicates an
initially rising temperature and luminosity as the supernova explosion shock
breaks out from a dense layer of material surrounding the star with a radius
$R_{0}\approx 1.9\times 10^{14}\,$cm, significantly larger than typical
supergiants. Our HST spectra reveal the dynamics of the shock and the
expanding emitting material, as well as a rich spectrum of absorption lines
from heavy elements mixed into the gas surrounding the exploding star. Our
data uniquely provide a temporally-resolved description of a massive-star
explosion, characterise the pre-explosion evolution of the doomed star, and
demonstrate a method to directly measure the pre-explosion chemical
composition of supernova progenitors.

Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, 76100 Rehovot, Israel

The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova,
SE-106 91 Stockholm, Sweden

Astrophysics Research Institute, Liverpool John Moores University, IC2,
Liverpool Science Park, 146 Browlow Hill, Liverpool L3 5RF, UK

The Oskar Klein Centre, Department of Astronomy, Stockholm University,
AlbaNova, SE-106 91 Stockholm, Sweden

Department of Astronomy, University of California, Berkeley, CA 94720-3411,
USA

Departamento de Astrof’isica, Centro de Astrobiolog’ia (CSIC-INTA), Ctra.
Torrej’on a Ajalvir km 4, 28850 Torrej’on de Ardoz, Spain

The School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978,
Israel

European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei
München, Germany

Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix,
Switzerland

Université Clermont Auvergne, CNRS/IN2P3, LPC, F-63000 Clermont-Ferrand,
France

Department of Physics and Astronomy, Texas A&M University, 4242 TAMU, College
Station, TX 77843, USA

MIT-Kavli Institute for Astrophysics and Space Research, 77 Massachusetts
Ave., Cambridge, MA 02139, USA

NASA Einstein Fellow

School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2,
Ireland

Caltech Optical Observatories, California Institute of Technology, Pasadena,
CA 91125, USA

Division of Physics, Mathematics and Astronomy, California Institute of
Technology, Pasadena, CA 91125, USA

Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem
91904, Israel

Cahill Center for Astronomy and Astrophysics, California Institute of
Technology, 1200 E. California Blvd, MC 249-17, Pasadena, CA 91125, USA

W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI 96743, USA

Department of Physics and Astronomy, Northwestern University, 2145 Sheridan
Rd, Evanston, IL 60208, USA

Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA),
Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA

Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720,
USA

Post Observatory, Lexington, MA 02421, USA

IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena,
CA 91125, USA

Department of Astronomy, Harvard University, Cambridge, MA 02138, USA

Isaac Newton Group (ING), Apt. de correos 321, E-38700, Santa Cruz de La
Palma, Canary Islands, Spain

Department of Physics, Bar-Ilan University Ramat-Gan 52900, Israel

Department of Astronomy & Astrophysics, University of California, San Diego,
La Jolla, CA, USA

These authors contributed equally: E. A. Zimmerman, I. Irani

On 2023, May 19, SN 2023ixf was discovered in the nearby (distance
$6.9\rm~{}Mpc$[7]) galaxy Messier 101 by K. Itagaki[8] at 17:27:15 (UTC dates
are used throughout this paper) and was reported to the Transient Name Server
(TNS; https://www.wis-tns.org/object/2023ixf) at 21:42:21. The new object is
located at right ascension $\alpha=14^{h}03^{m}38^{s}.56$ and declination
$\delta=54^{\circ}18^{\prime}41^{\prime\prime}.94$ (J2000), in the outskirts
of its host (Methods §1). Upon receiving this report, we rapidly obtained a
classification spectrum[9] at 22:23:45. The spectrum showed narrow emission
lines often seen[10] in early-time spectra of Type II SNe, known as flash
features[11, 12], including[9] H, He I, He II, N III, N IV, C III, and C IV.
We initiated a multiwavelength follow-up campaign for this event using the
methodology of ref[13]. Details about the collected observations are provided
in Methods $\S 2-\S 4$. In particular, we triggered UV spectroscopic
observations using our HST target-of-opportunity (ToO) program that proved
critical to reliably track the early UV evolution of this object. Additional
studies of this object have been promptly conducted and presented; a summary
of these studies is provided in Methods $\S 5$.

#### The extended shock-breakout phase:

As our HST spectra illustrate (Fig. 1), the emission from the SN peaked during
the first few days in the far-UV. The proximity of this event led to
saturation of the UV photometers onboard the Neil Gehrels Swift Observatory,
hindering standard photometric measurements. Anchoring our continuous optical-
UV HST spectra to unsaturated visible-light photometry, we were able to
extract reliable synthetic UV photometry and used it to independently confirm
our custom analysis of saturated Swift observations (Methods $\S 2$), and
provide reliable UV coverage of the rise and fall of the SN light. Using the
critical UV data, combined with visible-light and infrared (IR) observations
(Methods $\S 2$), we calculate a bolometric light curve (Methods $\S 2$) that
is shown in Fig. 2 along with the derived blackbody temperature and
photospheric radius.

Following the core collapse of a massive star, a shock wave propagates at
velocity $v_{\rm s}$ from the centre of the star outward, heating the material
it goes through. As the shock front reaches an area where the optical depth
$\tau$ of the material above the shock is low enough ($\tau<c/v_{\rm s}$), the
first photons escape the system, informing distant observers of the onset of
an SN explosion, and the shock dissipates[1]. For stars with a well-defined
outer radius $R_{*}$, the duration of this shock-breakout burst[1] is
$R_{*}/c$, which for supergiant stars ($R_{*}>10^{13}$ cm) is at most a few
hours, after which the shocked material expands and cools. During most of this
shock-cooling phase, the bolometric luminosity is approximately constant, the
emitting material expands, and hence its temperature decreases[4].
Alternatively, if the exploding star is engulfed by optically-thick
circumstellar material (CSM), the shock breakout occurs at a much larger
radius $r_{0}$ and the shock-breakout phase extends for a few days[5]. During
that period, photons from increasingly deeper layers escape, leading to a rise
in luminosity. The emitting radius is $r_{\rm s}=r_{0}+v_{\rm s}t$; for
typical values of $v_{\rm s}\approx 10,000\rm~{}km~{}s^{-1}$, initially
$r_{0}>v_{\rm s}t$, so $r_{\rm s}$ rises slowly. The temperature is therefore
expected to rise. In this manner, early bolometric observations of SNe can
differentiate between stars exploding in low-density environments, which
expand and cool, and those exploding within a thick CSM, which are expected to
initially get brighter and hotter, with an approximately constant emitting
radius.

Inspecting Fig. 2, we find that during the first three days of the SN
evolution, we measure an almost constant photospheric radius, with a mean
value of $(1.9\pm 0.2)\times 10^{14}$ cm. During this phase, the temperature
rises from $13,000$ K to $\sim 32,000$ K at peak, and the blackbody luminosity
rises from $5.2\times 10^{42}$ $\rm~{}erg~{}s^{-1}$ to $3.9\times
10^{43}\,\rm~{}erg~{}s^{-1}$. For an extended shock breakout, the rise time is
expected to be[5] $t_{\rm rise}=r_{\rm s}/v_{\rm s}$. Adopting the constant-
radius value we measure as the shock-breakout radius $r_{\rm s}=(1.9\pm
0.2)\times 10^{14}$ cm, and approximating the shock velocity $v_{\rm s}$ by
our measured photospheric velocity ($v_{\rm s}\approx v_{\rm
phot}=8000\rm~{}km~{}s^{-1}$; Methods $\S 3$), we find $t_{\rm rise}\approx
2.75\pm 0.3$ d, in excellent agreement with the data, suggesting that the
radiation-mediated explosion shock is indeed breaking out from a dense CSM
distribution[5].

The total bolometric luminosity at breakout can be estimated as the deposition
of kinetic energy into the shocked CSM ($L_{\rm bo}\approx Mv_{\rm
s}^{2}/t_{\rm rise})$, where we assume a shock velocity of
$8000\rm~{}km~{}s^{-1}\ $ and the rise time $t_{\rm rise}$ as the relevant
duration. We can therefore estimate the mass of the shocked CSM contained
within the breakout radius $r_{0}$ to be $M\approx L_{\rm bo}t_{\rm
rise}/v_{\rm s}^{2}\approx(7.4\pm 2)\times 10^{-3}~{}{\rm M}_{\odot}$, which
suggests a characteristic density of $\rho\approx 5\times
10^{-13}\rm~{}g~{}cm^{-3}$. Pre-explosion studies of the progenitor (Methods
$\S 5$) indicate that its size was much smaller[14] than $r_{0}$ . Assuming an
expansion velocity of $\approx 30\rm~{}km~{}s^{-1}$ , derived from the
blueshifted center of the narrow emission lines (Methods $\S 3$), we find the
mass-loss rate responsible for the confined CSM to be $\dot{M}\approx 0.011\pm
0.005\,~{}{\rm M}_{\odot}\,\rm yr^{-1}$. The velocity and mass-loss rate also
match independent estimates[15]. Following the end of the extended shock
breakout from the dense inner CSM, the blackbody emission radius increases
rapidly, and the temperature and luminosity drop. The continued rise of the
light curves in visible light (Methods $\S 2$; Fig. Resolving the explosion of
supernova 2023ixf in Messier 101 within its complex circumstellar environment)
is due to the emission peak moving redward from the far-UV and the growing
radius.

#### Origin and evolution of the narrow emission lines:

As the shock breakout is extended, the radiation-mediated shock gradually
exits the thick CSM behind the breakout region, transitioning into a
collisionless shock in the CSM plasma above[16, 17]. This shock heats the CSM
it passes through, while the continuously escaping photons cool the CSM
electrons through inverse-Compton scattering. We show this cooling process to
be highly efficient (Methods $\S 6$), leading to a spectrum peaking in the
extreme UV (EUV; $\gtrsim 60$ eV) at radii of up to $7\times 10^{14}\,\rm cm$.
This EUV radiation is sufficient to produce the highly ionised (He II, N IV, C
IV) narrow flash features appearing in the early-time spectra of SN 2023ixf,
and the continuous source of ionising photons is actually required by the
total flux of lines from the ionised CSM that we see, as the integrated
luminosity of narrow-line components requires each atom to be ionised many
times (Methods §3). For instance, to emit the total energy of $(1.66\pm
0.04)\times 10^{45}\,\rm erg$ in H$\alpha$ between 1.38 and 6 days after
explosion would require $0.92\pm 0.02\,\rm M_{\odot}$ of hydrogen if only one
H$\alpha$ photon is emitted per atom, which is ruled out by our estimate of
the total CSM mass ($M<0.01~{}{\rm M}_{\odot}$).

During the initial extended shock breakout, the expanding shock in the CSM
increases the EUV flux, ionising the CSM further. This is observed through our
sequence of day $\sim 1$–2 spectra. We see an increase in species ionisation,
as N 3 $\lambda\lambda 4634,4641$, C 3 $\lambda\lambda 4647,4650,\lambda
5696$, and He I $\lambda 5876,\lambda 6678$ decrease in strength and
disappear, while higher ionisation N 4 $\lambda\lambda 7109,7123$, C 4
$\lambda\lambda 5801,5812$, and He 2 $\lambda 4686$ lines continue to increase
in flux (Fig. 2). At this early stage, all lines manifest two components, a
narrow core (full width at half-maximum intensity (FWHM) $\lesssim
100\rm~{}km~{}s^{-1}$ at day 2.2), and a broad Lorentzian-shaped base that
originates from electron scattering[18].

In the UV, our HST spectra (Fig. 1) reveal two lines with P Cygni profiles
developing in the near-UV (NUV). N 4$\lambda 1718$ with an absorption minimum
at $\sim 1500\rm~{}km~{}s^{-1}$ is apparent already in our first HST spectrum
on day 3.6, and C 3 $\lambda 2297$ appears on day 4.7 and rapidly develops an
absorption minimum that evolves to higher velocities of up to $\sim
1500\rm~{}km~{}s^{-1}$ by day 8.7.

Apart from H$\alpha$, the optical narrow lines disappear by day 5 (Methods $\S
3$) as the shock propagates out of the efficiently Compton-cooled region,
hardening the bremsstrahlung spectrum. The broad electron-scattering wings,
however, last until day 6, since they originate from scattered photons, which
can be delayed by up to the dynamical size of the shock transition layer
$R_{\rm bo}/v_{\rm bo}\approx 2.5$ days. The NUV N 4 $\lambda 1718$ line
disappears between days 5.5 and 8.7, consistent with the optical-line
timescale, while the C 3 $\lambda 2297$ line that is remarkably detected until
at least day 8.7. C 3 $\lambda 2297$ arises from a transition from an excited
state populated by the EUV C 3 $\lambda 412$ transition, which itself results
from the C 4 absorption of an electron into an excited C 3 state (a process
known as dielectronic recombination)[19]. Thus, the persistence of this C 3
$\lambda 2297$ transition reflects a parent population of excited C 4 ions,
lasting until at least day 8.7, likely still ionised by the hardening X-ray
flux (Methods $\S 8$). It would be very informative to model the exact
mechanism through which these highly-ionised species persist; however, such a
model is beyond the scope of this work.

#### Detection of a CSM density drop:

By day 4 after explosion, the collisionless shock continues to propagate into
further layers of CSM that are not efficiently Compton-cooled. The hardening
nonthermal spectrum from this lower-density CSM manifests in an escaping flux
of X-rays that are detected starting day 4 (Fig. 2). Additional evidence for a
sharp drop in density is provided by a decreasing neutral hydrogen column
density deduced from the NuSTAR X-ray observations[20], reducing X-ray
absorption. These column densities translate into a CSM density of $4\times
10^{-16}\rm~{}g~{}cm^{-3}$ at a distance of $2\times 10^{15}\,\rm cm$, about
three orders of magnitude below our density measurement of
$10^{-12}\rm~{}g~{}cm^{-3}$ at $2\times 10^{14}\,\rm cm$. Fig. 2 shows that
the X-ray flux remains approximately constant as the collisionless shock
continues to travel into the lower-density CSM. As the energy source for this
emission remains the deposition of kinetic energy into shocked CSM layers
($L\propto\dot{M}v_{\rm s}^{2}/t=2\pi r^{2}\rho v_{\rm s}^{3}$), the constant
nature of the X-ray luminosity requires (for a constant velocity) a density
profile falling as $\rho\propto r^{-2}$ (Methods $\S 4$). Assuming this
density profile, the density extrapolated back to the shock-breakout radius
would be $\rho\approx 10^{-14}\rm~{}g~{}cm^{-3}$. This density is 1.5 orders
of magnitude smaller than the density inferred above at the shock-breakout
region. We present the full mapping of the CSM structure in Fig. 3, inferred
from the different messengers described above.

#### Detection and implication of radiative acceleration:

During their escape, photons interact with the CSM through radiative
acceleration, dominated by the bound-free and bound-bound cross-sections
(Methods §7). We observe this acceleration as an increase in the FWHM of the
H$\alpha$ narrow component from $\lesssim 100\rm~{}km~{}s^{-1}$ initially to
$\sim 2000\rm~{}km~{}s^{-1}$ on day 11, and in the acceleration of He 2 to
$\sim 900\rm~{}km~{}s^{-1}$ on day 4.1, both of which are consistent with the
report by ref[21].

The last narrow CSM spectral component surviving until day 15 is the H$\alpha$
P Cygni profile that can be excited thermally and does not require either EUV
or X-ray radiation to form. This feature persists as regular broad Type II SN
photospheric features emerge in the SN spectrum, eventually disappearing as
the outer CSM is swept up by the ejecta on day 15, at a distance of $\sim
10^{15}\,\rm cm$, assuming an ejecta velocity of $\sim
10,000\rm~{}km~{}s^{-1}$ inferred from the developing photospheric H$\alpha$ P
Cygni profile. This distance is consistent with calculations of the radiative
acceleration of hydrogen in the CSM (Methods $\S 7$). Assuming standard red
supergiant (RSG) wind velocities (10–20$\rm~{}km~{}s^{-1}$), this outer layer
of CSM was expelled by the progenitor star a few decades before explosion.

Radiative acceleration explains the measurement of high-velocity ($\gtrsim
1000\rm~{}km~{}s^{-1}$) flash P Cygni profiles in H$\alpha$, the acceleration
in the He 2 and the NUV N 4, and C 3 lines without requiring a recent stellar
eruption to accelerate material to these velocities prior to the SN. This
reconciles the SN observations with the nondetection of recent precursor
eruptions from the progenitor[14]. The strong acceleration observed here can
only be achieved if the wind is optically thin, and provides additional
support for the drop in density following breakout.

#### Chemical composition:

The initial chemical composition of an exploding star has a strong impact on
the properties of the resulting explosion. However, as all modern well-studied
supernovae (SNe) occurred in external galaxies where properties of individual
stars are very challenging to measure, direct observations of the chemical
composition of SN progenitors do not exist. Instead, statistical studies have
used the integrated composition of the entire host galaxy, or of nearby star-
forming regions, as proxies for the properties of the actual exploding stars.
Such studies are hindered by chemical mixing within galaxies[22].

The NUV spectra obtained of SN 2023ixf reveal a plethora of narrow absorption
lines from iron-group elements, commonly found in the interstellar material
(ISM) of galaxies, including Fe 2, Mg 2, Mn 2, Zn 2, and Fe 3 (see
Supplementary Table 1). These lines are seen in addition to commonly observed
optical ISM lines, such as the Na I D and Ca II H&K doublets. While the UV
lines are blended with the expected Milky Way (MW) absorption, their centre
lies at the redshift of M101, and there is no significant MW Na I D absorption
in our high-resolution optical spectra that spectrally resolve the velocity
difference between M101 and the MW, suggesting that they originate mostly from
gas in M101.

The UV lines show no temporal development (change in equivalent width) during
our observations. However, monitoring of the Na I D doublet with telluric-
corrected high-resolution spectra (Methods $\S 8$) reveals that the Na I D
doublet is separated into two components – a strong central line spanning $0$
to $-10\rm~{}km~{}s^{-1}$ and a weak blueshifted absorption component at
$-30\rm~{}km~{}s^{-1}$ (Methods $\S 8$), a velocity shift similar to that of
the SN narrow ($\sim 100\rm~{}km~{}s^{-1}$) flash features. This component
shows a decrease in equivalent width starting on day $43$ after explosion,
suggesting that this component is influenced by the SN and hence originates
from the immediate CSM of the progenitor star. The SN lies in a relatively
isolated location, away from detected H 2 or star-forming regions (Method $\S
1$) that could account for strong ISM absorption, supporting a CSM origin for
the detected lines.

We probe the column density and line-of-sight metallicity by applying the
curve-of-growth technique[23] to the observed lines. The column-density
measurements are presented in Supplementary Table 1 (Methods $\S 8$). The
ratio of different metals in the CSM likely reflects the composition of the
progenitor prior to explosion. To calculate standard metallicity $Z$ (i.e.,
ratio of elements to hydrogen), a hydrogen column density is needed.
Unfortunately, our UV spectra do not extend far enough into the UV to cover
the Ly$\alpha$ line, which would have allowed us to derive this quantity.
However, using an empirical correlation[27] between the Zn/Fe ratio and $Z$,
We estimate that the natal metallicity of the progenitor of SN 2023ixf,
reflected in the CSM around it, was $Z=0.95$ solar.

As a sanity check, we use integral-field-unit Keck Cosmic-Web Imager
(KCWI)[24] observations of the nearest star-forming region (80 pc west of the
SN location), and a vigorously star-forming region 310 pc north of the SN. We
measure the most prominent emission lines ([O 3] $\lambda\lambda$4959, 5007,
H$\alpha$, H$\beta$, and [N 2] $\lambda$6584) for both regions (Methods $\S
8$), and infer the metallicity of these regions using the O3N2 and R3 strong-
line metallicity indicators. We find metallicities of $Z=1.00\pm 0.06$ and
$Z=0.68\pm 0.01$ solar for the region to the west and north of the SN
explosion site, respectively. While a moderate metallicity gradient is present
across this small region of the host galaxy, the range of metallicity provides
an indirect indicator of the metallicity for the progenitor of SN 2023ixf. A
solar to slightly sub-solar metallicity agrees with the inference from the UV
absorption line analysis above.

Future HST observations, in particular including coverage of the far-UV, could
further improve on the measurements we present here. Routine early detection
of events such as SN 2023ixf by the forthcoming ULTRASAT space mission[25] and
follow-up UV spectroscopy using the proposed UVEX mission[26] could provide in
the near future direct measurements of the chemical composition of SN
progenitors prior to the explosion .

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  * [27] De Cia, A., et al.  Dust-depletion sequences in damped Lyman-$\alpha$ absorbers. A unified picture from low-metallicity systems to the Galaxy.  _Astronomy and Astrophysics_ 596,A97 (2016)

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Figure 1: Hubble Space Telescope UV spectra trace evolving shock-ionised flash
lines and reveal metal absorption lines that probe the progenitor metallicity
of SN 2023ixf. All 5 epochs of coadded UV-optical spectra are shown in log
space to enhance the visibility of narrow features across the entire spectrum.
Prominent flash features and UV absorption lines are marked, notably C 3
$\lambda 2297$ and N 4 $\lambda 1718$.

---
Figure 2:  Bolometric evolution of SN 2023ixf (top three panels, see titles)
reveals the heating light curve expected from the extended breakout flare of
an explosion embedded within thick CSM. A rising X-ray luminosity (fourth
panel) combined with high-ionisation line flux decrease (traced by He II here,
last panel) trace the hardening of the SN SED as the explosion shock
progresses into thinner CSM.

---
Figure 3: A full mapping of the CSM structure. The stellar envelope inferred
from pre-explosion images extends[14] to $\sim 5\times 10^{13}\,\rm cm$, above
which thick ($\rho\approx 10^{-12}\rm~{}g~{}cm^{-3}$) and confined ($r\lesssim
2\times 10^{14}\,\rm cm$) CSM extends the shock breakout. The X-ray column-
density measurements then suggest a drop in density around the shock-breakout
region. A collisionless shock forms within the extended wind (after the drop
in density near the shock-breakout radius), heating the CSM. At confined radii
($R\lesssim 7\times 10^{14}\,\rm cm$), this heating is suppressed by Compton
scattering of the SN-emitted UV-optical photons, resulting in an EUV-dominated
spectrum. However, at more distant radii, Compton cooling is no longer
efficient, resulting in hard X-ray bremsstrahlung emission, followed by radio
synchrotron emission from the heated electrons.

###  0.1 1\. The environment of SN 2023ixf within its host:

The SN 2023ixf explosion site is located 9 kpc from the centre of M101
(redshift $z=0.00084$)[1] on the far side of an outer spiral arm (Extended
Data Figure Resolving the explosion of supernova 2023ixf in Messier 101 within
its complex circumstellar environment). This region of the host galaxy is a
site of ongoing star formation, with islands of enhanced star formation and
diffusely distributed lower-level star-formation activity extending to the SN
site (Extended Data Figure Resolving the explosion of supernova 2023ixf in
Messier 101 within its complex circumstellar environmentc). The most nearby
well-defined star-forming region lies only 80 pc west of the SN explosion
site. Another vigorously star-forming region lies about 310 pc north of the SN
explosion site. In $\S 8$, we show that these star-forming regions close to SN
2023ixf are of sub-solar and solar metalicities.

###  0.2 2\. Photometry:

All phases in this manuscript refer to ${\rm JD}=2,460,083.3$ as the SN
explosion time, as inferred from early analysis of amateur-astronomer pre-
explosion observations of M101[2].

We obtained a full multiband light curve of SN 2023ixf, covering the early
rise, plateau, and radioactive decline. We present the full light curve in
Extended Data Figure Resolving the explosion of supernova 2023ixf in Messier
101 within its complex circumstellar environment. The data will be available
from the WISeREP database and in a digital format as supplementary material.
Standard reduction procedures are described in the supplementary online
methods, as are our methods for reduction of saturated Swift photometry using
the methods of ref[3].

####  56Ni mass and ejected mass estimate:

By $t=90$ days, the light curve has fully settled onto the radioactive 56Co
tail (Extended Data Figure Resolving the explosion of supernova 2023ixf in
Messier 101 within its complex circumstellar environment). The 56Ni mass can
be directly estimated from the bolometric luminosity (e.g., refs[4, 5]), which
is equal to the energy deposition from 56Co decay, assuming full trapping of
$\gamma$-ray photons as is typical for SNe II at this phase[6]. We calculate
bolometric corrections at late times using spectra of SN 2017ahn[7], which
shows similar persistent narrow features[8] and peak luminosity, with a
slightly shorter plateau duration. We stitch together nebular spectra of this
event obtained after the fall from the plateau in the optical ($t=72$ days)
and IR ($t=65$ days) to match each other. We then calculate the bolometric
correction to the $uBgriz$ photometry from the combined spectrum, extended
assuming a blackbody extrapolation outside the observed band. This estimate is
within 15% of the bolometric luminosity calculated directly using a blackbody
extrapolation to the SN 2023ixf $uBgriz$ photometry. We find that the
bolometric light curve of SN 2023ixf is well described by the energy
deposition from a 56Ni mass of $M_{{}^{56}\rm Ni}=0.071\pm 0.005\,{\rm
M}_{\odot}$. A 5% systematic error is included to account for the scatter in
the IR contribution ($>9000$ Å) to the bolometric flux at $t=90$ days[9]. Our
estimate places SN 2023ixf at the upper end of the Type II 56Ni mass
distribution[5]. We show the full bolometric light curve, bolometric
correction, and 56Ni deposition in Supplementary Figure 2.

Using the bolometric luminosity and the deposition from 56Ni, we calculate the
time-weighted integrated bolometric luminosity $ET=\int Lt\,dt-\int
Q_{{}^{56}{\rm Ni}}t\,dt$, where $Q_{{}^{56}{\rm Ni}}$ is the deposition from
56Ni. Note that $ET=0.1\,M_{\rm ej}R_{*}v_{\rm ej}$ is an observable that is
directly related to the ejected mass $M_{\rm ej}$, progenitor radius $R_{*}$,
and typical velocity $v_{\rm ej}$; it is dependent only on the hydrodynamical
profiles and is insensitive to the exact details of radiation transport and
CSM mass[10, 11]. We find $ET=(3.3\pm 0.2)\times 10^{55}\,\rm{erg\,s}$. We
estimate[11] $v_{\rm ej}=v_{\lambda 5169,50\,{\rm d}}=3400\pm
200\rm~{}km~{}s^{-1}$ and $v_{\rm env}=8000\rm~{}km~{}s^{-1}$, and we use the
four estimates existing for the progenitor radius from pre-explosion data[12,
14, 15]. Here we derived the radius from the effective temperature and
luminosity of the progenitor using $R=[L/(4\pi\sigma T_{\rm eff}^{4})]^{1/2}$.
We obtain a combined constraint on the ejected mass of $7.5\pm 2.5~{}{\rm
M}_{\odot}$ and a stellar envelope mass of $M_{\rm env}=0.5\pm 0.2~{}{\rm
M}_{\odot}$. The measurements of ref[13] are omitted, as they are both an
outlier and lead to an estimate of $M_{\rm ej}=13.6\pm 1.9~{}{\rm M}_{\odot}$,
which is inconsistent with their implied initial mass of $11~{}{\rm
M}_{\odot}$. To reconcile the initial mass implied by other works of $\sim
17~{}{\rm M}_{\odot}$ with our ejected-mass estimate, the progenitor needed to
either lose a large fraction of its mass before explosion, or leave a black-
hole remnant. The former option requires that the mass-loss rate of $\sim
10^{-4}~{}{\rm M}_{\odot}\ \rm yr^{-1}$ inferred from the CSM extended wind
would persist for $10^{4}$–$10^{5}\ \rm yr$, most of the red-supergiant phase
of the progenitor star.

###  0.3 3\. Spectroscopy:

We obtained a total of 112 spectra of SN 2023ixf, taken with the instruments
and configurations shown in Supplementary Table 2. The sequence of spectra in
different phases is shown in Extended Data Figures Resolving the explosion of
supernova 2023ixf in Messier 101 within its complex circumstellar environment
and Resolving the explosion of supernova 2023ixf in Messier 101 within its
complex circumstellar environment, and in Supplementary Figure 2. All
reduction notes are available as supplementary methods.

#### Hubble Space Telescope:

Immediately following the classification of SN 2023ixf as a Type II event
showing flash-ionisation features[9], less than 90 min from the SN discovery,
we initiated our disruptive HST program GO-17205 (PI: E. Zimmerman) as part of
the follow-up campaign. Rapid response and observation scheduling resulted in
the first epoch of HST spectroscopy being obtained less than 50 hr after the
SN discovery, providing the first-ever NUV spectrum of a Type II SN during the
flash-feature phase. Furthermore, as photometry from Swift saturated after
$t=2$ days, the HST spectra were the only reliable source of UV information by
providing synthetic UV photometry. This synthetic photometry validated the
readout-streak Swift photometry reduction and, thus, was the basis of the
entire SN UV light curve.

Five visits of SN 2023ixf were carried out as planned, covering a total of 22
HST orbits. All visits in the program were weighted toward the NUV, obtaining
several orbits with the G230LB grism, but also included coverage of the
optical part of the spectrum with single-orbit exposures using the G430L and
G750L grisms. A full list of the program HST visits and exposures is shown in
Supplementary Table 3. Our sequence of HST spectra is presented in Fig. 1,
showing a coadded spectrum for each of our five HST program visits. Reduction
notes for the HST spectra are presented in the supplementary methods.

A strong N 4 $\lambda 1718$ P Cygni profile persists throughout the first
three visits (days 3.5–5.5 after explosion), reaching very high velocities of
$>2500\rm~{}km~{}s^{-1}$ (Extended Data Figure Resolving the explosion of
supernova 2023ixf in Messier 101 within its complex circumstellar
environment). This phase is roughly consistent with the peak flux in the
strong flash features (Balmer lines and He 2 $\lambda 4686$), providing the
first observation of flash features in the NUV. Starting from the second visit
(+4.63 days), a C 3 $\lambda 2297$ P Cygni profile starts to develop, which
persists on top of the broad photospheric features that emerge in visit 4
(+8.5 days). During this visit, the optical spectrum exhibits a “blue
continuum,” while the UV spectrum develops photospheric features, implying
that the continuum originates by day 8.5 from the SN ejecta rather than from
the CSM. The broad photospheric features are enhanced through visit 5 (+11.5
days), during which no flash features appear in the UV spectra.

Notable in their absence in the UV spectra are He 2 $n\rightarrow 3$ series
lines, clearly seen in the early-time optical spectra ($4\rightarrow
3\,\,\lambda 4686$). This is likely due to insufficient sensitivity to detect
these weaker lines, as evident from the marginal detection of the
$5\rightarrow 3\,\lambda 3203$ line in visits 1 and 2. Another notably missing
feature in the UV spectra is the predicted Fe 4 absorption-line forest at
1600–1800 Å which appeared in previously published model CSM UV spectra[16].
To assess whether this absence is caused by a low Fe abundance or the CSM
temperature regime, we calculate non-LTE (local thermodynamic equilibrium)
stellar models using the Potsdam Wolf-Rayet (PoWR) code, shown in $\S 8$, that
indicate that these lines are likely missing owing to the CSM temperatures.

Along with the strong N 4 and C 3 emission lines in the UV spectra, we find
prominent line-of-sight absorption lines of Fe ii and Mg ii, as well as weaker
Al 3, Zn 2, Mn 2, and Cr 2 (shown in a zoom-in in Fig. 1). These are known ISM
lines[17] and are accompanied by absorption lines of Ca and Na seen in high-
resolution optical spectra from HARPS-N. These absorption features also appear
in spectra of other Type II SNe (see Extended Data Figure Resolving the
explosion of supernova 2023ixf in Messier 101 within its complex circumstellar
environment), but they appear stronger in SN 2023ixf.

In Extended Data Figure Resolving the explosion of supernova 2023ixf in
Messier 101 within its complex circumstellar environment, we compare the
photospheric features of SN 2023ixf to other early-time UV spectra of Type II
SNe. The UV photospheric features of SN 2023ixf show a resemblance to those
appearing in the early UV spectra of SN 2022wsp[18], SN 2021yja[19], and SN
2022acko[20], suggesting some uniformity among Type II-P SNe. On the other
hand, the photospheric features of SN 2023ixf develop later than in SN
2022acko, for which we have early-time NUV spectra, and lack a strong emission
feature at $\sim 1910$ Å appearing in the early NUV spectra of SN 1999em[21]
and SN 2005ay[22]. To assess the diversity of Type II SN NUV photospheric
spectra, a larger sample of early-time NUV spectra must be collected. As some
uniformity is observed in at least four recent SNe, these photospheric
features must arise from the progenitor natal chemical composition and not
from species synthesised during the SN explosion itself, whose parameters
differ among explosions. It would thus be very informative to probe the
progenitor chemical composition by modeling these early-time NUV spectra.

#### Narrow-line velocities:

Using our sequence of high-resolution ($R=45,000$) NOT/FIES spectra, we
measure the velocities of the fully-resolved strongest narrow flash features,
namely H$\alpha$, H$\beta$, He 2 $\lambda 4686$, C 4 $\lambda\lambda
5801,5812$, and N 4 $\lambda\lambda 7109,7123$. For the Balmer lines, we also
measure the corresponding He 2 $n\rightarrow 4$ transition velocity. We
present our velocity measurements in Supplementary Table 4.

Depending on the epoch, we fit different phenomenological models to the narrow
features in order to measure the velocity of different spectral components. To
remove the local continuum, we fit a third-degree polynomial to the continuum
(masking the line), and divide the spectrum by it. In Extended Data Figure
Resolving the explosion of supernova 2023ixf in Messier 101 within its complex
circumstellar environment, we present our fit for the $t=2.23$ days spectrum,
for which we fit a model consisting of a Lorentzian base (representing the
electron-scattering wing) with Gaussian components representing the narrow
lines themselves. For He 2 $\lambda 4686$ we also add a blueshifted Lorentzian
base, improving our overall fit to this line. We interpret this blue excess as
originating from the scattering caused by the (by this epoch) ionised N 3
$\lambda 4640$, which we have shown can be delayed by up to $t_{\rm bo}$ in
evolution. Overall, we find all narrow lines to be consistent with velocities
of $\sim 100\rm~{}km~{}s^{-1}$ at this epoch. All narrow lines are shifted
blueward by $\sim 30\rm~{}km~{}s^{-1}$, except for H$\alpha$, which may be
contaminated by M101 emission.

In Supplementary Figure 2, we present our fit for the $t=4.3$ and $t=5.11$
days spectra. We fit Lorentzian profiles to the flash features, as no clear
narrow components are visible (except for He 2 $\lambda 4686$ at $t=4.3$ d,
for which we add a narrow Gaussian component to the fit as well). Starting
from $t=8.13$ days, as no other narrow features appear in the optical spectra,
we only fit a model to H$\alpha$, showing a narrow P Cygni profile. We
represent this profile with a redshifted Lorentzian emission component
adjacent to a blueshifted Gaussian absorption component. To avoid introducing
a saddle mimicking the absorption component, we use a first-degree polynomial
to remove the continuum from these later spectra. These fits are presented in
Extended Data Figure Resolving the explosion of supernova 2023ixf in Messier
101 within its complex circumstellar environment. We define the P Cygni
velocity to be 3 times the standard deviation of the Gaussian absorption
component, as this corresponds to the absorption profile blue edge well in
cases where this edge is clear (see arrows in Extended Data Figure Resolving
the explosion of supernova 2023ixf in Messier 101 within its complex
circumstellar environment) and can also be measured when the blue-edge
measurement is hindered by uncertain continuum levels. A velocity of $v\approx
2000\rm~{}km~{}s^{-1}$ is inferred from the mean of all four epochs. All fits
were made using the lmfit[23] package.

#### Narrow-line fluxes:

In order to probe the strength of the narrow lines, we measure their flux
using the sequence of medium-resolution spectra. To do so, we fit a first-
degree polynomial to the continuum in the emission-line vicinity and remove it
from the spectrum. We define the borders of the nearby continuum by drawing
its edge from a uniform distribution. The emission flux is then obtained using
trapezoidal integration. We repeat this process 1000 times, drawing different
edges to the continuum, and take the mean value as the measurement and the
standard deviation as the measurement uncertainty.

The total emitted flux from He 2 and H$\alpha$ cannot be explained by a single
ionisation event, as it corresponds to the emission of one photon per atom by
$\sim 1\,\rm M_{\odot}$ of material. We present He 2 flux measurements in Fig.
2 and the other line fluxes in Supplementary Table 5.

#### Measuring distant CSM and ISM properties:

Utilising high-resolution ($R=115,000$) spectra obtained with the northern
High Accuracy Radial velocity Planet Searcher (HARPS-N), we observe the fully
resolved Ca 2 K ($\lambda 3934$) & H ($\lambda 3968$), as well as Na 1 D
($\lambda\lambda 5890,5896$), ISM absorption lines. We find that both Ca
features show a significant MW component, which would not be resolved in low-
resolution spectra. However, there are no significant Na D MW components.
Therefore, we conclude that some of the UV ISM lines observed with HST/STIS
could be blended with a MW component. However, note that all UV ISM features
are centred at the M101 redshift. Ca 2 is no longer detected after the $t=21$
days spectrum, as the SN light curve reddens, decreasing the signal at blue
wavelengths.

We continue to monitor the Na 1 D component until $t=85$ days. Both
absorptions in the doublet show four significant components centred at
blueshifts of $v\approx
0\rm~{}km~{}s^{-1},\,4\rm~{}km~{}s^{-1},\,8\rm~{}km~{}s^{-1},$ and a shallower
$30\rm~{}km~{}s^{-1}$ with respect to the M101 frame (see Extended Data Figure
Resolving the explosion of supernova 2023ixf in Messier 101 within its complex
circumstellar environment). At each epoch, we measure the EW of each
component. To do so, we remove the local continuum around the D1 and D2 lines
separately by dividing the spectrum with a first-degree polynomial fit
calculated after removing outliers. We then use a bootstrapping algorithm to
calculate the component EW, by applying different Gaussian noise to the
spectra with the linear fit standard deviation to each run. We present the
result of this calculation in Supplementary Table 6.

Tracking the Na 1 D doublets, we find that starting from $t=43$ days, both the
$-30\rm~{}km~{}s^{-1}$ and $-8\rm~{}km~{}s^{-1}$ components weaken in EW. A
change in Na EW has been shown to suggest the presence of CSM around Type Ia
SNe[24]. While in the case of SNe Ia, Na variability shows recombination of
ionised Na to neutral Na (i.e., an increase in the Na 1 D absorption), in the
case of SN 2023ixf we see the ionisation of neutral Na. Nonetheless,
ionisation of Na indicates that the changing Na components are in the vicinity
of the SN explosion since the number of ionising photons decreases with
distance squared. Further ionisation of extended CSM is but another indication
of the hard spectral energy distribution (SED) of SN 2023ixf, even at later
phases. As the SN is located away from its nearby star-forming region (Methods
$\S 1$), and the progenitor star experienced elevated mass loss for up to
$\sim 10^{4}$–$10^{5}$ yr (Methods $\S 2$), it is likely that most of the
strong ISM absorption lines originate from CSM around the progenitor star,
suggesting that the metal abundance we measure from these lines traces the
progenitor chemical composition (as shown in Methods $\S 8$).

To measure the EW of the UV absorption lines at each epoch, we remove the
local continuum by fitting a third-degree polynomial to the area adjacent to
the lines and fitting a Gaussian to each line. By fitting Gaussian profiles,
we resolve blended components, allowing us to measure each line in a blended
doublet individually. We used the lmfit package[23] to conduct the fits.
Lastly, we integrate over the best-fit model, producing an EW for each line.
An example fit is shown in Supplementary Figure 2. We use the EW of these
lines to probe the SN metal abundances as discussed in $\S 8$.

Unlike the optical Na 1 D, no significant change in the intensity of the UV
absorption lines can be measured. However, many of the weak lines fall below
the requisite signal-to-noise ratio (SNR) as the SN UV brightness fades, and
all lines are unresolved at later epochs so that any variable components would
be diluted by the strong saturated absorption.

###  0.4 4\. X-ray observations

The Swift satellite obtained multiple observations of SN 2023ixf at X-ray
energies with its X-ray telescope (XRT)[25] in photon-counting mode. We
present the early X-ray rise in Figure 2, while the full X-ray light curve is
shown in Extended Data Figure Resolving the explosion of supernova 2023ixf in
Messier 101 within its complex circumstellar environment. The individual
measurements are summarised in Supplementary Table 7. After an initial rise in
the X-rays, SN 2023ixf settles onto an X-ray plateau. As the luminosity of a
cooling forward shock is given by $L=2\pi r^{2}\rho v_{\rm sh}^{3}$, and the
shock velocity $v_{\rm sh}$ is only slowly decreasing (i.e., staying roughly
constant at early times), a constant X-ray light curve suggests a steady mass-
loss and a density profile of $\rho\propto r^{-2}$, where $r$ is the distance
from the progenitor star. Some variability is seen in the X-ray plateau,
suggestive of deviations from the $\rho\propto r^{-2}$ profile, which can
arise due to variable progenitor mass loss (see Methods $\S 5$ progenitor
properties). We note that at the time of writing SN 2023ixf is still X-ray
bright. Detailed reduction notes of the XRT data are shown as Supplementary
methods.

###  0.5 5\. Summary of additional studies of SN 2023ixf at the time of
writing:

Located in the nearby, well-known galaxy M101, SN 2023ixf attracted
significant attention from the community. In this section, we summarise the
main results published thus far.

#### Studies of the narrow lines:

Several papers have discussed the early-time optical flash features and their
temporal progression. Refs[15, 26] present early flash spectrocsopy of SN
2023ixf, and discuss the increase in the flash-feature ionisation (He 1, N 3,
C 3$\rightarrow$ He 2, N 4, C 4), unique to this event. Both studies also
compare the early spectra to non-LTE models, reaching a mass-loss-rate
estimate of the same order of magnitude as that inferred by us for the
confined CSM up to the shock-breakout radius of $\dot{M}\approx 2\times
10^{-2}~{}{\rm M}_{\odot}\rm~{}yr^{-1}$. Ref[15] also finds a density of
$\rho\approx 10^{-12}\rm~{}g~{}cm^{-3}$ at a radius of $r\approx 10^{14}\,{\rm
cm}$, consistent with our results. Both studies suggest that the CSM is
confined to $(0.5-1)\times 10^{15}\,{\rm cm}$. We find that low-density CSM
extends further to explain the persistent X-ray luminosity and narrow
spectroscopic features until at least day 8.7 (C 3 $\lambda 2297$) and day 15
(H$\alpha$). Ref[15] presents their best-fit model (r1w6b) from Ref[27]
extending to the UV. We illustrate a comparison of the NUV spectra we obtained
to this model in Supplementary Figure 2. Both studies also compare SN 2023ixf
to other SNe displaying flash features and note the similarity of SN 2023ixf
to SN 1998S[28, 29], SN 2017ahn[7], SN 2020pni[30], and SN 2020tlf[31], all of
which show long-lasting flash features compared to most other SNe[10]. We also
note that SN 2017ahn, SN 2020pni, and SN 2020tlf all exhibit a rise in their
bolometric luminosity, suggesting that they experience an extended shock
breakout in a wind, similar to SN 2023ixf.

Ref[21] studies the narrow lines in detail using high-resolution spectroscopy,
noting that the narrow lines exhibit broadening between days 2 and 4. Our
high-resolution velocity measurements from $t=2.3$ days are consistent with
this study, and the broadening of narrow lines is consistent with our results
on radiative acceleration of the CSM. We add the velocity measurements by this
study into Extended Data Figure Resolving the explosion of supernova 2023ixf
in Messier 101 within its complex circumstellar environment. Ref[21] also
discusses the possibility of the CSM being asymmetric, citing the difference
in blueshift between highly-ionised species and lack of narrow ($\sim
100\rm~{}km~{}s^{-1}$) P Cygni profiles. In our independent analysis, we also
observe a blueshift of the narrow lines in the $t=2.3$ day NOT/FIES spectrum
(see Extended Data Figure Resolving the explosion of supernova 2023ixf in
Messier 101 within its complex circumstellar environment and Supplementary
Table 4). We note, however, that UV spectroscopy reveals deep, narrow P Cygni
profiles, unlike the optical H$\alpha$ profile.

#### Spectropolarimetry:

Ref[32] conducted early-time spectropolarimetry of SN 2023ixf and observes a
high continuum polarization of $p_{\rm cont}\approx 1\%$ until day $2.5$,
dropping to $0.5\%$ on day $3.5$ after explosion. This timeline is consistent
with our extended shock-breakout time $R_{\rm bo}/v_{\rm s}\approx 2.5$ days.
As discussed in ref[32], as the shock moves within optically thick CSM, the
photosphere is expected to originate from a radius at which the optical
thickness $\tau\approx 2/3$. This is consistent with the roughly constant
radius obtained through our blackbody measurements, suggesting that at this
radius ($\sim 1.9\times 10^{14}\,{\rm cm}$), the CSM is aspherical. We note,
however, that this asymmetry is only measured in layers above the breakout
radius. The sudden change in polarization between days $2.5$ and $3.5$ is also
consistent with a change in the CSM structure, which coincides with the drop
in density we derive as the SN shock moves from the confined CSM into the
extended wind (with the measurement of first X-ray emission). Ref[32] also
observes a depolarization in the centre of the narrow He 2 $\lambda 4686$ and
H$\alpha$ emission, suggesting that they form above the early polarized
photosphere. This is consistent with our results, showing that many of the
narrow features form in the extended wind.

#### X-ray and other messengers:

As demonstrated through our CSM model, X-ray and (mostly lack of) radio
detections of SN 2023ixf have been critical messengers in the mapping of CSM
structure.

Ref[20] provided two epochs of X-ray observations with the Nuclear
Spectroscopic Telescope Array (NuSTAR). While both epochs are covered by the
Swift /XRT as well, NuStar provides several important insights unavailable
through XRT. Both epochs fit a bremsstrahlung spectrum, with a peak at $>25$
and $34^{+22}_{-12}\,{\rm keV}$, respectively, showing that the X-ray spectrum
is hard. The inferred NuSTAR unabsorbed bolometric X-ray luminosity in both
epochs of $L_{X}\approx 2.5\times 10^{40}\rm~{}erg~{}s^{-1}$ is consistent
with our XRT measurements, taking into account a bolometric correction.
Lastly, the decrease in soft X-ray absorption between the first NuStar epcoh
($\sim 4$ days) and the second epoch ($\sim 11$ days) traces the extended-wind
profile and is consistent with the drop in density we expect after the shock-
breakout region. We adopt the mass-loss rate and densities reported by this
study as the extended wind density.

Ref[33] reports limits on millimeter observations of SN 2023ixf, consistent
with the high mass-loss rate we infer for the confined CSM, but inconsistent
with the lower mass-loss rates we infer for the extended wind. Ref[20],
however, shows that lower mass-loss rates consistent with our extended-wind
model can also be allowed by the radio data.

#### Photometric evolution and models:

Ref[34] shows a fit to a recent shock-cooling model[4] for photometry starting
at $t=1$ day. As we measure the SN heating for the first 2.5 days, we find
this result to be inconsistent with our data. While shock cooling can describe
the unsaturated early-time Swift /UVOT photometry, it is in conflict with the
full SN light curve, including our HST data and Swift streak-photometry, which
shows a prolonged rise in all UV bands. Ref[34] also discusses a change in the
light-curve behaviour between the first day before discovery by Itagaki and
later photometry. This is consistent with our pre-discovery P48 $g$-band
photometry. Further discussion of the change in slope between very early times
and the +1 day light curve will be presented in a future paper[35] analysing
more amateur-astronomer photometry.

Ref[36] performs blackbody SED fits to the $UBVRI$ light curve (i.e.,
excluding the saturated Swift data), reaching the conclusion that SN 2023ixf
experiences heating during the first $\sim 5$ days after explosion. Ref[36]
therefore concludes that the SN goes through an extended shock breakout in a
wind, consistent with our results. However, owing to the inaccessibility of
the saturated UV light curve, the bolometric luminosity is only sampled in the
Rayleigh-Jeans regime, causing an overestimation of the luminosity rise time
(by $\sim 2.5$ days) and an underestimation of the SN heating (by $\sim
16,000\,{\rm K}$). Ref[36] therefore concludes that the confined CSM extends
much farther than our results [$(3-7)\times 10^{14}\,{\rm cm}$ compared to
$\sim 2\times 10^{14}\,{\rm cm}$].

Ref[37] conduct a multiband study of SN 2023ixf as well, presenting the Swift
grism NUV observations and three epochs of photospheric (starting from $+6.9$
days) FUV spectra using the UltraViolet Imaging Telescope (UVIT) onboard the
AstroSat satellite. Ref[37] also perform a light-curve study without the
saturated early-time UV data, inferring a more extended dense CSM (results
consistent with those of ref[36]).

#### Progenitor properties:

Several studies have pointed out a candidate progenitor star using pre-
explosion images[14, 12, 13, 14, 15], indicating that the progenitor candidate
was an RSG experiencing periodic ($\sim 1000$ days) mass-loss episodes.
Refs[12, 14, 15] also constrain the mass-loss parameters from the progenitor
IR variability and SED fits. This mass loss should reflect the extended wind,
as the confined CSM must have been ejected closer to the time of explosion
owing to its much larger density. The inferred CSM densities from these
studies are indeed consistent with the values we measure for the extended
wind. However, it is interesting to note that no eruptive IR source has been
identified for the confined CSM (below the breakout radius), as IR
observations were obtained a mere 10 days before the explosion by ref[12].
Ref[14] also conducted a study of nearby H 2 regions and found the metallicity
to be solar to slightly subsolar, consistent with our findings in Methods $\S
8$.

An interesting possibility from the variable nature of the progenitor mass
loss described in the above studies is the expectation for variability in the
X-ray light curve. This would arise due to the X-ray luminosity being a tracer
of the density, as the source of the emission is momentum transfer to the CSM
($L_{\rm X-ray}\propto\rho v^{3}_{\rm s}$). Such periodicity would be hard to
trace in detail as the light-travel time from the shock edge increases by
$R/c$, smearing any such effect. We note that some variability might exist in
the X-ray light curve, which shows dimming of the X-ray flux between 10 and 20
days after explosion, followed by a rebrightening (see Extended Data Fig.
Resolving the explosion of supernova 2023ixf in Messier 101 within its complex
circumstellar environment).

###  0.6 6\. Evidence for shock breakout in a wind:

#### Blackbody fits:

Starting at the first time when multiband photometry is available at JD =
2,460,084.42, we linearly interpolate our photometry onto a grid with uniform
steps in log time. For each grid point we fit the resulting SED with a
blackbody (BB) function to recover the blackbody bolometric luminosity,
radius, and temperature of SN 2023ixf. We also compare the optical-UV HST
spectra to a grid of BB temperatures and luminosities and calculate the best
fit and the corresponding uncertainties, assuming a likelihood
$\log{\mathcal{L}}\approx\chi^{2}$. Both methods are in good agreement. We
report our results in Table 8 and in Supplementary Figure 2. A full bolometric
light curve is shown in Supplementary Figure 2. During the first three days of
its evolution, the photospheric radius is approximately constant, with a mean
value of $(1.9\pm 0.2)\times 10^{14}$ cm. During this phase, the temperature
rises from $13,000$ K to $\sim 32,000$ K at peak, and the blackbody luminosity
rises from $5.19\times 10^{42}$ $\rm~{}erg~{}s^{-1}$ to $3.9\times 10^{43}$
$\rm~{}erg~{}s^{-1}$. As the radius begins to increase, the temperature and
luminosity drop, and the continued rise of the light curves is powered by
cooling, as the SED peak moved from the unobserved FUV into initially the UV
and later the optical bands.

#### Compton cooling of the shock-heated wind at early times:

As the radiation-mediated shock breaks out of the dense CSM, the accelerated
ejecta slam into the low-density and optically thin wind, creating a
collisionless shock[16, 1] that is mediated by collective plasma
instabilities. The average proton temperature at the shocked region[16, 17] is
$T_{p}\approx 0.2$ MeV. The electrons are heated by the shock passage and
Coulomb interactions with the protons to temperatures of $\gtrsim 60$ keV.
These energetic electrons inverse-Compton scatter on the flux of UV photons
from the underlying breakout region, suppressing the X-ray flux at early
times. Owing to the large density difference between the breakout region and
the extended encompassing wind, there are not enough hot electrons to
significantly impact the UV-optical spectral peak. Rather, the plasma energy
is carried away by EUV photons created through inverse-Compton scattering. The
spectrum will continuously harden as the radiation flux decreases with time
and distance, making this cooling channel inefficient.

The cooling rate of a plasma with energy $E_{p}$ of hot electrons at $\tau<1$
and at a temperature $T$ by photons at much lower energies is given by

$\displaystyle dE_{p}=-\frac{\sigma_{T}}{4\pi
r^{2}}\frac{4TL_{\gamma}\,dt}{m_{e}c^{2}}\,,$ (1)

where $\sigma_{T}$ is the Thompson cross section and $m_{e}c^{2}$ is the
electron rest-mass energy. Assuming $E_{p}=3T$, we find

$\displaystyle\frac{d\log E_{p}}{dt}=-\frac{\sigma_{T}}{3\pi
r^{2}}\frac{L_{\gamma}}{m_{e}c^{2}}\,.$ (2)

Using the observed bolometric light curve, we calculate the cooling rate of
the plasma by the breakout burst photons at various locations and times since
the explosion. We show these results in Supplementary Figure 2; they indicate
that during the first day of the explosion, Compton cooling is not yet
significant enough to prevent X-ray emission, but no observations exist at
this time. During days $t=2$–6, X-ray emission is suppressed at the
collisionless shock front, but the electrons remain hot enough to allow EUV
emission. After day 6, the density drops and X-ray emission is no longer
suppressed by inverse-Compton scattering. Our results are broadly consistent
with the observed He 2 emission (ionised efficiently by $\lesssim 100$ eV
photons) before day 6 and the emergence of X-rays on days 4–5. The continued
presence of C 3 $\lambda 2297$ after the disappearance of He 2 is consistent
with the hardening of the illuminating spectrum rather than an effect of
recombination time, as discussed in the main text.

###  0.7 7\. Radiative acceleration:

Radiative acceleration of CSM has been calculated[38] and shown to have
occurred in previous SNe, such as SN 2010jl[39]. Here, we calculate the
velocity gained by radiative acceleration at a given radius using the time-
dependent blackbody spectral luminosity that we measured (Methods $\S 2$) for
SN 2023ixf.

The acceleration a spherical shell of material at $\tau<1$ experiences due to
a flux $f_{\nu}$ passing through it is given by

$\displaystyle\frac{d^{2}r}{dt^{2}}=\frac{1}{4\pi r^{2}\mu
c}\int\limits_{0}^{\infty}L_{\nu}\sigma_{\nu}\,d\nu\,,$ (3)

where $L_{\nu}$ is the spectral luminosity of the internal source,
$\mu=0.7m_{p}$ is the average particle mass assuming solar mixture, and
$\sigma_{\nu}$ is the total cross section at a given frequency. To estimate
the total cross section, we use the open-source opacity table described in
ref[4] and based on atomic line lists[40] by assuming CSM with a density of
$10^{-15}\,\rm{g\,cm^{-3}}$ and a temperature of $5\,\rm eV$, which would
generate a thermal electron velocity distribution consistent with the FWHM of
the electron-scattering wings of the narrow emission lines observed during the
first few days. The effective cross section due to bound-bound and bound-free
processes dominates the effective cross-section for acceleration (but not for
diffusion) and is much larger than the Thompson cross section. While the CSM
is not in LTE with the incoming radiation from the underlying SN, the cross
section is most affected by bound-free interactions in the plasma. A full
steady-state solution of the fractions of different plasma species is outside
the scope of this work.

In Extended Data Figure Resolving the explosion of supernova 2023ixf in
Messier 101 within its complex circumstellar environment, we show the
velocities measured from various narrow lines as they evolve until
photospheric features emerge. Our calculation demonstrates that with a
fraction of neutral elements achieved at $5\,\rm eV$, the absorption cross
section is sufficiently high to fully explain the observed line velocities
with radiative acceleration from the SN UV–optical emission at a single
location. The first high-resolution spectrum capable of resolving lines with
$v<100\rm~{}km~{}s^{-1}$ was taken at $t=2.3$ days. At this time, radiative
acceleration was already high enough to dominate the observed velocity,
indicating that the CSM velocity is not measured from these features. As a
consequence of our calculation, we also determine the position and radiative
acceleration experienced by shells immediately above the breakout region.
Given sufficient time, matter below the shock-breakout region (instead of the
ejecta) would accelerate enough to shock the matter. In our case, significant
acceleration does not occur before day 3. By this time, the ejecta themselves
will sweep up all matter, which can be accelerated to
$>5000\rm~{}km~{}s^{-1}$, indicating a collisionless shock had, in fact,
formed above the breakout region.

###  0.8 8\. Constraining the progenitor Metallicity:

#### Nearby H II regions:

Using the integral-field-unit Keck Cosmic Web Imager KCWI[24], we measure
prominent emission lines from the two nearby star-forming regions 80 pc west
and 310 pc north of the SN explosion site (see $\S 1$). Supplemantary Table 9
summarises the line fluxes of the most prominent emission lines ([O 3]
$\lambda\lambda$4959, 5007, H$\alpha$, H$\beta$, and [N 2] $\lambda$ 6584) of
the two regions. Using the O3N2 and R3 strong-line metallicity indicators and
the calibrations from ref[41], we infer metallicities of $1.00\pm 0.06$ and
$0.68\pm 0.01$ solar for the regions to the west and north of the SN explosion
site (respectively), indicating that there is a moderate metallicity gradient
across this small region of the host galaxy.

In ref[42], multi-object spectroscopy of M101 was performed with the Large
Binocular Telescope. They obtained a spectrum of the H 2 region at
$\alpha=14^{\rm hr}03^{\rm m}38.562^{\rm s}$,
$\delta=+54^{\circ}18^{\prime}41.94^{\prime\prime}$ (J2000) that is $\sim
10^{\prime\prime}$ ($\sim 340$ pc) north of SN 2023ixf (spectrum ID
NGC5457+225.6-124.1). Their deep observation also recovered emission from the
weak auroral line [O 3] $\lambda$ 4363, which allows a measurement of the
metallicity using the $T_{e}$ method. These authors determine a metallicity of
0.64 solar, corroborating our value.

Most SNe will be detected at larger distances than SN 2023ixf, and the H II
regions to the north and west would be indistinguishable from the ground.
Adopting the weighted mean of the two regions from our KCWI observation
($0.69\pm 0.01$) as a proxy of the metallicity at the SN site.

#### ISM lines:

To infer the column densities from the ISM EWs, we use the curve-of-growth
(CoG) technique[43, 23], though see ref[44] for the limitations and perils of
this method. We present the resulting column densities in Supplementary Table
1 and the fit to the CoG in Supplementary Figure 2. From the series of Fe 2
absorption lines, we measure an instrumental plus intrinsic line-broadening
parameter $b$ of $70\rm~{}km~{}s^{-1}$. Assuming that all other species have
the same $b$ value, we can measure the column densities $N$ of all species. To
determine the metallicity of the SN line of sight, the neutral hydrogen column
density of the line of sight has to be known. However, the HST/STIS spectra do
not cover Ly$\alpha$, commonly used for measuring $N$(H 1). Hence, in order to
estimate the metallicity, we use an empirical relation [Zn/Fe] =0.73 [Zn/H] +
1.26 from ref[27]. Given the observed [Zn/Fe] $=1.25$, we estimate the
metallicity of the neutral ISM to be $Z=0.95$ solar, using the solar
abundances for H and Zn from ref[45]. While this relation has a scatter of
$0.3$ dex, it rules out extremely low progenitor metallicity ($\lesssim 0.5$
solar), while large extrasolar metallicities are ruled out by the environment
measurements, which are consistent with the latter estimate.

In addition, we calculate the mass dust-to-metals ratio DTM $\approx 0.43$,
based on the observed [Zn/Fe] and using the formalism of ref[46], also used by
ref[47]. This DTM value is very close to the Milky Way value (0.45). Finally,
we analyse the metal pattern using the method of ref[48], but adapted for the
case where no H measurements are available[49]. We find no evidence for alpha-
element enhancement or Mn underabundance in the gas, their metal columns
aligning well with what is expected from only dust depletion. This suggests
that the neutral ISM in this galaxy has not been enriched by a recent burst of
star formation. Either the star formation in this galaxy has been rather
continuous (with contributions from SNe Ia as well, in addition to core-
collapse SNe), or the recent star formation did not yet have the time or
extent to enrich the neutral ISM in the host galaxy.

#### Comparison to PoWR models:

A full modelling of our spectral datasets requires detailed non-LTE time-
dependent radiative-transfer calculations in expanding atmospheres[50], which
is beyond the scope of this work. Here, we use stationary non-LTE PoWR
models[51, 52] to obtain a better understanding of the conditions that lead to
the formation of diagnostic spectral lines in the UV, and to constrain the
iron content in the progenitor’s wind. Briefly, the PoWR code solves the one-
dimensional non-LTE radiative transfer and rate equations in an expanding
atmosphere while ensuring energy conservation. Models are defined via the
chemical abundances, surface effective temperature $T_{*}$, bolometric
luminosity $L$, mass-loss rate $\dot{M}$, terminal wind speed $v_{\infty}$,
and clumping factor $D$ (which describes the ratio of the clumped material to
an equivalent smooth wind). The surface effective temperature $T_{*}$ is
defined at a mean Rosseland optical depth of $\tau_{\rm Ross}=20$; the
photospheric effective temperature at $\tau_{\rm Ross}=2/3$ is denoted as
$T_{2/3}$. The stellar radius $R_{*}$ follows from the Stefan-Boltzmann
relation. The wind is assumed to follow a standard $\beta$ law[53] with
$\beta=1$. While the models do not provide a full representation of the
physical conditions in the CSM (e.g., ionising bremsstrahlung radiation), we
can use the effective temperature as a proxy for the ionisation balance in the
emitting layers.

A more convenient parameter for describing the wind is given by the
transformed radius[54],

$R_{\text{t}}=R_{*}\left[\frac{v_{\infty}}{2500\,{\rm km}\,{\rm
s}^{-1}\,}\middle/\frac{\dot{M}\sqrt{D}}{10^{-4}\,{\rm M}_{\odot}\,{\rm
yr}^{-1}}\right]^{2/3}\,.$ (4)

For given $T_{*}$, $R_{\rm t}$, and chemical abundances, the strength of
emission lines remains similar, regardless of the luminosity.

We do not attempt to accurately reproduce the spectral line profiles, which
would likely require more sophisticated velocity fields than the $\beta$ law
adopted here. Instead, we focus on the impact of varying $T_{*}$ on the iron
diagnostics. Supplementary Figure 2 shows a comparison between the UV data and
three PoWR models. The PoWR models were computed with abundances following
ref[27] and a solar iron composition, $\log R_{\rm t}=1.5\,[{\rm R}_{\odot}]$,
$\log L=5.3\,[{\rm L}_{\odot}]$, $v_{\infty}=1500\,\rm~{}km~{}s^{-1}$, and
$D=4$, as well as varying values of $T_{*}=35$, 40, and 45 kK. As
Supplementary Fig. 2 illustrates, the cooler model produces a wealth of Fe iv
lines in the range 1650–1900 Å. However, this model also greatly overpredicts
the strength of the C iii $\lambda 2297$ line, and predicts the presence of
lines such as N iii $\lambda 1747$, which is not observed. A better match is
obtained for models with $T_{*}\approx 45\,$kK (amounting to $T_{2/3}\approx
43\,$kK). However, models with $T_{*}\gtrsim 45\,$kK no longer show
significant Fe iv lines, which inhibits their usage as abundance diagnostics.
At these temperatures, the Fe v lines, which dominate the range 1400–1500 Å,
would have provided useful abundance diagnostics, but unfortunately this range
cannot currently be observed with disruptive ToO HST observations. We conclude
that the subsolar iron abundance implied by the nearby H 2 regions is
consistent with the data, but we cannot provide more accurate constraints
using the available spectra.

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Correspondence and requests for materials should be addressed to Erez A.
Zimmerman (email: erezimm@gmail.com).

We thank Prof. Boaz Katz and Prof. Eli Waxman for their advice on the physical
interpretation of SN 2023ixf. This work is based on observations obtained with
HST as part of proposal GO-17205 (PI E. Zimmerman). Based in part on
observations obtained with the Samuel Oschin 48-inch Telescope and the 60-inch
Telescope at the Palomar Observatory as part of the Zwicky Transient Facility
(ZTF) project. ZTF is supported by the U.S. National Science Foundation (NSF)
under grant 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 SED Machine is
based upon work supported by the NSF under grant 1106171. Based in part on
observations made with the Nordic Optical Telescope (NOT), 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, respectively), 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. These
data were obtained 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 (NASA); the Observatory was made
possible by the generous financial support of the W. M. Keck Foundation. This
work includes observations obtained at the Liverpool Telescope, which 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. A major upgrade of the Kast spectrograph on the
Shane 3 m telescope at Lick Observatory, led by Brad Holden, was made possible
through generous gifts from the Heising-Simons Foundation, William and Marina
Kast, and the University of California Observatories. Research at Lick
Observatory is partially supported by a generous gift from Google. This work
benefited from the OPTICON telescope access program (https://www.astro-
opticon.org/index.html), funded from the European Union’s Horizon 2020
research and innovation programme under grant agreement 101004719. Based in
part on observations made with the Italian Telescopio Nazionale Galileo (TNG)
operated on the island of La Palma by the Fundación Galileo Galilei of the
INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque
de los Muchachos of the Instituto de Astrofisica de Canarias. We made use of
IRAF, which is distributed by the NSF NOIR Lab. The Gordon and Betty Moore
Foundation, through both the Data-Driven Investigator Program and a dedicated
grant, provided critical funding for SkyPortal. A.V.F.’s supernova group at
U.C. Berkeley has been supported by Steven Nelson, Gary and Cynthia Bengier,
Clark and Sharon Winslow, Sanford Robertson, Briggs and Kathleen Wood, the
Christopher R. Redlich Fund, and numerous other donors. T.S. acknowledges
support from the Comunidad de Madrid (2022-T1/TIC-24117).

E. A. Z. is the PI of the HST proposal, triggered the HST observations,
reduced the HST spectra, conducted follow-up observations, conducted
spectroscopic and physical analysis, and wrote the manuscript.
I. I. helped develop the manuscript, contributed to follow-up design and
execution, conducted photometric analysis and physical analysis, reduced the
saturated Swift data, and contributed to the physical interpretation of the
data.
P. C. contributed to follow-up design and execution, reduced photometry, and
reduced the Swift spectra.
A. G.-Y. leads the Weizmann research group, provided mentorship, and edited
the manuscript.
S. S. conducted spectroscopic analysis on the ISM lines and reduced the XRT
data.
D. A. P. classified the supernova and provided LT data.
J. S. helped develop the manuscript, provided the NOT data, and is a ZTF
builder.
A. V. F. leads the U.C. Berkeley research group, is PI of the Lick program,
and thoroughly edited the manuscript.
T. S. modeled the SN spectra with PoWR and advised about spectral features.
O. Y. helped develop the manuscript and created several figures.
S. S. provided microtelluric models for the HARPS spectra and advised on
statistical tests.
R. B. contributed to follow-up design and execution, and helped develop the
manuscript.
E. O. O. advised on the physical interpretation and helped develop the
manuscript.
A. D. C. conducted an analysis of the ISM absorption lines.
T. G. B. obtained and reduced all of the Lick data.
Y. Y. and S. S. V. helped develop the manuscript.
S. B. contributed to follow-up design and execution.
M. A. is part of the ZTF calibration team.
A. B. advised on the measurement of absorption-line EWs.
J. S. B. and K. Zhang obtained data for this study.
P. J. B. and M. R. consulted on Swift data reduction and helped develop the
manuscript.
M. M. K. and K. D. provided infrared data from Gattini and IRTF.
G. D., J. H. T., and K. M. S. obtained the INT spectrum.
C. F. requested and shared the KCWI data.
A. H. and I. S. advised on radio data.
K. H. and J. W. reduced LT data.
J. P. J. obtained the NOT/FIES spectra.
S. R. K. is the ZTF PI and contributed Keck data.
D. K., J. M., and T. W. advised on the physical interpretation and models. J.
D. N. Obtained and reduced the KCWI data.
C. M., M. M., N. Z. P., and R. C. M. are part of the KCRM commissioning team
and were involved in obtaining the KCWI data.
P. E. N. provided the DESI data and contributed to follow-up design and
execution.
L. Y. and A. A. M. helped develop the manuscript and are coauthors of HST
proposal GO-17205.
R. S. P. provided photometry from the Post Observatory, Mayhill, NM.
Y. Q. and R. D. S. provided and reduced Keck spectroscopic data.
A. R. reduced the DESI spectra.
R. R. and B. R. are ZTF builders.
M. S. is a coauthor of HST proposal GO-17205.
A. W. is part of the ZTF data-system team.

The authors declare that they have no competing financial interests.

Photometry and spectra used in this study will be made available on
WISeREP[55, 56]. A log of the available spectra can be found in Supplementary
Table 2. All scripts used to conduct the analyses presented in this paper are
available from the corresponding author upon request. Opticon observations
were obtained under Program ID OPT/2023A/001, PI Erez Zimmerman.

Relevant software sources have been provided in the text, web locations
provided as references, and are publicly available.

(a) A composite $ugi$ image taken by the Liverpool Telescope (LT) showing the
location of SN 2023ixf within the Messier 101 galaxy. The SN is clearly seen
to be very blue (the most blue object within the inset rectangle). It is
located within a spiral arm at the outskirts of the galaxy. (b) The LT
$g$-band image showing the area marked in the composite image. Nearby star-
forming regions are clearly seen. (c) H$\alpha$ image of the direct vicinity
of SN 2023ixf, as marked in the LT $g$-band image, constructed from
observations with the integral field unit KCWI. The SN explosion site is
embedded in a region of ongoing star formation, including diffusely
distributed lower-level star-formation activity and islands of enhanced star
formation.

---

UV–optical light curves of SN 2023ixf using photometry reported in this work.
PSF or aperture photometry are shown with star symbols, UVOT streak photometry
is indicated using a plus symbol, and synthetic photometry from HST is
indicated using squares.

---

The early-time spectral sequence of SN 2023ixf. A plethora of narrow flash-
ionisation lines is seen in the earliest spectra, including the H Balmer
series, H Paschen series, several series of He 2 ($n\rightarrow 3,4,5,6$), He
1, C 4, N 4, N 3, and C 3. Lower-ionisation species (C 3, N 3, He 1) weaken
until they disappear in the optical spectra by day 2.

---

The spectral sequence of SN 2023ixf after $t=6$ days, and until broad features
appear. All narrow lines except narrow $H\alpha$ P Cygni disappear by day 6 in
the optical. The narrow P Cygni no longer appears by day $\sim 16$.

Narrow P Cygni profiles from the NOT/FIES spectra (optical, H$\alpha$) and
HST/STIS (UV, N 4, C 3). We model (dashed lines) the H$\alpha$ profile to
measure the blue-edge velocity, which is shown in corresponding colours. The
original spectrum (binned, transparent) is shown as well. We do not model the
N 4 feature as it lies at the edge of the STIS CCD, and one cannot determine
the exact blue edge of the P Cygni profile. We therefore adopt a
phenomenological value of 3 times the minimum velocity of the profile as the
blue-edge velocity. We also do not model the C 3 feature as it is blended with
a photospheric absorption in the $t=8.74$ day spectrum. However, the blue edge
is clearly seen to be $\sim 1500\rm~{}km~{}s^{-1}$ (marked).

Comparison of photospheric UV spectra of SN 2023ixf with the early-time UV
spectra of SN 2022wsp[18], SN 2021yja[19], SN 2022acko[20], SN 1999em [21],
and SN 2005ay [22]. All spectra were taken by HST except for the SN 2005ay
spectrum obtained with GALEX. The prominent Mg 2 $\lambda 2798$ line is marked
in yellow. Its double-peaked shape is similar to that of SN 2022wsp,
indicating an Fe 2 transition[18]. The broad absorption across 1800–2100 Å is
marked as well and arises from a mix of metals. The features in SN 2023ixf are
similar to those of SN 2022wsp, SN 2021yja, and SN 2022acko, yet shifted due
to different velocity regimes. The uniformity of these features suggests that
they originate from the exploding star’s natal chemical composition. However,
a strong emission feature at $\sim 1910$ Å appears in the spectra of SN 1999em
and SN 2005ay, suggesting some diversity in SN II UV photospheric features
exists as well.

Narrow flash features from the $t=2.3\,d$ NOT/FIES spectrum. We fit a model
consisting of a Lorentzian base along narrow Gaussian components for the
respective narrow lines. For He 2 we fit a second Lorentzian component to
account for the blue edge.

The Na I D1 $\lambda 5890$ complex from the first four HARPS-N spectra. Three
distinct clouds of Na I D are seen in the strong line-of-sight ISM line.
Another weaker component is visible at $-30\rm~{}km~{}s^{-1}$. Components that
show a significant decrease in EW are marked with a red arrow. The decrease in
EW is caused by ionisation of neutral Na, which requires the changing
component to come from the SN CSM. The component in the $t=56.07$ day spectrum
at $\sim 40\rm~{}km~{}s^{-1}$ arises from a calibration lamp due to the SN
becoming fainter (marked in grey).

Absorbed X-ray luminosity from the XRT 0.–10 keV band. Each point represents a
binned measurement, with an error bar representing the measurement period. The
relatively constant X-ray luminosity suggests an average CSM density profile
of $\rho\propto r^{-2}$. The variability around the mean luminosity between
$t\approx 10\,$–$\,20$ days could correspond to variability in the CSM
density, as suggested by progenitor papers (Methods $\S 5$).

---

Feature velocities and appearance times for H$\alpha$, He 2 $\lambda 4686$, N
4 $\lambda 1718$, and C 3 $\lambda 2297$. The dashed lines show the expected
radiative acceleration of stationary matter at $\tau<1$, calculated from the
cumulative radiated energy at different radii. We indicate the region excluded
by ejecta starting at $R_{\rm bo}/2=0.8\times 10^{14}\,\rm cm$ with a velocity
of $10^{9}\,\rm{cm\,s^{-1}}$, estimated from the blue edge of the photospheric
spectrum. Approximate measurements from ref[21] are added as well.

#### Supplementary Methods

##  1 Photometric reduction:

#### P48:

A pre-discovery $g$-band image of SN 2023ixf was obtained by the Zwicky
Transient Facility (ZTF)[1, 2] with the Palomar 48-inch Schmidt telescope
(P48) on 2023-05-19 07:45:07. The image was reduced through an automatic
pipeline[3]. Follow-up photometry was not obtained with P48 owing to
saturation.

#### Liverpool Telescope:

We carried out an extensive campaign of imaging observations of SN 2023ixf,
beginning from 2023-05-19 at 22:06:49.78 (within a few hours of the initial
report by Itagaki et al.) and continuing on all operational nights of the
telescope thereafter. Observations were acquired in all SDSS filters ($u$,
$g$, $r$, $i$, $z$) with varying exposure times. For the first few nights when
the SN was rapidly rising we acquired 3–5 sets of observations each night with
a cadence of about 1 hr; after peak brightness our cadence dropped to 2
observations per night (cadence 4 hr) and then 1 observation per night. There
are two significant gaps in our coverage: the gap shortly after peak
brightness is due to inclement weather (high humidity); the subsequent gap was
primarily due to a power-supply issue.

In all epochs and filters to date, the SN is sufficiently bright that host-
galaxy contamination is not a significant source of error, and we measure the
magnitudes of the source using standard aperture photometry with a radius
matched to the seeing conditions. However, neither the SDSS nor PS1
photometric catalogues are reliable in this field owing to the complex M101
background. We constructed our own photometric catalog of secondary standards
using data from the night of discovery and observations of the standard stars
PG1047, BD+18, and SA 114 (on this night, several images of the field were
taken at varying airmasses, the night was photometric, and exposures were long
as the SN was still well below peak). This secondary-standard catalogue was
then used to calibrate all other observations of the field.

A small number of observations close to peak light are (mildly) saturated.
Frames for which the peak counts exceed 46,000 DN were identified and
excluded.

The statistical error on virtually all measurements is negligible. The
photometric uncertainty is approximated using the scatter in the magnitude
measurements of the calibration stars relative to our secondary calibration.
This does not include systematic uncertainties in the calibration of the
field, which are estimated to be about 0.05 mag (but will affect all
measurements in each filter in the same way).

#### P60/SEDM:

We observed SN 2023ixf with the robotic 60-inch telescope at Palomar
Observatory (P60)[4] using the Spectral Energy Distribution Machine (SEDM)[5].
We obtained 31 epochs of $g$, $r$, and $i$-band images and 30 epochs of
$u$-band images between 2023 May 20 and 2023 June 30. Short exposure times of
around 10 s were adopted to avoid saturation when SN 2023ixf was bright. The
first-epoch images taken on 2023 May 20 were chosen as reference images in
each band. We manually selected a few isolated stars in the reference image
with negligible host flux contamination to build the effective point-spread
function (PSF) for each image. We performed PSF photometry for all the
detected sources in each image and relative photometry calibration against the
reference image in each band. The relative photometry was performed with the
sources used for PSF construction. Source detection, PSF construction, and PSF
photometry were done with IRAF (IRAF 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 U.S.
National Science Foundation NSF.) using the daophot package[6]. We then
derived the photometric zero points of the reference image and applied the
zero points to the relative photometry to get the final light curves. For
photometry calibration, we used the AAVSO Photometric All-sky Survey
(APASS[7]) catalogue for the $g$, $r$, and $i$ bands, and the secondary
magnitudes derived for LT photometry above for the $u$ band. We checked and
confirmed that the differences between APASS and LT secondary standards are
smaller than 0.05 mag in the $g$, $r$, and $i$ bands.

#### RC32 telescope at Post Observatory:

We started the follow-up campaign of SN 2023ixf on 2023 May 23 with an RC32
telescope operated by Post Observatory at Mayhill, New Mexico. The telescope
has an aperture size of 80 cm and is equipped with an FLI CCD. Observations
were obtained with Astrodon Photometric filters, specifically Johnson[8] $B$
and Sloan[9] $g,\,r,\,i,\,z$. We followed the PSF photometry method as
described in ref[10]. The final $B$-band magnitudes in the Vega system were
obtained by calibrating them to the APASS catalog, and the $g,\,r,\,i$, and
$z$-band magnitudes in the AB system [11] by calibrating them to the ATLAS
All-Sky Stellar Reference Catalog [12].

#### Las Cumbres Observatory 0.4 m telescope

We used public images of SN 2023ixf taken between 2023 May 20 and 2023 June 4
with a Las Cumbres Observatory 0.4 m telescope[13]. The reduced images with
BANZAI [14] were downloaded from the Las Cumbres Observatory data archive. We
performed photometry in the same way as we did for RC32 images and obtained
$B,\,V$ (Vega system) and $g,\,r,\,i$ (AB system) magnitudes. For $V$-band
calibration, we used the APASS catalog.

####  Swift /UVOT photometry:

Immediately following classification, UV photometry was acquired using the UV-
Optical Telescope (UVOT) onboard the Neil Gehrels Swift Observatory [15, 16].
UVOT observed the field 46 times during the first 3 weeks of the event and
documented the rise and decline of the UV light curve. During most of the UV-
bright phase of the evolution ($\lesssim 13$ AB mag), SN 2023ixf saturated the
UVOT images. However, the data were recoverable using readout-streak
photometry. We reduced these data using the methods of ref[3]. We masked the
raw images for bright and faint sources, corrected them for large-scale and
small-scale sensitivity variations, and corrected them for sensitivity loss
using the latest calibration files from September 2020. We then calculated the
coincidence-loss-corrected mean count rates across the pixel columns and
converted these to magnitudes using the zero point reported by ref[3],
depending on the appropriate readout mode of the detector. A 0.1 mag
systematic calibration error is assumed in all observations, and data are
binned within intervals of 0.5 day to reduce scatter.

In epochs during which observations did not saturate (both before and after
peak brightness), we reduced images using the Swift HEAsoft toolset[17].
Source counts were extracted using uvotmaghist from the individual exposures
images using a circular aperture with a $5^{\prime\prime}$ radius. The
background was estimated from several larger regions surrounding the host
galaxy. These counts were then converted to magnitudes using the photometric
zero points of ref[18] with the latest calibration files from September 2020.
We did not attempt to subtract the host-galaxy flux at the location of the SN
under the assumption that the SN dominates the flux at the SN location, which
is verified by the $\sim 4$–7 mag decline observed by UVOT after peak
brightness. By integrating the UVOT transmission filters, we estimate that the
red leak from optical wavelengths becomes important ($\sim 2.5\%$) at UV minus
optical colours of $\sim 4$ mag.

#### Gattini:

The position of SN 2023ixf was covered as part of survey operations of the
Palomar Gattini-IR $J$-band survey[19, 20]. We retrieved $J$-band photometry
of SN 2023ixf by performing forced PSF photometry at the SN position on the
difference images generated in the data-reduction pipeline[21]. We show the
resulting light curves in Extended Data Figure Resolving the explosion of
supernova 2023ixf in Messier 101 within its complex circumstellar environment.

###  1.1 2\. Spectroscopy reduction:

We have made use of the skyportal[22] instance fritz for observation planning
and collaboration within ZTF for this study.

#### P60/SEDM:

We obtained a series of spectra with the P60 using the SEDM (see previous
section for photometry). All spectra were automatically reduced [23].

#### Liverpool Telescope:

Following the classification spectrum, we obtained a spectral series of SN
2023ixf with the SPRAT robotic spectrograph [24] mounted on the Liverpool
Telescope (LT). A spectrum was taken on all operational nights of the
telescope since the SN discovery. To improve the flux calibration and SNR of
the spectra, we manually ran the reduction pipeline on all spectra instead of
relying on the automatic pipeline. To improve the flux calibration, we
reapplied standard-star correction with ESO reference standard-star spectra.
As the trace was unusually bright, we could not have corrected the spectra for
cosmic rays using a standard procedure such as the LACosmic algorithm. We
therefore correct for cosmic rays by manually removing any obvious cosmic-ray
outliers in the extracted spectra.

#### Lick Observatory:

We obtained a series of early-time spectra, starting from $t=1.3$ days, using
the Kast double spectrograph[25] mounted on the Shane 3 m telescope at Lick
Observatory. These observations were made using both arms of the instrument
and in several configurations, providing a resolving power of $R=970$–1940 for
the blue side and $R=690$–4180 for the red side (see Table 2 for exact
details). To minimise slit losses caused by atmospheric dispersion[26], the
slit was oriented at or near the parallactic angle. The data were reduced
following standard techniques for CCD processing and spectrum extraction[28]
utilising IRAF routines and custom Python and IDL codes[27]. Low-order
polynomial fits to comparison-lamp spectra were used to calibrate the
wavelength scale, and small adjustments derived from night-sky emission lines
in the target frames were applied. The spectra were flux calibrated using
observations of appropriate spectrophotometric standard stars observed on the
same night, at similar airmasses, and with an identical instrument
configuration.

#### Swift/UVOT grism:

We requested Swift/UVOT grism spectra of SN 2023ixf shortly after the
discovery. The first epoch of UVOT observation started on MJD = 60084.47 (1.67
days after the estimated explosion time). We acquired the UVOT grism data
taken between 2022 May 20 and 2022 July 5 from the Swift Archive Download
Portal[30] and extracted the spectra with uvotpy[29] which utilises
calibration presented in ref[31]. The observation information is listed in
Supplementary Table 10 and the spectra are presented in Supplementary Figure
2. We summed all the exposures belonging to the same epoch (with the same
OBSID) except for the first epoch (OBSID = 00032481003), for which we summed
the second, third, and fourth exposures and kept the individual spectra from
the first and the fifth exposure. As a result, we report 20 grism spectra of
SN 2023ixf in this paper.

#### Dark Energy Spectroscopic Instrument (DESI):

Starting from $t=4.3$ days, the DESI collaboration[32] observed and published
spectra of SN 2023ixf using the Dark Energy Spectroscopic Instrument (DESI) at
the 4 m Mayall telescope at Kitt Peak National Laboratory[33]. Daily spectra
were obtained when allowed by weather until $t=25.34$ days, after which two
more epochs were taken, resulting in a total of 21 spectra, which have been
processed with the standard DESI pipeline[34]. We use these spectra in our
general spectroscopic analysis. As these spectra are well flux-calibrated, we
compared our photometric flux calibration to them as a sanity check for our
process.

#### Hubble Space Telescope:

We reduced all spectra using the standard STIStools pipeline and its Python
interface. The reduction process included alignment and combination of the
dithered spectra, cosmic-ray rejection, exposure combination, and fringe-flat
correction for the G750L exposures. We manually removed any strong cosmic rays
still showing after the pipeline process. While single-orbit combined spectra
produce an hourly temporal resolution spectrum, we found no significant
spectral development between orbits in any visit. Therefore, we chose to
combine all exposures in single visits, yielding improved cosmic-ray and bad-
pixel rejection.

During the HST visits, some guiding and saturation issues emerged. In visit 1,
the guide-star acquisition was delayed, causing a failure of two exposures and
an off-centre slit acquisition of all later exposures during the visit. This
created an inaccurate absolute flux calibration during this visit, which we
mitigated using a photometric flux correction of the optical grisms, applying
the same shift to the NUV spectra during the visit. Visit 3 suffered guiding
issues as well, resulting in the loss of five G230L exposures and one G750L
exposure.

During visits 2–4 some parts of the spectrum were degraded owing to
saturation, causing unreliable flux calibration because of the nonlinearity of
the CCD response function. As these pixels are counted by the STIStools
pipeline, they only represent a lower limit of the exact flux at the pixel. We
note, however, that the severity of the issue mostly affects the G430L
spectrum, covered by ground-based spectra, while the saturated NUV parts of
the spectrum show counts much closer to the instrumental limit (of 33,000
counts). We also note that saturation does not affect the bluest wavelengths
of the NUV spectra critical for blackbody fits (away from the Rayleigh-Jeans
tail). While we cannot fully mitigate this issue, we find the overall quality
of the NUV spectra to be good, as no clear nonlinearity is seen between the
saturated red part of the NUV spectra and the unsaturated blue wavelengths. To
correct the overall flux calibration, we applied a photometric correction to
the optical parts of the spectra, adding the same shift to the NUV part of the
spectrum. Lastly, to avoid saturation in the fifth HST visit, we increased the
CCD gain.

#### HARPS-N:

We obtained a total of five spectra with the northern High Accuracy Radial
velocity Planet Searcher (HARPS)[35] mounted on the 3.58 m Galileo National
Telescope (TNG) in the Roque de los Muchachos Observatory, La Palma. These
spectra were obtained through the OPTICON telescope access program 23A001 (PI:
E. Zimmerman). The echelle spectrograph provides high-resolution ($R=115,000$)
spectra. All spectra were reduced by the observatory using the standard
pipeline. We used non-blaze-corrected spectra in our analysis to avoid
artifacts from SN features that are broader than typical stellar features
expected by the deblazing algorithm.

To account for atmospheric microtelluric absorption, we calculate a telluric
model using the TelFit code[36] with the atmospheric conditions at the
observatory at the time of observations. For the $t=21$ and $t=56$ day
spectra, we manually increased the strength of the calculated telluric model
by a factor of 2 and 2.5, respectively, as the untreated calculated model did
not result in a complete removal of the identified telluric absorption. Since
this correction is linear and applies to all calculated microtelluric
absorption lines, we find the correction to be robust.

#### Nordic Optical Telescope:

We obtained a series of low-resolution spectra with the Alhambra Faint Object
Spectrograph and Camera (ALFOSC)[37] on the 2.56 m Nordic Optical Telescope
(NOT) at the Observatorio del Roque de los Muchachos on La Palma (Spain). The
spectra were obtained with grism #4, providing a wavelength range of 3300–8500
Å, and either a 1$.\\!\\!^{\prime\prime}$0 or 1$.\\!\\!^{\prime\prime}$3 wide
slit, depending on the weather conditions. The resolving power is $R\approx
340$ for the 1$.\\!\\!^{\prime\prime}$0-wide slit. The ALFOSC data were
reduced using PypeIt[38, 49], version 1.8.1. The reduction includes cosmic-ray
rejection, bias corrections, flat fielding, and wavelength calibration using
spectra of He-Ne comparison lamps obtained immediately after the target. The
relative-flux calibration was done with spectrophotometric standard stars
observed during the same night.

With the NOT, we also obtained high-resolution spectroscopy using the FIbre-
fed Echelle Spectrograph FIES[40] in its medium-resolution mode, using fiber
#3 (diameter 1$.\\!\\!^{\prime\prime}$0) covering the wavelength interval
3700–8300 Å at $R\approx 46,000$. The data were reduced using the FIEStool
software[41] provided by the observatory. The reduction includes cosmic-ray
rejection, bias corrections, flat fielding, wavelength calibration with a Th-
Ar lamp, and merging the individual orders. The relative-flux calibration was
done with spectrophotometric standard stars observed during the same night.

#### IRTF:

On 2023-05-25, we obtained a near-infrared (NIR) spectrum of SN 2023ixf using
SpeX on the NASA Infrared Telescope Facility[42] as part of program 2023A070
(PI K. De). The observations were acquired in the SXD mode, consisting of a
series of dithered exposures in the ABBA pattern, amounting to a total
exposure time of 600 s. The spectra were reduced and extracted using the
spextool package[43]. The extracted spectra were flux calibrated and corrected
for telluric absorption with observations of the A0 V star HIP 65280 using the
xtellcor package[44].

#### KCWI:

On 2023-06-05, we observed SN 2023ixf with the Keck Cosmic Web Imager[24]
(KCWI) on a night of commissioning for the newly installed red arm (Keck
Cosmic Reionization Mapper, KCRM). The Medium slicer with a field of view of
$16^{\prime\prime}\times 20^{\prime\prime}$ was in place. The blue arm was
configured with the BL grating yielding an average resolution of $R\approx
1800$ with a bandpass of 3580–5578 Å. The red arm was configured with the RL
grating yielding an average resolution of $R\approx 1000$ and a bandpass of
6465–9777 Å. We acquired four images with the blue arm and exposure times of
10 s each, and three exposures with the red arm and exposure times of 60 s
each. A nearby flux standard was observed right after the SN. The red-arm SN
exposures were median stacked to mitigate cosmic rays.

The data were reduced using a development version of the Keck KCWI
pipeline[45] to perform instrument signature removal and the construction of
geometrically-corrected and flux-calibrated data cubes. The wavelength
solution for the BL grating had an average residual of $0.089\pm 0.012$ Å, and
the RL grating solution yielded an average residual of $0.100\pm 0.014$ Å. The
flux-calibration residuals had a root-mean-square (RMS) scatter of $\sim 2\%$
in both channels.

#### INT/IDS:

We obtained one spectrum with the Intermediate Dispersion Spectrograph (IDS)
on the 2.5 m Isaac Newton Telescope. The spectrum was reduced using standard
iraf/pyraf and IDL/Python routines for bias/overscan subtractions and flat
fielding, while the wavelength solution was derived with comparison lamps and
verified against bright night-sky emission lines. The final flux calibration
and telluric-line removal were performed using spectrophotometric standard-
star spectra obtained on the same night.

#### Keck/LRIS:

We acquired a set of three spectra using the Low Resolution Imaging
Spectrometer (LRIS)[46] on the W. M. Keck I telescope as part of program C256
(PI: S. Kulkarni), during the same night. The first spectrum was taken on
2023-05-21.29, approximately 2.46 days after the explosion, and the last
spectrum was taken at 2.58 days post-explosion. All spectra were taken using a
$1^{\prime\prime}$ slit oriented at the parallactic angle to minimise slit
losses due to atmospheric dispersion. We chose a short exposure time of 20 s
to prevent saturation issues. The observational configuration provided a
median resolving power of $R\approx 2000$ on the blue side (3000–5600 Å) and
$R\approx 6700$ on the red side (5600–10,000 Å).

We reduced the data using LPipe[47]. First, the science exposures are flat-
fielded using dome flats taken immediately after the observing run, cleaned
for cosmic rays, and sky-subtracted. Owing to the lack of spectral evolution
during the night, we stack three separate exposures into one. Then, a one-
dimensional spectrum was extracted from the stacked science exposure,
wavelength calibrated using comparison-lamp frames taken before the observing
run, and flux calibrated using standard stars taken during the same night.
Finally, the flux-calibrated spectra from both the blue and red sides were
joined.

#### Keck/Deimos:

On 2023-05-23, we obtained three spectra of SN 2023ixf using the DEep Imaging
Multi-Object Spectrograph (DEIMOS)[48] on the 10 m Keck II Telescope (PI: R.
Stein). We performed observations with multiple long-slit configurations, to
maximise wavelength coverage. We obtained one medium-resolution 30 s spectrum
with the 600ZD grating, and two higher-resolution 30 s spectra with the 1200G
grating (one with the GG455 filter, and another with the redder OG550 filter).
The data were reduced using PypeIt [49, 38], with the dedicated DEIMOS
instrument default settings. Flux calibration was performed with the built-in
PypeIt DEIMOS sensitivity functions.

#### Extinction estimate:

To probe the host-galaxy extinction, we adopt the relations in ref[50] between
the equivalent width (EW) of Na 1 D absorption and extinction. We use our
sequence of HARPS-N spectra, which resolve the Na absorption completely. The
EW is measured directly from the spectra after removing any microtelluric
absorption, taking the mean of the first HARPS-N spectra (for which we measure
no decrease in Na component EW. We determine $E(B-V)=0.034$ mag, which we
multiply by a factor of 0.86 to correct for the conversion in ref[51]; the
final result is $E(B-V)=0.029\pm 0.005$ mag. Systematics dominate the
uncertainty.

#### Photometric flux calibration:

We calibrate the full spectral sequence to our photometric observations.
First, we corrected the absolute-flux calibration by interpolating the
$r$-band light curves using a fifth-order polynomial. We then corrected the
relative-flux calibration by fitting a free linear function multiplying the
flux of each spectrum. We find the best-fit parameters so that the
interpolated $g,\,r,$ and $V$ photometry matches with the synthetic photometry
calculated by integrating the spectra with the appropriate transmission
filters acquired from the Spanish virtual observatory filter profile
service[52]. For the HST and the IR spectra, we do not apply a photometric
relative-flux correction.

###  1.2 Reduction of XRT X-ray data

The Swift satellite also obtained multiple epochs of SN 2023ixf at X-ray
energies with its X-ray telescope (XRT)[25] in photon-counting mode. In
addition, Swift observed this field extensively between January 2005 and
December 2019, long before the SN explosion, for 476 ks in the windowed-timing
(20 ks) and the photon-counting (455 ks) modes. We analysed all photon-
counting data with the online tools of the UK Swift team that use the methods
described by refs[53, 54, 55] and the software package HEAsoft[17].

An image constructed from all pre-SN XRT data reveals no credible source
within a radius of $19.8^{\prime\prime}$ from the SN position (for comparison,
XRT’s half-power radius[25] is $9^{\prime\prime}$ at 1.5 keV). Some diffuse
emission is present at the SN site. The inverse-variance-weighted count rate
is $(3.8\pm 0.5)\times 10^{-4}~{}\rm ct\,s^{-1}$. Using the dynamic binning
method of the Swift online tools, we measure the first $3\sigma$ detection of
SN 2023ixf with XRT on 2023-05-22 at 01:07:06. The uncertainty is +0.9 day,
$-1.8$ day, reflecting the time interval of the XRT observations used to
obtain this detection in the dynamic binning mode. The count rate at the time
of discovery is $(10\pm 3)\times 10^{-4}\,\rm ct\,s^{-1}$, before removing the
contribution from the diffuse emission. About 19 days later, the light curve
reached a maximum of $0.011\pm 0.002\,\rm ct\,s^{-1}$. Since then, the light
curve has been relatively stable, with a small oscillation in the X-ray flux,
reaching a minimum on day 17, and peaking on day 23.5 again. The early X-ray
rise is shown in Fig. 2, while the full X-ray light curve is shown in Extended
Data Figure Resolving the explosion of supernova 2023ixf in Messier 101 within
its complex circumstellar environment. The individual measurements are
summarised in Supplementary Table 7.

We convert the Swift XRT 0.3–10 keV count rate to absorbed flux using the
HEASARC webPIMMS calculator[56], assuming a thermal bremsstrahlung spectrum
with a temperature of $T=34$ keV and the hydrogen column $N(H)=5.6\times
10^{22}\,\rm cm^{-2}$ as reported by NuStar in ref[20]. These values are
broadly consistent with a conversion factor acquired by fitting the XRT data
with an absorbed power-law spectrum.

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