# From Discovery to the First Month of the Type II Supernova 2023ixf: High and
Variable Mass Loss in the Final Year before Explosion

Daichi Hiramatsu Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA  The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, USA Daichi Hiramatsu<EMAIL_ADDRESS>Daichi Tsuna TAPIR, Mailcode 350-17, California Institute of Technology, Pasadena, CA 91125-0001, USA  Research Center for the Early Universe, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan  Edo Berger Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA  The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, USA  Koichi Itagaki  Itagaki Astronomical Observatory, Yamagata 990-2492, Japan  Jared A. Goldberg Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010-5902, USA  Sebastian Gomez Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2410, USA  Kishalay De NASA Einstein Fellow  MIT-Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA  Griffin Hosseinzadeh Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA  K. Azalee Bostroem LSSTC Catalyst Fellow  Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA  Peter J. Brown Department of Physics and Astronomy, Texas A&M University, 4242 TAMU, College Station, TX 77843-4242, USA  George P. and Cynthia Woods Mitchell Institute for Fundamental Physics & Astronomy, College Station, TX 77843, USA  Iair Arcavi School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel  Allyson Bieryla Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA  Peter K. Blanchard Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy,
Northwestern University, 1800 Sherman Avenue, 8th Floor, Evanston, IL 60201, USA  Gilbert A. Esquerdo Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA  Joseph Farah Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA  Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA  D. Andrew Howell Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA  Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA  Tatsuya Matsumoto Department of Physics and Columbia Astrophysics Laboratory, Columbia University, Pupin Hall, New York, NY 10027, USA  Curtis McCully Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA  Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA  Megan Newsome Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA  Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA  Estefania Padilla Gonzalez Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA  Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA  Craig Pellegrino Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA  Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA  Jaehyon Rhee Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA  Department of Physics, Yonsei University, Seoul 03722, Republic of Korea  Giacomo Terreran Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA  Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA  József Vinkó University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA  Konkoly Observatory, CSFK, MTA Center of Excellence, Konkoly-Thege M. út 15-17, Budapest, 1121, Hungary  ELTE Eötvös Loránd University, Institute of Physics and Astronomy, Pázmány Péter sétány 1/A, Budapest, 1117 Hungary  Department of Experimental Physics, University of Szeged, Dóm tér 9, Szeged, 6720, Hungary  J. Craig Wheeler University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA

###### Abstract

We present the discovery of the Type II supernova SN 2023ixf in M101 and
follow-up photometric and spectroscopic observations, respectively, in the
first month and week of its evolution. Our discovery was made within a day of
estimated first light, and the following light curve is characterized by a
rapid rise ($\approx 5$ days) to a luminous peak ($M_{V}\approx-18.2$ mag) and
plateau ($M_{V}\approx-17.6$ mag) extending to $30$ days with a fast decline
rate of $\approx 0.03$ mag day-1. During the rising phase, $U-V$ color shows
blueward evolution, followed by redward evolution in the plateau phase.
Prominent flash features of hydrogen, helium, carbon, and nitrogen dominate
the spectra up to $\approx 5$ days after first light, with a transition to a
higher ionization state in the first $\approx 2$ days. Both the $U-V$ color
and flash ionization states suggest a rise in the temperature, indicative of a
delayed shock breakout inside dense circumstellar material (CSM). From the
timescales of CSM interaction, we estimate its compact radial extent of
$\sim(3-7)\times 10^{14}$ cm. We then construct numerical light-curve models
based on both continuous and eruptive mass-loss scenarios shortly before
explosion. For the continuous mass-loss scenario, we infer a range of mass-
loss history with $0.1-1.0\,M_{\odot}\,{\rm yr}^{-1}$ in the final $2-1$ yr
before explosion, with a potentially decreasing mass loss of
$0.01-0.1\,M_{\odot}\,{\rm yr}^{-1}$ in $\sim 0.7-0.4$ yr toward the
explosion. For the eruptive mass-loss scenario, we favor eruptions releasing
$0.3-1\,M_{\odot}$ of the envelope at about a year before explosion, which
result in CSM with mass and extent similar to the continuous scenario. We
discuss the implications of the available multiwavelength constraints obtained
thus far on the progenitor candidate and SN 2023ixf to our variable CSM
models.

Supernovae (1668); Core-collapse supernovae (304); Type II supernovae (1731);
Massive stars (732); Supergiant stars (1661); Red supergiant stars (1375);
Stellar mass loss (1613); Circumstellar matter (241)

††facilities: ADS, ATLAS, FLWO:1.2m (KeplerCam), FLWO:1.5m (FAST, TRES), IRSA,
LCOGT (SBIG, Sinistro), MMT (Hectoechelle), NED, NEOWISE, PS1, Swift (UVOT),
WISE, ZTF††software: Astropy (Astropy Collaboration 2018), atlas-fp
(https://gist.github.com/thespacedoctor/86777fa5a9567b7939e8d84fd8cf6a76),
BANZAI (McCully et al., 2018), CHIPS (Takei et al., 2022), emcee (Foreman-
Mackey et al., 2013), lcogtsnpipe (Valenti et al., 2016), Matplotlib (Hunter,
2007), MESA (Paxton et al., 2011, 2013, 2015, 2018, 2019), NumPy (Oliphant,
2006), photutils (Bradley et al., 2022), PyRAF (Science Software Branch at
STScI, 2012), SciPy (Virtanen et al., 2020), seaborn (Waskom et al., 2020),
SExtractor (Bertin & Arnouts, 1996), STELLA (Blinnikov et al., 1998, 2000;
Blinnikov & Sorokina, 2004; Baklanov et al., 2005; Blinnikov et al., 2006),
SNEC (Morozova et al., 2015)

##  1 Introduction

The majority of massive stars (zero-age main-sequence (ZAMS) masses of
$M_{\mathrm{ZAMS}}\approx 8-25\,M_{\odot}$) end their lives when their iron
cores collapse, leading to explosions as hydrogen-rich (H-rich) Type II
supernovae (SNe II; see, e.g., Smartt 2009, 2015; Arcavi 2017, for reviews).
While there is consensus that the progenitors of SNe II are red supergiants
(RSGs) from direct progenitor identifications (e.g., Van Dyk 2017), evidence
is mounting that they experience elevated mass loss in the final months to
decades before explosion, which is not predicted from standard stellar
evolution theory. The presence of circumstellar material (CSM) from such mass
loss can be inferred from early light-curve excess above shock-cooling
emission (e.g., Morozova et al. 2018; Moriya et al. 2018; Förster et al. 2018;
Hiramatsu et al. 2021b), narrow high-ionization emission lines, so-called
flash features excited by the radiation from shock breakout or the SN–CSM
shock interface (e.g., Gal-Yam et al. 2014; Yaron et al. 2017; Dessart et al.
2017; Boian & Groh 2019, 2020; Terreran et al. 2022; Jacobson-Galán et al.
2022), and/or bremsstrahlung X-ray and synchrotron radio/millimeter emission
from SN–CSM shock interaction (e.g., Chevalier & Fransson 2017).

The recent discovery of Type II SN 2023ixf in M101 (§2) has allowed intensive
multiwavelength observations. A dust-obscured RSG progenitor candidate with a
possible periodic variability ($\approx 1000$-day period over the past
$\approx 13$ yr) has been identified in pre-explosion Hubble Space Telescope,
Spitzer Space Telescope, and ground-based optical to infrared images (Pledger
& Shara, 2023; Kilpatrick et al., 2023; Jencson et al., 2023; Neustadt et al.,
2023; Soraisam et al., 2023). Post-explosion observations have revealed early
light curve excess in the optical bands (Jacobson-Galan et al., 2023;
Hosseinzadeh et al., 2023; Teja et al., 2023b; Sgro et al., 2023), flash
features in low-to-high-resolution optical spectra (Yamanaka et al., 2023;
Jacobson-Galan et al., 2023; Smith et al., 2023; Bostroem et al., 2023; Teja
et al., 2023b), polarization measurements in spectropolarimetry (Vasylyev et
al., 2023), early X-ray detections (Grefenstette et al., 2023), and millimeter
nondetections (Berger et al., 2023).

Here we report our discovery of SN 2023ixf in §2 and follow-up photometric and
spectroscopic observations, respectively, in the first month and week of its
evolution in §3. In §4, we analyze the light curves and spectra. We then
construct numerical light-curve models based on both continuous and eruptive
mass-loss scenarios to extract the CSM formation history shortly before
explosion in §5. Using this information, we discuss a consistent picture among
the available observational constraints. Finally, we summarize our findings in
§6.

##  2 Discovery and Classification

Figure 1: Left: last nondetection (top) and discovery (bottom) images taken by
the Itagaki Astronomical Observatory. The red circle marks the position of SN
2023ixf. Right: color-composite ($griz$-band) image of SN 2023ixf and its host
galaxy M101 taken by the Fred Lawrence Whipple Observatory. SN 2023ixf is
located on a spiral arm with a nearby H ii region roughly at solar metallicity
and young ($10^{6}-10^{7}$ yr) stellar population (Hu et al., 2018).

We (Itagaki, 2023) discovered SN 2023ixf in M101 (Figure 1) on 2023 May 19.727
(UT dates are used throughout; ${\rm MJD}=60083.727$) at an unfiltered
magnitude of $14.9\pm 0.1$ at
$\text{R.A.}=14^{\text{h}}03^{\text{m}}38^{\text{s}}.580$ and
$\text{decl.}=+54^{\circ}18^{\prime}42".10$ with the Itagaki Astronomical
Observatory 0.35 m telescope (Okayama, Japan) + KAF-1001E CCD. The last
nondetection was obtained with the same setup at 17.5 mag on 2023 May 17.749
(${\rm MJD}=60081.749$). Subsequently, several other observers have reported
limits and detections in the time window between our last nondetection and
discovery (Perley & Irani, 2023; Filippenko et al., 2023; Fulton et al., 2023;
Mao et al., 2023; Koltenbah, 2023; Chufarin et al., 2023). By taking the
midpoint of the $\gtrsim 20.4$ mag limit on 2023 May 18.660 (${\rm
MJD}=60082.660$) from Mao et al. (2023) and the detection with $18.76\pm 0.25$
mag on 2023 May 18.826 (${\rm MJD}=60082.826$) from Chufarin et al. (2023), we
estimate the epoch of SN first light (i.e., the moment when the SN light
emerges from the stellar/CSM surface)111This estimate is somewhat limited by
the instrument sensitivity since the magnitude limit of $20.4$ corresponds to
a luminosity limit of $\sim 2\times 10^{39}$ erg s-1, roughly five times the
estimated progenitor luminosity (Neustadt et al., 2023). We assume that the SN
light reached the luminosity limit from the progenitor luminosity within the
error in the epoch of SN first light ($\approx 2$ hr). to be ${\rm
MJD}=60082.743\pm 0.083$, corresponding to $0.98$ days before our discovery.

Perley et al. (2023) obtained an optical spectrum of SN 2023ixf on 2023 May
19.93 (${\rm MJD}=60083.93$; 4.9 hr after discovery) with the SPectrograph for
the Rapid Acquisition of Transients (SPRAT; Piascik et al. 2014) on the
Liverpool Telescope (LT; Steele et al. 2004), classifying it as an SN II with
flash ionization features. Subsequent spectra have been obtained and reported
by Teja et al. (2023a) with the Himalaya Faint Object Spectrograph (HFOSC) on
the 2 m Himalayan Chandra Telescope
(HCT),222http://www.iiap.res.in/iao/hfosc.html Stritzinger et al. (2023) with
the Alhambra Faint Object Spectrograph and Camera (ALFOSC) on the Nordic
Optical Telescope (NOT), Zhang et al. (2023) with the Beijing Faint Object
Spectrograph and Camera (BFOSC) on the Xinglong 2.16 m Telescope (Fan et al.,
2016), and Yaron (2023) with the Spectral Energy Distribution Machine (SEDM;
Blagorodnova et al. 2018) on the Palomar 60-inch telescope, confirming the SN
II classification.

In this work, we adopt a redshift of $z=0.000804\pm 0.000007$ (de Vaucouleurs
et al., 1991)333Via the NASA/IPAC Extragalactic Database:
http://ned.ipac.caltech.edu/ and luminosity distance of $d_{L}=6.9\pm 0.1$ Mpc
(distance modulus of $\mu=29.194\pm 0.039$ mag; Riess et al. 2022) to M101. We
use the estimated first light (${\rm MJD}=60082.743\pm 0.083$) as the
zeropoint reference for all phases unless otherwise specified.

##  3 Observations and Data Reduction

###  3.1 Photometry

Following the discovery, we continued monitoring SN 2023ixf with the Itagaki
Astronomical Observatory 0.35, 0.5, and 0.6 m telescopes (Kochi, Okayama, and
Yamagata, Japan) + unfiltered KAF-1001E CCD. Using our custom software, the
aperture photometry was extracted and calibrated to Vega magnitudes from the
Fourth US Naval Observatory CCD Astrograph Catalog (Zacharias et al., 2013).
Additionally, we include the prediscovery amateur points listed in §2 in the
subsequent analysis.

Through the Global Supernova Project (Howell & Global Supernova Project,
2017), we obtained Las Cumbres Observatory (LCO; Brown et al. 2013) UBVgriz-
band imaging with the SBIG and Sinistro cameras on the network of 0.4 and 1.0
m telescopes at the Haleakal$\bar{\text{a}}$ Observatory (Hawaii, USA),
McDonald Observatory (Texas, USA), and Teide Observatory (Canary Islands,
Spain) starting on 2023 May 19.97 (${\rm MJD}=60083.97$; 5.8 hr after
discovery). The initial photometry up to 2023 May 30 (${\rm MJD}=60094$) has
been presented in Hosseinzadeh et al. (2023), and we extend it to 2023 June 18
(${\rm MJD}=60113$; 30 days after discovery) following the same reduction
procedures; UBV and griz point-spread function (PSF) photometry was calibrated
to Vega (Landolt, 1983, 1992) and AB (SDSS Collaboration et al., 2017)
magnitudes, respectively, using
lcogtsnpipe444https://github.com/LCOGT/lcogtsnpipe (Valenti et al., 2016).

In addition, we triggered the Neil Gehrels Swift Observatory
Ultraviolet/Optical Telescope (UVOT) follow-up imaging starting on 2023 May
20.27 (${\rm MJD}=60084.27$; 13 hr after discovery). The aperture photometry
was conducted and calibrated to Vega magnitudes using the pipeline for the
Swift Optical Ultraviolet Supernova Archive (SOUSA; Brown et al. 2014),
including the zero-points from Breeveld et al. (2011) and the sensitivity
correction from 2020 September. The initial photometry up to 2023 May 21
(${\rm MJD}=60085$) has been presented in Hosseinzadeh et al. (2023), and we
extend it to 2023 June 9 (${\rm MJD}=60104$). Unfortunately, the majority of
the early Swift UVOT observations were saturated, and we only include the
unsaturated observations analyzed by the standard SOUSA procedures.

Through our FLEET program (Gomez et al., 2020, 2023), we also obtained griz-
band imaging with KeplerCam (Szentgyorgyi et al., 2005) on the 1.2 m Telescope
at the Fred Lawrence Whipple Observatory (FLWO; Arizona, USA) starting on 2023
May 21.32 (${\rm MJD}=60085.32$; 1.59 days after discovery). The PSF
photometry was extracted and calibrated to AB magnitudes from the Pan-STARRS1
(Chambers et al., 2016) Data Release 2 (Flewelling et al., 2020).

To explore possible pre-explosion variability of SN 2023ixf, we also processed
and examined the Zwicky Transient Facility (ZTF; Bellm et al. 2019; Graham et
al. 2019), Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al.
2018; Smith et al. 2020), and Wide-field Infrared Survey Explorer (WISE;
Wright et al. 2010; Mainzer et al. 2014) survey data. ZTF and ATLAS photometry
were retrieved, respectively, from the ZTF forced-photometry
service555https://ztfweb.ipac.caltech.edu/cgi-bin/requestForcedPhotometry.cgi
(Masci et al., 2019, 2023) in the $g$, $r$, and $i$ bands (from 2018 March 21;
${\rm MJD}=58198$) and the ATLAS forced photometry
server666https://fallingstar-data.com/forcedphot/ (Shingles et al., 2021) in
the $c$ and $o$ bands (from 2015 December 30; ${\rm MJD}=57386$). Then, we
stacked them with a time window of every 10 days ($\lesssim 1\%$ of the
observed progenitor period; Kilpatrick et al. 2023; Jencson et al. 2023;
Soraisam et al. 2023) for each filter to obtain deeper measurements.

For the ongoing NEOWISE all-sky survey in the W1 ($3.4$ $\mu$m) and W2 ($4.5$
$\mu$m) bands, we retrieved time-resolved coadded images of the field created
as part of the unWISE project (Lang, 2014; Meisner et al., 2018; UnWISE Team,
2021). With the custom code (De et al., 2020) based on the ZOGY algorithm
(Zackay et al., 2016), we performed image subtraction on the NEOWISE images
using the full-depth coadds of the WISE and NEOWISE mission (obtained during
2010–2014) as reference images. Photometric measurements were obtained by
performing forced PSF photometry at the transient position on the subtracted
WISE images until the epoch of the unWISE data release (data acquired until
2022 May).

###  3.2 Spectroscopy

Through our FLEET program, we obtained optical spectra of SN 2023ixf with the
FAST spectrograph (Fabricant et al., 1998; Tokarz & Roll, 1997) on the FLWO
1.5 m Telescope starting on 2023 May 21.33 (${\rm MJD}=60085.33$; 1.60 days
after discovery). The combination of the 300 grating with a $3"$-wide slit was
used for dispersion, resulting in a wavelength coverage of
$4000\,\textup{\AA}$ with a resolution $R\approx 1000$. One-dimensional
spectra were extracted, reduced, and calibrated following standard procedures
using PyRAF and flux-calibrated to a standard taken during the same night.
Additionally, we retrieved the public LT/SPRAT, HCT/HFOSC, NOT/ALFOSC,
Xinglong/BFOSC, and P60/SEDM spectra (listed in §2) via the Transient Name
Server (TNS)777https://www.wis-tns.org/ and include them in the subsequent
analysis.

We also obtained high-resolution optical spectra on 2023 May 25 and 26 (${\rm
MJD}=60089$ and $60090$; 5.6 and 6.5 days after discovery, respectively) with
the Tillinghast Reflector Echelle Spectrograph (TRES; Szentgyorgyi & Furész
2007) on the FLWO 1.5 m Telescope ($3850-9100\,\textup{\AA}$ coverage with
$R\approx 44$,$000$), and on 2023 May 27, 28, and 29 (${\rm MJD}=60091$,
$60092$, and $60093$; 7.4, 8.4, and 9.4 days after discovery, respectively)
with Hectoechelle (Szentgyorgyi et al., 1998) on the 6.5 m MMT Observatory
($6460-6660\,\textup{\AA}$ coverage centered on H$\alpha$ with $R\approx
34$,$000$). The reduced data products were provided by the Smithsonian
Astrophysical Observatory Optical/Infrared Telescope Data Center (Mink et al.,
2005, 2007; Buchhave et al., 2010; Mink, 2011). Given the lack of narrow flash
features in the TRES and Hectoechelle spectra,888This is due to the
combination of a low signal-to-noise ratio (the first TRES spectrum) and a
late phase coverage (the later TRES and Hectoechelle spectra). we only used
them to measure the equivalent widths (EWs) of Na i D lines at the M101
redshift, yielding ${\rm EW(D1)}=0.121\,\textup{\AA}$ and ${\rm
EW(D2)}=0.180\,\textup{\AA}$. Using the Poznanski et al. (2012) calibration,
these EWs translate to an M101 extinction of $E(B-V)=0.032$ mag, in agreement
with Lundquist et al. (2023) and Smith et al. (2023).

##  4 Analysis

Figure 2: Left: pre-explosion differential flux measurements at the SN 2023ixf
position in the ZTF g,r,i bands; ATLAS c,o bands; and WISE W1,W2 bands. Error
bars denote $1\sigma$ uncertainties, and filled and open symbols are used for
detections and $5\sigma$ limits, respectively. No significant variability
($>5\sigma$) is seen in the ZTF and ATLAS bands, except the two possible ZTF
i-band detections. The few sporadic WISE W1 detections are due to imperfect
subtraction of the nearby H ii region (Figure 1). Right: multiband light curve
(top) and $U-V$ color evolution (bottom) of SN 2023ixf. Amateur detections and
limits are shown as the open plus signs and downward-pointing triangles,
respectively, with no vertical offset. The gray vertical dashed lines mark the
epochs of flash spectroscopy (Figure 3). SN 2023ixf is characterized by a
rapid rise to the peak with blueward color evolution and a following bright
plateau with redward color evolution. (The data used to create this figure
will be available upon publication.)  Figure 3: Spectral time series of SN
2023ixf. The phase of each spectrum with respect to first light is noted on
the right side. The prominent flash features of H, He i, He ii, C iii, C iv, N
iii, and N iv persist up to 5.6 days, after which a blue featureless continuum
dominates the spectra. A transition to a higher ionization state (i.e., C iii
$\rightarrow$ C iv) happens during the first two spectra, in line with the
blueward $U-V$ color evolution (Figure 2). (The data used to create this
figure will be available upon publication.)

All photometry and spectroscopy of SN 2023ixf are presented in Figures 2 and
3, respectively. We correct the photometry and spectroscopy for the Milky Way
(MW) extinction of $E(B-V)=0.0089$ mag (Schlafly & Finkbeiner, 2011),999Via
the NASA/IPAC Infrared Science Archive:
https://irsa.ipac.caltech.edu/applications/DUST/ as well as the M101
extinction of $E(B-V)=0.032$ mag (measured in §3), assuming the Fitzpatrick
(1999) reddening law with $R_{V}=3.1$ and extended to the WISE bands with the
relative optical to infrared extinction values from Wang & Chen (2019).

###  4.1 Light-curve Evolution

We determine the detection significance ($\sigma$) of the ZTF, ATLAS, and WISE
forced photometry from the ratio of measured flux ($f$) to its uncertainty
($f_{\rm err}$), i.e., $\sigma=|f|/f_{\rm err}$, where the absolute value of
$f$ is taken to account for both positive and negative variability. As shown
in Figure 2, no significant pre-explosion variability ($\sigma>5$) is seen at
the SN 2023ixf position in ZTF ($m_{g,r,i}\gtrsim 21.8,22.0,21.3$) and ATLAS
($m_{c,o}\gtrsim 20.2,20.3$) within $\approx 2700$ days before first light
(except the two possible ZTF i-band detections at $22.5\pm 0.2$ mag
($\sigma=6.1$) and $22.6\pm 0.2$ mag ($\sigma=5.3$) from $\approx 1205$ to
$1195$ days before first light, which may allow short-term variability). This
is consistent with the lack of long-term pre-explosion variability reported by
Neustadt et al. (2023) in the deep Large Binocular Telescope (LBT) UBVR-band
observations ($m\gtrsim 25-26$) from $\approx 5600$ to $400$ days before first
light and extends such a coverage to $0.32$ days before first light, albeit
with the shallower limits. The few sporadic (nonperiodic) detections in the
WISE $W1$ band ($\sim\pm 25$ uJy) are much higher than the variability
($\sim\pm 8$ uJy) seen in the Spitzer/IRAC Chanel 1 ($3.6\,\mu{\rm m}$)
observations with a similar temporal coverage ($\sim 11.3–3.6$ yr before
explosion; Kilpatrick et al. 2023; Jencson et al. 2023; Soraisam et al. 2023),
and visual inspections of the WISE difference images indicate imperfect image
subtraction of the nearby H ii region (Figure 1) due to its relatively large
PSF size (${\rm FWHM}\sim 6"$). Thus, we do not consider these as significant
detections.

The light curve of SN 2023ixf is characterized by a rapid rise to a bright
peak, from $M_{V}\approx-10$ to $-18.2$ mag within $\approx 5$ days of first
light (Figure 2), with a possible deviation from a single power-law rise in
the first $\approx 1$ day as noted by Hosseinzadeh et al. (2023). During this
rising phase, the $U-V$ color shows blueward evolution101010Similar blueward
color evolution is also seen in the other bands (e.g., $g-r$), indicating a
change in the continuum rather than in specific lines. from $-0.86\pm 0.03$
mag (1.26 days) to $-1.01\pm 0.04$ mag (3.39 days). Assuming a blackbody
spectral energy distribution (SED), this color evolution corresponds to a
temperature rise from $\approx 12$,$000$ to $16$,$000$ K. Such
color/temperature evolution is not expected from pure shock cooling, and
instead suggests a possible delayed shock breakout through dense CSM, in which
a photosphere initially forms inside the unshocked optically thick CSM (e.g.,
Moriya et al. 2018; Förster et al. 2018; Haynie & Piro 2021; Khatami & Kasen
2023), as similarly seen in SN II 2018zd (Hiramatsu et al., 2021b).

Following the bright peak, the light curve of SN 2023ixf settles on a plateau
($M_{V}\approx-17.6$ mag) extending to 30 days with a smooth decline rate of
$\approx 0.03$ mag day-1 from the peak (Figure 2). This plateau is on the
bright and fast-declining ends of the SN II population (e.g., Anderson et al.
2014; Valenti et al. 2016; see also Jacobson-Galan et al. 2023; Teja et al.
2023b; Sgro et al. 2023). Coinciding with the peak, the $U-V$ color shows a
transition to redward evolution, reaching $\approx 0.5$ mag ($\approx 6000$ K)
at 30 days. In contrast to the early blueward evolution, this redward
evolution is typical of the SN II population (e.g., Valenti et al. 2016),
likely suggesting a switch in the location of the photosphere to the SN ejecta
as it overruns the dense CSM. If we assume a typical SN shock velocity of
$\sim 10$,$000-15$,$000$ km s-1 (e.g., Chevalier & Fransson 1994; Matzner &
McKee 1999), the $U-V$ color transition at $3.4-4.4$ days corresponds to a CSM
radial extent of $\sim(3-6)\times 10^{14}$ cm.

To extract CSM properties from the light-curve modeling in §5, we construct a
pseudobolometric light curve of SN 2023ixf by fitting a blackbody SED to every
epoch of photometry containing at least three filters obtained within 30
minutes of each other and then integrating the fitted blackbody SED over the
optical coverage (UBVRI: $3250-8900\textup{\AA}$). For the unfiltered Itagaki
photometry with a similar wavelength coverage ($\approx
3200-9900\textup{\AA}$), we estimate its pseudobolometric correction at each
epoch by linearly extrapolating or interpolating the blackbody temperatures
from the multiband fits and taking the ratio of the integrated blackbody to
observed flux convolved with the CCD response function. For the pre-discovery
amateur points, we crudely estimate their pseudobolometric luminosity assuming
the same correction as the Itagaki discovery point, albeit with
uncharacterized CCD/filter response functions. We note that the observed SED
peaks are bluer than the $U$ band in the first $\approx 10$ days ($\gtrsim
10$,$000$ K), and without reliable Swift UVOT coverage (§3.1) during this
phase, the fitted blackbody temperatures may be underestimated by up to $\sim
10$,$000$ K (e.g., Valenti et al. 2016; Arcavi 2022), which translates to a
factor of $\sim 2$ in the pseudobolometric luminosity.

###  4.2 Spectral Evolution

As shown in Figure 3, the early spectral evolution of SN 2023ixf is
characterized by the flash features of H, He i, He ii, C iii, C iv, N iii, and
N iv on top of a blue continuum (see also Yamanaka et al. 2023; Jacobson-Galan
et al. 2023; Smith et al. 2023; Bostroem et al. 2023; Teja et al. 2023b;
Vasylyev et al. 2023). The N iii $\lambda\lambda\,4634,4641$ and C iii
$\lambda\lambda\,4647,4650$ complex is comparable in line strength to He ii
$\lambda\,4686$ in the first spectrum at 1.19 days but then quickly fades in
the second spectrum at 2.13 days. Similarly, C iii $\lambda\,5696$ is
comparable to C iv $\lambda\lambda\,5801,5812$ at 1.19 days but fades at 2.13
days. This transition to a higher ionization state (C iii $\rightarrow$ C iv)
likely indicates a rise in temperature if we assume a smooth CSM density
profile with constant elemental abundance (Boian & Groh, 2019, 2020), as
similarly seen in SN II 2018zd (Hiramatsu et al., 2021b). This is consistent
with the possible delayed shock breakout seen in the $U-V$ color evolution
(Figure 2).

The prominent flash features of He ii $\lambda\,4686$ and C iv
$\lambda\lambda\,5801,5812$ persist until $\approx 5.6$ and $4.5$ days,
respectively (Figure 3). Using these timescales as an alternate proxy to the
color evolution111111These two estimates do not agree perfectly, likely due to
the difference in the optical depths they probe (i.e., optically thick
photosphere and optically thin line emission; e.g., Morozova et al. 2020). for
the SN ejecta overrunning the dense CSM and assuming the same SN shock
velocity as in §4.1, a radial CSM extent can be estimated as $\sim(4-7)\times
10^{14}$ cm. Together with the $U-V$ color evolution, the flash spectral
series indicates a confined CSM, broadly in agreement with the estimates from
Jacobson-Galan et al. (2023), Smith et al. (2023), Bostroem et al. (2023), and
Teja et al. (2023b).

##  5 Light-curve Modeling with CSM Interaction

Motivated by the early light curve (§4.1) and spectral evolution (§4.2), we
consider two different mass-loss scenarios for producing the confined CSM:
continuous and eruptive.

###  5.1 Continuous Mass Loss

As a first scenario, we explore “superwind” mass loss (e.g., Moriya et al.
2018; Förster et al. 2018) in which the continuous mass-loss rate is enhanced
in the final years to decades before explosion by a few orders of magnitude
($\gtrsim 10^{-3}\,M_{\odot}\,{\rm yr}^{-1}$) compared to a typical RSG mass-
loss rate ($\sim 10^{-5}\,M_{\odot}\,{\rm yr}^{-1}$; e.g., Goldman et al.
2017; Beasor et al. 2020). We assume a constant wind velocity of $v_{\rm
wind}=115$ km s-1 inferred by Smith et al. (2023) from the narrow H$\alpha$
line profile in a high-resolution spectrum taken at 2.62 days. As noted by
Smith et al. (2023), this velocity is higher than a typical RSG wind velocity
of $\sim 20$ km s-1, and likely requires radiative acceleration (Moriya et
al., 2018; Tsuna et al., 2023a) or eruptive mass loss (§5). With the estimated
CSM radial extent of $\sim(3-7)\times 10^{14}$ cm from the $U-V$ color and
flash spectral evolution (§4), the velocity gives a mass-loss duration of
$\sim 0.8-1.9$ yr before explosion, which we explore here.

A broad range of possible masses, $8-20\,M_{\odot}$, has been reported for the
progenitor of SN 2023ixf from pre-explosion imaging (Pledger & Shara, 2023;
Kilpatrick et al., 2023; Jencson et al., 2023; Soraisam et al., 2023). As a
base model, we use an RSG model with a low mass ($M_{\rm ZAMS}=12\,M_{\odot}$
and $M_{\rm final}=11\,M_{\odot}$) and large radius ($907\,R_{\odot}$) from
Goldberg & Bildsten (2020) given the observed bright light-curve plateau
(§4.1). We note that according to the scaling relations for SN II light-curve
plateaus (e.g., Popov 1993; Kasen & Woosley 2009; Sukhbold et al. 2016;
Goldberg et al. 2019), these progenitor properties are degenerate with the
explosion energy, which we vary to account for the uncertainties in the
progenitor properties. A better characterization of the progenitor properties
from light-curve modeling should be explored when the light curve is sampled
to the radioactive tail. We also note that CSM estimates are less sensitive to
a particular choice of degenerate parameters that result in a comparable CSM-
free light curve (e.g., Goldberg & Bildsten 2020; Hiramatsu et al. 2021b;
Khatami & Kasen 2023).

Figure 4: Comparison of the pseudobolometric light curve of SN 2023ixf with
CSM-free light-curve models with varying explosion energies ($E_{\rm exp}$;
top left), and continuous mass-loss light-curve models with a single $E_{\rm
exp}$ ($1.0\times 10^{51}$ erg) and CSM velocity ($v_{\rm wind}=115$ km s-1):
two fixed mass-loss rates ($\dot{M}_{\rm wind}$) with varying mass-loss
durations ($t_{\rm wind}$; top right), two fixed $t_{\rm wind}$ with varying
$\dot{M}_{\rm wind}$ (bottom left), and a zoomed-in version to the early phase
(bottom right). The phase is shifted with respect to the Itagaki discovery.
CSM-free and minimal-CSM models are unable to reproduce the luminous
peak/plateau up to $20$ days, and the luminosity excess requires $\dot{M}_{\rm
wind}=0.3-1\,M_{\odot}\,{\rm yr}^{-1}$ for $t_{\rm wind}=2-1$ yr before
explosion. The mismatch in the prediscovery excess may suggest a decreasing
$\dot{M}_{\rm wind}$ toward the explosion.

We use a combination of MESA (Paxton et al., 2011, 2013, 2015, 2018, 2019) and
STELLA (Blinnikov et al., 1998, 2000; Blinnikov & Sorokina, 2004; Baklanov et
al., 2005; Blinnikov et al., 2006) for SN explosion and light-curve
calculations, respectively. For the MESA explosion models, we vary the
explosion energy ($E_{\rm exp}=(1.0-2.0)\times 10^{51}$ erg with $0.2\times
10^{51}$ erg increments), with a single value for the synthesized 56Ni mass
($M_{\rm Ni}=0.1\,M_{\odot}$), as it has no significant effect on the early
light-curve evolution (e.g., Kasen & Woosley 2009; Goldberg et al. 2019;
Kozyreva et al. 2019; Hiramatsu et al. 2021a). We then use the output of these
explosion models as an input to STELLA by adding a wind density profile,
$\rho_{\mathrm{wind}}(r)=\dot{M}_{\mathrm{wind}}/4\pi r^{2}v_{\mathrm{wind}}$,
with varying mass-loss rates ($\dot{M}_{\mathrm{wind}}=10^{-4}-1\,M_{\odot}$
yr-1 with $0.5$ dex increments) and durations ($t_{\mathrm{wind}}=0.5-2.0$ yr
with $0.5$ yr increments); the CSM mass is given by $M_{\rm
CSM}=\dot{M}_{\mathrm{wind}}t_{\mathrm{wind}}$. We use 700 and 100 spatial
zones for the SN ejecta and CSM, respectively, and 100 frequency bins. A more
detailed description of the MESA+STELLA workflow can be found in previous
works (e.g, Paxton et al. 2018; Goldberg et al. 2019; Hiramatsu et al. 2021a).

In Figure 4, the pseudobolometric light curves of both CSM-free models and a
subset of CSM models are compared with that of SN 2023ixf. In these model
comparisons, we reference the phase with respect to the Itagaki discovery (the
first point with the known CCD characteristic; §4.1) and shift the model phase
to match the discovery point since the “first light” is not well defined in
the models. The CSM-free models fail to reproduce the first $\approx 20$ days
of light-curve evolution (Figure 4, top left), which highlights the need for
CSM interaction.

In the following CSM model comparison, we choose an explosion energy of
$E_{\rm exp}=1.0\times 10^{51}$ erg, as its CSM-free model provides the best
fit at 30 days after discovery when the effect of CSM interaction is expected
to become less significant (but we do not intend to constrain $E_{\rm exp}$
given its degeneracy with the progenitor properties). For a given mass-loss
rate of $\dot{M}_{\mathrm{wind}}=1.0\,M_{\odot}\,{\rm yr}^{-1}$, the peak
luminosity is reasonably well reproduced by a mass-loss duration of $t_{\rm
wind}=1.0$ yr (Figure 4, top right). A longer (shorter) $t_{\rm wind}$ results
in a peak that is too wide (narrow). For
$\dot{M}_{\mathrm{wind}}=0.3\,M_{\odot}\,{\rm yr}^{-1}$, $t_{\rm wind}=2.0$ yr
better reproduces the peak luminosity. With a higher explosion energy of
$E_{\rm exp}=2.0\times 10^{51}$ erg, these mass-loss estimates are reduced by
$\approx 0.5$ dex (i.e., $\dot{M}_{\mathrm{wind}}\approx
0.1-0.3\,M_{\odot}\,{\rm yr}^{-1}$ for $t_{\rm wind}=2.0-1.0$ yr), albeit with
an overshoot of the plateau luminosity at $>20$ days (as already seen in
Figure 4, top left). Below $\dot{M}_{\mathrm{wind}}=0.1\,M_{\odot}\,{\rm
yr}^{-1}$, we do not find any models with a peak luminosity $\gtrsim 3\times
10^{42}$ erg s-1 (even with $E_{\rm exp}=2.0\times 10^{51}$ erg) in this
confined CSM configuration with $v_{\rm wind}=115$ km s-1. Thus, we estimate a
range of mass-loss history to be $\dot{M}_{\rm wind}=0.1-1.0\,M_{\odot}\,{\rm
yr}^{-1}$ for $t_{\rm wind}=2.0-1.0$ yr to reproduce the peak luminosity.

For $t_{\rm wind}=1.0$ and $2.0$ yr, the effect of varying
$\dot{M}_{\mathrm{wind}}$ is shown in Figure 4 (bottom left). As discussed,
$\dot{M}_{\mathrm{wind}}\geq 0.3\,M_{\odot}\,{\rm yr}^{-1}$ is required to fit
the peak luminosity well, and $\dot{M}_{\mathrm{wind}}\lesssim
10^{-3}\,M_{\odot}\,{\rm yr}^{-1}$ has negligible effects on the peak
luminosity. However, one difficulty with
$\dot{M}_{\mathrm{wind}}=0.3-1.0\,M_{\odot}\,{\rm yr}^{-1}$ for $t_{\rm
wind}=2.0-1.0$ yr is that it overproduces the luminosity in prediscovery
phase. The zoomed-in version to this phase is shown in Figure 4 (bottom
right). For $t_{\rm wind}=1.0$ yr,
$\dot{M}_{\mathrm{wind}}=0.1\,M_{\odot}\,{\rm yr}^{-1}$ captures the early
excess (whose presence is first noted by Hosseinzadeh et al. 2023) down to
$\approx 0.6$ days before discovery and the following rise up to $\approx 1$
day after discovery, albeit with an overshoot of the deep upper limit at
$\approx 8\times 10^{38}$ erg s-1 by $\approx 0.3$ days. Therefore, we
speculate a time-variable mass-loss rate in the final $2-1$ years before
explosion in which a lower mass-loss rate ($\approx 0.01-0.1\,M_{\odot}\,{\rm
yr}^{-1}$) follows a higher one ($\approx 0.1-1\,M_{\odot}\,{\rm yr}^{-1}$)
within the explosion properties explored here.121212This may also be possible
with an increasing $v_{\rm wind}$, as
$\rho_{\mathrm{wind}}\propto\dot{M}_{\mathrm{wind}}/v_{\mathrm{wind}}$. But it
would create wind-to-wind collision, which should be explored in future work.
Assuming that the lower mass-loss rate is responsible for the first $\sim 2$
days of light-curve evolution after first light as the SN–CSM shock layer
propagates through, this transition might have happened at $\sim 0.7-0.4$ yr
before explosion (with an SN shock velocity of $\sim 10$,$000-15$,$000$ km
s-1). Then, the total CSM mass could be estimated as $\sim
0.1-0.7\,M_{\odot}$.

###  5.2 Eruptive Mass Loss

Another scenario usually considered for a dense CSM is eruptive mass loss
months to years before core collapse. Such outbursts are observed for a
significant fraction of SNe IIn (strongly interacting SNe with narrow H
emission lines throughout their evolution; Ofek et al. 2013, 2014; Margutti et
al. 2014; Strotjohann et al. 2021; Hiramatsu et al. 2023). While such
detections are limited for more standard SNe II (Kochanek et al. 2017; Johnson
et al. 2018; but see Jacobson-Galán et al. 2022), this scenario is recently
found to be favored over the superwind model from modeling of SN II
progenitors (Davies et al., 2022).

For this scenario, we follow and extend the methods of Tsuna et al. (2023a),
which used the open-source code CHIPS (Takei et al., 2022) to simulate mass
eruption, followed by SNEC (Morozova et al., 2015) for calculating the
bolometric light curve. The mass loss is assumed to be triggered by energy
injection at the base of the envelope, which forms a shock that propagates to
the surface and expels a part of the envelope. The model is characterized by
two parameters: the injected energy and the amount of time from injection to
core collapse. These control the mass, velocity, and extent of the CSM at core
collapse.

We extend the model grid from Tsuna et al. (2023a) while using the same RSG
progenitor as the earlier work evolved by MESA of 15 $M_{\odot}$ at ZAMS,
which is within the mass range inferred for the SN 2023ixf progenitor. The
progenitor has a mass of $12.8\,M_{\odot}$ and a radius of $670\,R_{\odot}$ at
the time of energy injection, with modeling of mixing length and stellar wind
outlined in Tsuna et al. (2023a). This progenitor choice is different from
that in §5.1; however, as noted, a variation in explosion energies should
account for the difference given the light-curve scaling relations. For the
injected energy, the CHIPS code uses the injected energy scaled to the binding
energy of the envelope ($\approx 4.86\times 10^{47}$ erg for this progenitor
star), defined in the code as $f_{\rm inj}$. A fraction of the envelope is
unbound for $f_{\rm inj}\gtrsim 0.2$, and we thus consider a range of values
$f_{\rm inj}=[0.2,0.25,0.3,0.4,0.5,0.6,0.7]$.

---
Figure 5: Optical light curves of interacting SNe for the eruptive mass-loss
model, color-coded by the normalized injected energy, $f_{\rm inj}$. The
dashed and dotted lines are explosions with CSM formed by energy injections 1
and 2 yr before the SN, respectively. The black dashed-dotted lines show the
case for the progenitor with no CSM. The green (black) solid lines in the left
panels are models with an extended CSM expelled 10 (5) yr before the SN with
$f_{\rm inj}=0.3\ (0.25)$. The data points from observations are the same as
in Figure 4.

We first simulate explosions with CSM models of 1 and 2 yr from energy
injection, motivated by the deep LBT limits on apparent pre-SN outbursts of
the progenitor up to about 400 days prior to the SN (Neustadt et al.,
2023).131313We note that it takes several months from energy injection for the
shock to propagate to the surface and be seen as an outburst. For the range of
$f_{\rm inj}$, the dense CSM expands in these 2 yr to radii of
$\approx(2-7)\times 10^{14}$ cm, with respective masses of $\approx
0.04-1.7\,M_{\odot}$. As in Tsuna et al. (2023a), we then attach the evolved
envelope to the core of the MESA model and simulate the explosion using SNEC.
We excise the inner $1.8\,M_{\odot}$ of the star and inject energy at the
inner $0.1\,M_{\odot}$ as a thermal bomb. We consider the final explosion
energy of $E_{\rm exp}=10^{51}$ and $2\times 10^{51}\ {\rm erg}$.

Using the bolometric luminosity output by SNEC, we assume that the emission is
thermalized at the photosphere and calculate the luminosities in the UBVRI
bands ($3250-8900$ Å). The resulting light curves for injections 1 and 2 yr
before the SN are shown as dashed and dotted lines in Figure 5, respectively;
a reference model with no CSM is shown with a dashed-dotted line. As in §5.1,
the model with no CSM has a monotonic rise and thus significantly
underestimates around the peak. The bright plateau of SN 2023ixf after the
peak is well reproduced by the cooling emission of the shocked CSM (e.g.,
Piro, 2015; Piro et al., 2021), for a range of $f_{\rm inj}\approx
0.25$–$0.4$. While no perfect match to observations is found, the 1 yr model
with $E_{\rm exp}=10^{51}$ erg appears to best reproduce both the rise
timescale and the plateau luminosity.141414For a fixed $f_{\rm inj}$, the CSM
extent generally scales with the escape velocity of the RSG progenitor. Thus,
the required timescale can change for different progenitor models, but this
change is expected to be of order unity. The masses of the CSM in these models
are $M_{\rm CSM}\approx 0.3$–$1\ M_{\odot}$, which agrees with the masses
estimated in §5.1.

A notable problem for the models is that they generally overpredict the
luminosity around the peak. One possible reason for this is a departure from
thermal equilibrium in the postshock region around breakout. In our CSM model,
the density sharply drops around the breakout radius, which can also
accelerate the shock. A low CSM density and, more importantly, a large shock
velocity ($\gtrsim 10^{4}$ km s-1) can both lead to deviation from thermal
equilibrium (Nakar & Sari, 2010; Svirski et al., 2012; Haynie & Piro, 2021),
which reduces the optical luminosity.

Another possibility for the eruptive case is a much earlier mass ejection,
where breakout from a less dense CSM powers the peak and continued interaction
powers the plateau. We explored models with longer intervals between the
eruption and SN and found that two models, with energy injection 10 (5) yr
before the SN with $f_{\rm inj}=0.3$ (0.25), reproduce the light curves better
around the peak than the $1$–$2$ yr models for $E_{\rm exp}=10^{51}$ erg.
These light curves are shown with solid lines in Figure 5. In these models,
the CSM extends to $\approx 10^{15}\ (5\times 10^{14})$ cm with mass of
$\approx 0.06\ (0.04)\,M_{\odot}$, which is consistent with the results of
Jacobson-Galan et al. (2023). However, as we explain in the next section, this
model of early eruption is less preferred, as it may face two challenges: the
nearly periodic pre-explosion variability (Kilpatrick et al., 2023; Jencson et
al., 2023; Soraisam et al., 2023) and the early X-ray detection (Grefenstette
et al., 2023).

###  5.3 Comparison to Other Observational Constraints

In both the continuous and eruptive mass-loss cases, we find that the dense
CSM ($0.1-1\,M_{\odot}$) confined within $(3-7)\times 10^{14}$ cm is able to
reproduce the early light-curve evolution. Here we discuss other observational
constraints obtained thus far and their implications for our mass-loss models.

The infrared light curve of the progenitor from 2012 is consistent with a
nearly regular periodic variability, with no evidence of an additional
precursor emission (Kilpatrick et al., 2023; Jencson et al., 2023; Soraisam et
al., 2023). The absence of change in dust optical depth between $5600$ and
$400$ days before explosion has independently constrained the potential
outburst to be $\lesssim 3-5$ times brighter than the progenitor’s original
luminosity of $\sim 10^{5}L_{\odot}=4\times 10^{38}$ erg s-1 (Neustadt et al.,
2023).

The continuous mass-loss scenario may result in precursor emission owing to
the sustained energy injection that generates the outflow. With the inferred
CSM properties, we estimate the luminosity of a possible precursor to be
$\lesssim 2\times 10^{39}$ erg s-1 from Matsumoto & Metzger (2022) in a dust-
free environment. This is within the allowed luminosity range without altering
the dust optical depth too much ($\tau_{V}=10-13$; Neustadt et al. 2023), and
such an obscured precursor is consistent with the pre-explosion limits
presented in Figure 2, as well as the deeper LBT limits (Neustadt et al.,
2023) if the mass loss happens within $\approx 400$ days before explosion
(i.e., outside the LBT coverage). If the mass loss is instead driven by
pulsation (e.g., Yoon & Cantiello 2010), a less luminous precursor is likely
expected, which may provide a more natural explanation for the periodic
progenitor variability.

For the eruptive mass-loss scenario, the precursor models of Tsuna et al.
(2023b) are dust-free and cannot be directly constrained from the pre-
explosion observations of SN 2023ixf. However, the precursor emission for
models with $f_{\rm inj}=0.25$ and $0.3$, which lasts for $\lesssim 1$ yr from
eruption, is only $\lesssim 1$ mag brighter (Tsuna et al., 2023b) and is thus
within the allowed luminosity range to avoid significant change of the dust
optical depth (Neustadt et al., 2023). A caveat is that for eruptions much
earlier than 2 yr before the SN, this scenario does not explain the observed
periodic variability after the eruption. An eruption perturbs and oscillates
the leftover envelope, which may cause the variability, but there is no
evident reason that both its timescale and amplitude would be similar to the
observed one. It is worthwhile to study the precursor emission in dusty
environments, as well as its later oscillation, to quantitatively constrain
this scenario.

The X-ray data obtained $4$ days after explosion suggest a rather tenuous CSM,
with a mass-loss rate of $\approx 3\times 10^{-4}\ \,M_{\odot}\,{\rm yr}^{-1}$
for an assumed CSM velocity of $50\ {\rm km\ s^{-1}}$ (Grefenstette et al.,
2023). Since the radius of the CSM probed is $\gtrsim(4-6)\times 10^{14}$ cm
for an SN shock velocity of $\sim 10$,$000-15$,$000$ km s-1, this does not
contradict our preferred models (both continuous and eruptive) of the compact,
dense CSM. However, this is in tension with an eruption $5-10$ yr before
explosion, whose density is higher than this by about an order of magnitude.
An asymmetric CSM may alleviate this problem, if the line of sight is not
subtended by the dense CSM inferred from the optical light curve. An
asymmetric CSM is also suggested from early spectra (Smith et al., 2023;
Vasylyev et al., 2023) and millimeter nondetections ($\lesssim$ mJy at 230
GHz; Berger et al. 2023), although the latter require the line of sight to be
in the dense part of the CSM assuming a single wind density profile.
Alternatively, the millimeter nondetections may be explained with our confined
CSM models in that the early nondetections ($\lesssim 4$ days) are due to
free-free absorption by the confined CSM, while the later nondetections are
due to weaker synchrotron emission from the underlying extended CSM ($\sim
10^{-4}$ to $10^{-5}\,M_{\odot}\,{\rm yr}^{-1}$). This merits further
investigations, especially with a recent fainter radio detection ($\approx
40\,\mu{\rm Jy}$ at 10 GHz) at 29 days after first light (Matthews et al.,
2023).

Estimates based on spectra (Jacobson-Galan et al., 2023; Bostroem et al.,
2023) find a CSM of $M_{\rm CSM}\approx 10^{-2}-0.1M_{\odot}$, much smaller
than our results of $0.1-1M_{\odot}$. This may be due to the light curve and
spectra probing different parts of the CSM (see e.g. Morozova et al. 2020 for
a similar discussion on SN II 2013fs). Indeed, the light-curve model from
Jacobson-Galan et al. (2023) with such a lower $M_{\rm CSM}$ underproduces the
luminosity in the first week of its evolution (see also Teja et al. 2023b for
a similar discrepancy even with extremely large explosion energies of
$(2-5)\times 10^{51}$ erg). Furthermore, the density profiles of our preferred
CSM models are different from a wind profile assumed in spectral modeling. As
in §5.1, a single wind profile has difficulties explaining both the light-
curve rise and plateau. The CSM from mass eruption is also expected to have a
profile much different from that of a wind (Kuriyama & Shigeyama, 2020;
Morozova et al., 2020; Tsuna et al., 2021; Tsang et al., 2022).

##  6 Summary

We have presented our discovery of SN 2023ixf in M101 at $6.9$ Mpc and follow-
up photometric and spectroscopic observations, respectively, in the first
month and week of its evolution. Our observations have revealed the following:

  * •

A rapid rise ($\approx 5$ days) to a luminous light-curve peak
($M_{V}\approx-18.2$ mag) and plateau ($M_{V}\approx-17.6$ mag) extending to
$30$ days after first light with a fast decline rate of $\approx 0.03$ mag
day-1.

  * •

Blueward $U-V$ color evolution in the first $4.4$ days toward the light-curve
peak followed by redward color evolution during the light-curve plateau.

  * •

Prominent flash spectral features of H, He i, He ii, C iii, C iv, N iii, and N
iv persisting up to $4.5-5.6$ days after first light.

  * •

A transition to a higher spectral ionization state (i.e., C iii $\rightarrow$
C iv) in the first $2.1$ days after first light.

These observed properties are not expected from pure shock-cooling emission,
and instead suggest a delayed shock breakout from a dense CSM confined within
$\sim(3-7)\times 10^{14}$ cm. Motivated by this, we have constructed numerical
light-curve models based on both continuous and eruptive mass-loss scenarios
shortly before explosion. Our key findings are as follows:

  * •

For the continuous mass-loss scenario, we infer a range of mass-loss history
with $0.1-1.0\,M_{\odot}\,{\rm yr}^{-1}$ in the final $2.0-1.0$ yr before
explosion, with a potentially decreasing mass loss of
$0.01-0.1\,M_{\odot}\,{\rm yr}^{-1}$ toward the explosion ($\lesssim 0.7-0.4$
years) to reproduce the rapid rise and luminous peak.

  * •

For the eruptive mass-loss scenario, we favor energy injections of $\sim
10^{47}$ erg at the base of the envelope in the final year before explosion,
which releases $0.3$–$1\ M_{\odot}$ of the envelope as CSM. The mass and
extent of the CSM broadly agree with the continuous scenario.

These confined, dense CSM models are compatible with the early X-ray
detections, millimeter nondetections, and possibly the progenitor variability
without clear outbursts before the final $\sim 1$ yr of explosion.

Given its proximity, SN 2023ixf will remain as a primarily SN target for
multiwavelength follow-up observations in the coming years. In ultraviolet to
infrared, a fully sampled light curve up to a radioactive tail will allow
better characterizations of the progenitor and explosion properties. Nebular-
phase spectra have potential to constrain the progenitor mass, possibly better
than the current pre-explosion estimates. Deviations from a smooth light-curve
plateau and/or tail, or continued X-ray and/or radio/millimeter emission, will
provide unique opportunities to probe the mass-loss history in the final years
to decades of its progenitor evolution.

##  7 acknowledgments

We are grateful to Anthony Piro, Jim Fuller, Ken’ichi Nomoto, Takashi Moriya,
and Charles Kilpatrick for useful discussions; to Warren Brown, Pascal Fortin,
Murdock Hart, David Latham, and Jessica Mink for scheduling and reducing FLWO
KeplerCam, FAST, and TRES observations; and to Nelson Caldwell, Daniel
Fabricant, and Sean Moran for scheduling the MMT Hectochelle observations.

The Berger Time-Domain research group at Harvard is supported by the NSF and
NASA. The LCO supernova group is supported by NSF grants AST-1911151 and
AST-1911225. D.T. is supported by the Sherman Fairchild Postdoctoral
Fellowship at the California Institute of Technology. The Flatiron Institute
is supported by the Simons Foundation. This publication was made possible
through the support of an LSSTC Catalyst Fellowship to K.A.B., funded through
grant 62192 from the John Templeton Foundation to LSST Corporation. The
opinions expressed in this publication are those of the authors and do not
necessarily reflect the views of LSSTC or the John Templeton Foundation. I.A.
acknowledges support from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation program (grant agreement
No. 852097), from the Israel Science Foundation (grant No. 2752/19), from the
United States - Israel Binational Science Foundation (BSF), and from the
Israeli Council for Higher Education Alon Fellowship. J.C.W. and J.V. are
supported by NSF grant AST1813825. J.V. is also supported by by OTKA grant
K-142534 of the National Research, Development and Innovation Office, Hungary.

This work makes use of observations from KeplerCam on the 1.2 m telescope and
FAST and TRES on the 1.5 m telescope at the Fred Lawrence Whipple Observatory.
Observations reported here were obtained at the MMT Observatory, a joint
facility of the Smithsonian Institution and the University of Arizona. This
paper uses data products produced by the OIR Telescope Data Center, supported
by the Smithsonian Astrophysical Observatory.

This work makes use of observations from the Las Cumbres Observatory global
telescope network. The authors wish to recognize and acknowledge the very
significant cultural role and reverence that the summit of
Haleakal$\bar{\text{a}}$ has always had within the indigenous Hawaiian
community. We are most fortunate to have the opportunity to conduct
observations from the mountain.

We thank the support of the staffs at the Neil Gehrels Swift Observatory.

This work has made use of data from the Zwicky Transient Facility (ZTF). ZTF
is supported by NSF grant No. AST-1440341 and a collaboration including
Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at
Stockholm University, the University of Maryland, the University of
Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los
Alamos National Laboratories, the TANGO Consortium of Taiwan, the University
of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories.
Operations are conducted by COO, IPAC, and UW. The ZTF forced-photometry
service was funded under the Heising-Simons Foundation grant No. 12540303 (PI:
Graham).

This work has made use of data from the Asteroid Terrestrial-impact Last Alert
System (ATLAS) project. ATLAS is primarily funded to search for near-Earth
asteroids through NASA grant Nos. NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575;
byproducts of the NEO search include images and catalogs from the survey area.
This work was partially funded by Kepler/K2 grant No. J1944/80NSSC19K0112 and
HST grant No. GO-15889, and STFC grant Nos. ST/T000198/1 and ST/S006109/1. The
ATLAS science products have been made possible through the contributions of
the University of Hawaii Institute for Astronomy, the Queen’s University
Belfast, the Space Telescope Science Institute, the South African Astronomical
Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.

The PS1 and the PS1 public science archives have been made possible through
contributions by the Institute for Astronomy, the University of Hawaii, the
Pan-STARRS Project Office, the Max-Planck Society and its participating
institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max
Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins
University, Durham University, the University of Edinburgh, the Queen’s
University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las
Cumbres Observatory Global Telescope Network Incorporated, the National
Central University of Taiwan, the Space Telescope Science Institute, NASA
under grant No. NNX08AR22G issued through the Planetary Science Division of
the NASA Science Mission Directorate, NSF grant No. AST-1238877, the
University of Maryland, Eotvos Lorand University, the Los Alamos National
Laboratory, and the Gordon and Betty Moore Foundation.

This publication makes use of data products from the Wide-field Infrared
Survey Explorer, which is a joint project of the University of California, Los
Angeles, and the Jet Propulsion Laboratory/California Institute of Technology,
funded by the National Aeronautics and Space Administration.

This research has made use of the NASA Astrophysics Data System (ADS), the
NASA/IPAC Extragalactic Database (NED), and NASA/IPAC Infrared Science Archive
(IRSA, which is funded by NASA and operated by the California Institute of
Technology), and IRAF (which is distributed by the National Optical Astronomy
Observatory, NOAO, operated by the Association of Universities for Research in
Astronomy, AURA, Inc., under cooperative agreement with the NSF).

TNS is supported by funding from the Weizmann Institute of Science, as well as
grants from the Israeli Institute for Advanced Studies and the European Union
via ERC grant No. 725161.

.

.

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