#  NEOWISE-R Caught the Luminous SN 2023ixf in Messier 101

Schuyler D. Van Dyk Caltech/IPAC, Mailcode 100-22, Pasadena, CA 91125, USA
Tamás Szalai Department of Experimental Physics, Institute of Physics,
University of Szeged, Dóm tér 9, Szeged, 6720, Hungary  MTA-ELTE Lendület
”Momentum” Milky Way Research Group, Hungary  Roc M. Cutri Caltech/IPAC,
Mailcode 100-22, Pasadena, CA 91125, USA  J. Davy Kirkpatrick Caltech/IPAC,
Mailcode 100-22, Pasadena, CA 91125, USA  Carl J. Grillmair Caltech/IPAC,
Mailcode 314-6, Pasadena, CA 91125, USA  Sergio B. Fajardo-Acosta
Caltech/IPAC, Mailcode 100-22, Pasadena, CA 91125, USA  Joseph R. Masiero
Caltech/IPAC, Mailcode 100-22, Pasadena, CA 91125, USA  Amy K. Mainzer
University of Arizona, 1629 E. University Boulevard, Tucson, AZ 85721, USA
Department of Earth, Planetary, and Space Sciences, The University of
California, Los Angeles, 595 Charles E. Young Drive East, Los Angeles, CA
90095, USA  Christopher R. Gelino Caltech/IPAC, Mailcode 100-22, Pasadena, CA
91125, USA  József Vinkó Department of Experimental Physics, Institute of
Physics, University of Szeged, Dóm tér 9, Szeged, 6720, Hungary  HUN-REN CSFK
Konkoly Observatory, Konkoly Th. M. út 15-17, Budapest, 1121, Hungary  CSFK,
MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, Budapest, 1121,
Hungary  ELTE Eötvös Loránd University, Institute of Physics and Astronomy,
Pázmány Péter sétány 1/A, Budapest, 1117, Hungary  András Péter Joó HUN-REN
CSFK Konkoly Observatory, Konkoly Th. M. út 15-17, Budapest, 1121, Hungary
CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, Budapest, 1121,
Hungary  ELTE Eötvös Loránd University, Institute of Physics and Astronomy,
Pázmány Péter sétány 1/A, Budapest, 1117, Hungary  András Pál HUN-REN CSFK
Konkoly Observatory, Konkoly Th. M. út 15-17, Budapest, 1121, Hungary  CSFK,
MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, Budapest, 1121,
Hungary  Réka Könyves-Tóth HUN-REN CSFK Konkoly Observatory, Konkoly Th. M. út
15-17, Budapest, 1121, Hungary  CSFK, MTA Centre of Excellence, Konkoly Thege
Miklós út 15-17, Budapest, 1121, Hungary  Department of Experimental Physics,
Institute of Physics, University of Szeged, Dóm tér 9, Szeged, 6720, Hungary
ELTE E tv s Lor nd University, Gothard Astrophysical Observatory, Szombathely,
Hungary  Levente Kriskovics  HUN-REN CSFK Konkoly Observatory, Konkoly Th. M.
út 15-17, Budapest, 1121, Hungary  CSFK, MTA Centre of Excellence, Konkoly
Thege Miklós út 15-17, Budapest, 1121, Hungary  Róbert Szakáts HUN-REN CSFK
Konkoly Observatory, Konkoly Th. M. út 15-17, Budapest, 1121, Hungary  CSFK,
MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, Budapest, 1121,
Hungary  Krisztián Vida HUN-REN CSFK Konkoly Observatory, Konkoly Th. M. út
15-17, Budapest, 1121, Hungary  CSFK, MTA Centre of Excellence, Konkoly Thege
Miklós út 15-17, Budapest, 1121, Hungary  WeiKang Zheng Department of
Astronomy, University of California, Berkeley, CA 94720-3411, USA  Thomas G.
Brink Department of Astronomy, University of California, Berkeley, CA
94720-3411, USA  Alexei V. Filippenko Department of Astronomy, University of
California, Berkeley, CA 94720-3411, USA

###### Abstract

The reactivated Near-Earth Object Wide-field Infrared Survey Explorer
(NEOWISE-R) serendipitously caught the Type II supernova SN 2023ixf in Messier
101 on the rise, starting day 3.6 through day 10.9, and on the late-time
decline from days 211 through 213 and days 370 through 372. We have considered
these mid-infrared (mid-IR) data together with observations from the
ultraviolet (UV) through the near-IR, when possible. At day 3.6 we
approximated the optical emission with a hot, $\sim 26,630$ K blackbody, with
a notable UV excess likely from strong SN shock interaction with circumstellar
matter (CSM). In the IR, however, a clear excess is also obvious, and we fit
it with a cooler, $\sim 1,620$ K blackbody with radius of $\sim 2.6\times
10^{15}$ cm, consistent with dust in the progenitor’s circumstellar shell
likely heated by the UV emission from the CSM interaction. On day 10.8, the
light detected was consistent with SN ejecta-dominated emission. At late times
we also observed a clear NEOWISE-R excess, which could arise either from newly
formed dust in the inner ejecta or in the contact discontinuity between the
forward and reverse shocks, or from more distant pre-existing dust grains in
the SN environment. Furthermore, the large 4.6 $\mu$m excess at late times can
also be explained by the emergence of the carbon monoxide 1–0 vibrational
band. SN 2023ixf is the best-observed SN IIP in the mid-IR during the first
several days after explosion and one of the most luminous such SNe ever seen.

supernovae: general — supernovae: individual (SN 2023ixf) — stars: massive —
dust — circumstellar matter — infrared: stars

††facilities: NEOWISE, KAIT, Nickel, Shane, IRSA, RC80 (Konkoly)††software:
IRAF (Tody 1986; 1993), PyRAF (http://www.stsci.edu/institute/
software_hardware/pyraf)

##  1 Introduction

Supernova (SN) explosions are among the most powerful events in the Universe.
They serve as unique cosmic laboratories for studying processes in extreme
physical conditions and chemical feedback into the interstellar and
intergalactic media. Core-collapse supernovae (CCSNe), consequences of the
gravitational collapse of iron cores of massive ($\gtrsim 8\ M_{\odot}$)
stars, have been considered as possible sources of cosmic dust at high
redshifts for over $\sim 50$ yr (e.g., Cernuschi & Codina, 1967; Hoyle &
Wickramasinghe, 1970; Dwek et al., 2007). Observed dust in CCSNe may form
either in the (unshocked) ejecta or in a cold dense shell (CDS) across the
contact discontinuity between the shocked circumstellar matter (CSM) and
shocked ejecta. A late-time mid-infrared (mid-IR) excess may emerge from
either newly formed or heated pre-existing dust grains. In the shocked CSM,
heating can be collisional, and grains in the more distant, unshocked CSM are
assumed to be radiatively heated by the peak SN luminosity or by energetic
photons generated during CSM interaction (see, e.g., Gall et al. 2011 for a
review).

Observed properties and classifications of CCSNe, from H-rich to H-free
spectra (IIP, IIL, IIb, Ib/c; see Filippenko 1997 and Gal-Yam 2017 for
reviews), depend mainly on the degree of pre-explosion mass loss from the
progenitor star (and/or its companion in a binary system). H-rich Type II-
plateau (IIP) SNe, characterized by their $\sim 100$-day optical plateau-like
light curves, are the predominant CCSN subclass (corresponding to $\sim 55$%
of all CCSNe; Perley et al., 2020). Direct evidence exists that these SNe
arise from stars in the red supergiant (RSG) phase, with the star’s massive
hydrogen envelope remaining relatively intact at explosion (Smartt et al.,
2009; Smartt, 2015; Van Dyk, 2017). SNe IIP are known to form new dust in
their ejecta (see Gall et al. 2011 and references therein). Based on recent
model calculations, expanding SN IIP ejecta succeed in condensing sufficient
quantities (0.05–$1.0\ M_{\odot}$) of dust. Some of these models propose slow
and steady dust growth over several thousand days (e.g., Gall et al., 2014;
Wesson et al., 2015), while others suggest a more rapid dust growth (Dwek et
al., 2019; Sarangi, 2022). These theoretical expectations are also in
agreement with the far-infrared/submillimeter detection of a large amount of
cold ($\lesssim 50$ K) dust in hundreds-of-years-old Galactic SN remnants,
such as Cas A (Barlow et al., 2010; Sibthorpe et al., 2010; Arendt et al.,
2014) and the Crab Nebula (Gomez et al., 2012; Temim & Dwek, 2013; De Looze et
al., 2019), as well as the nearby ($\sim 50$ kpc) and famous SN 1987A
(Matsuura et al., 2011, 2019; Indebetouw et al., 2014). Very recently, the
James Webb Space Telescope (JWST) has offered a new opportunity to study the
late phases of cool ($\sim 100$–200 K) dust in extragalactic SNe and has
already led to the detection of a significant amount ($\gtrsim 10^{-3}\
M_{\odot}$) of dust in Type IIP SNe 2004et and 2017eaw (Shahbandeh et al.,
2023), and the Type IIL SN 1980K (Zsíros et al., 2024).

At the same time, only a handful of nearby young ($\lesssim$5 yr) SNe II
(primarily IIP) show direct observational evidence for dust condensation, and
these examples have all yielded two-to-three orders of magnitude less dust
($\sim 10^{-5}$–10${}^{-3}\ M_{\odot}$) than predicted by the models. Most of
these observations, however, were carried out in the wavelength range 3–$5\
\mu$m, and thus have been limited to just the warmer ($\gtrsim 500$ K) dust
grains. In the last quarter century, the primary source of mid-infrared (mid-
IR) data on SNe was NASA’s Spitzer Space Telescope, which resulted in valuable
data during both its cryogenic (2003–2009) and post-cryogenic (2009–2020)
missions. Except for several single-object studies — e.g., SN 1987A (Bouchet
et al., 2006; Dwek et al., 2010), SN 1993J (Zsíros et al., 2022), SN 1995N
(Van Dyk, 2013), SN 2003gd (Sugerman et al., 2006; Meikle et al., 2007), SN
2004dj (Szalai et al., 2011; Meikle et al., 2011), SN 2004et (Kotak et al.,
2009; Fabbri et al., 2011), SN 2005af (Kotak et al., 2006; Szalai & Vinkó,
2013), SN 2005ip (Fox et al., 2010), SN 2007it (Andrews et al., 2011), and SN
2007od (Andrews et al., 2010) — most of these Spitzer SN data were collected
in the post-cryogenic phase (at 3.6 and 4.5 $\mu$m). Studies during this phase
included either targeted surveys, such as the SPIRITS project (SPitzer
InfraRed Intensive Transients Survey, a systematic study of transients in
nearby galaxies; see Tinyanont et al., 2016; Kasliwal et al., 2017; Jencson et
al., 2019) and work focused on interacting SNe (Fox et al., 2011, 2013; Szalai
et al., 2021), or archival images for which the SNe were not the primary
target (Szalai et al., 2019). The latter work, including data from targeted
surveys, presents the most extensive analysis of mid-IR SN observations to
date, including $\sim 120$ positively detected objects from $\sim 1100$ SN
sites imaged by Spitzer.

Another very important tool for detecting early-time mid-IR radiation from SNe
has been the Wide-field Infrared Survey Explorer (WISE, both cryogenic and
post-cryogenic, 2009–2011; Wright et al. 2010). The post-cryogenic WISE
mission was reactivated in 2013 and has been monitoring the sky at 3.4 and 4.6
$\mu$m ever since, as the Near-Earth Object Wide-field Infrared Survey
Explorer Reactivation (NEOWISE-R, or NEOWISE for short; Mainzer et al. 2011,
2014). While the original aim of the reactivated mission is mainly
characterization of known Solar System objects, its database also serves as a
valuable source of information on a rich variety of transient objects, such as
cataclysmic variables, active galactic nuclei, tidal disruption events, and
SNe (e.g., Kokubo et al. 2019; Tartaglia et al. 2020; Sun et al. 2022; Moran
et al. 2023; Wang et al. 2024).

We as a community have been incredibly fortunate to have the recent, nearby SN
2023ixf occur in Messier 101 (M101; NGC 5457). Its proximity and brightness
have led to many investigators training various facilities at a range of
wavelengths at the event, which has exhibited a number of fascinating
properties. The SN was discovered by Itagaki (2023) on 2023 May 19.73 (UTC
dates are used throughout this paper) and classified as an SN II by Perley et
al. (2023) within hours of discovery. It was evident immediately that the
optical spectrum was dominated by “flash” emission features indicative of
interaction of the SN shock with pre-existing CSM (e.g., Jacobson-Galán et
al., 2023; Bostroem et al., 2023; Hiramatsu et al., 2023; Teja et al., 2023).
The SN’s light curves provided similar indications (e.g., Hosseinzadeh et al.,
2023; Hiramatsu et al., 2023; Martinez et al., 2024). Zimmerman et al. (2024),
from an analysis of early-time Hubble Space Telescope (HST) ultraviolet (UV)
spectroscopy of the SN, constrained the CSM to be dense and confined, with
$\sim 10^{-12}$ g cm-3 at $\lesssim 2\times 10^{14}$ cm; they concluded that
this dense CSM immediate to the progenitor prolonged the SN shock breakout by
$\sim 3$ d. Other indications of initial and longer-term CSM interaction for
SN 2023ixf come from observations at X-ray (Grefenstette et al., 2023; Chandra
et al., 2024) and radio (Berger et al., 2023) wavelengths.

A progenitor candidate was directly identified in archival HST, Spitzer, and
ground-based near-IR data (e.g., Pledger & Shara, 2023; Kilpatrick et al.,
2023; Jencson et al., 2023; Soraisam et al., 2023; Van Dyk et al., 2024).
These unprecedented data were plentiful enough that the star was shown in
astonishing detail to be a long-period variable, similar to what we expect for
many RSGs (Jencson et al., 2023; Soraisam et al., 2023). Additionally,
modeling of the star’s spectral energy distribution (SED), e.g., by Van Dyk et
al. (2024) revealed it to be quite dusty and luminous, and implied that the
star was surrounded by a dusty silicate-rich shell with an inner radius of
$\approx 10$ times the star’s radius, or $\approx 10^{15}$ cm.

Near-IR studies of SN 2023ixf have already been conducted and published
(Yamanaka et al., 2023; Teja et al., 2023), and others will likely emerge. The
SN has already been observed with the JWST, and those results are pending.
Here we describe and analyze observations by NEOWISE, which serendipitously
caught SN 2023ixf in the act between $\sim 3$ days and $\sim 372$ days in age.
This is among the earliest that an SN has been detected in the mid-IR (the
peculiar Type IIP SN 2009js was caught by Spitzer two days after discovery,
however, its explosion epoch is rather uncertain; Gandhi et al. 2013; Szalai
et al. 2019).

Following Hosseinzadeh et al. (2023), we have adopted 2023 May 18 18:00 UTC
(MJD 60082.75) as the explosion epoch. We assume throughout a distance to M101
of $6.85\pm 0.13$ Mpc (Riess et al., 2022).

##  2 Observations

###  2.1 NEOWISE

NEOWISE observed the SN site pre-explosion, as part of routine operations, 152
times between 2013 December 18.26 and 2022 December 18.99 UT (MJD 56644.2618
and 59931.9958, respectively). The first pre-SN pair of single exposures
occurred 3438.49 d prior to explosion, while the last was 150.75 d pre-SN. The
progenitor candidate was not detected in any of these observations (e.g.,
Hiramatsu et al., 2023; Jencson et al., 2023; Soraisam et al., 2023; Van Dyk
et al., 2024). See also Section A of this paper. The lack of detection tends
to rule out luminous eruptions or outbursts from the progenitor candidate
during that time period prior to explosion, as discussed in the above studies.
(The star was also not detected by WISE and the post-cryogenic NEOWISE prior
to reactivation, between 2009 and 2011; see Van Dyk et al. 2024.)

The SN itself was detected during the nineteenth sky coverage since the start
of the Reactivation Mission, from day 3.631 through 10.901 (2023 May 22.38
through May 29.65). We note that the gap of several days in the early-time
data is caused by a “Moon toggle” procedure, in which the spacecraft pointing
skips ahead to avoid the Moon, and then slews back to observe the “skipped”
area of sky (see Wright et al. 2010 for a description); in our case, this was
advantageous, as we were able to sample the SN’s evolution about a week after
the first detections. All of these data are publicly available via the
NASA/IPAC Infrared Science Archive (IRSA; https://irsa.ipac.caltech.edu/); see
Figure 1. The SN was then captured again at late times during the twenty-first
sky coverage, from day 211.736 through 213.351 (2023 December 16.49 through
18.10), and then again from day 370.875 through 372.476 (2024 May 23.63
through 25.23). Those data were still pre-release at the time of this writing
and were also obtained via IRSA. Given the separation of the SN from the
general environs of the neighboring giant H ii region NGC 5461, the detections
are relatively clean. The SN detections are listed in Table 1. The quantities
W1mpro and W2mpro are profile-fit photometry magnitudes at 3.4 and 4.6 $\mu$m
(bands W1 and W2), respectively, in the Vega system111See the NEOWISE Data
Release Explanatory Supplement,
https://wise2.ipac.caltech.edu/docs/release/neowise/expsup/., without any
further special processing or additional background subtraction applied. The
resulting light curves are shown in Figure 2.

Figure 1: Color-composite image of the NEOWISE-R detection of SN 2023ixf in
M101 at 3.4 and 4.6 $\mu$m (bands W1 and W2, respectively) in the day
3.361–4.995 range combined (see Table 1). The SN is indicated with tick marks.
The bright emission to the northeast of the SN is from the giant H ii region
complex NGC 5461.

Table 1: NEOWISE-R Observations of SN 2023ixf

MJD | Age | Scan | Frame | W1mpro | W1sigmpro | W2mpro | W2sigmpro
---|---|---|---|---|---|---|---
| (d) | ID | Num | (mag) | (mag) | (mag) | (mag)
60086.381 | 3.631 | 50719r | 232 | 11.352 | 0.020 | 11.287 | 0.026
60086.511 | 3.761 | 50723r | 159 | 11.301 | 0.022 | 11.262 | 0.036
60086.641 | 3.891 | 50727r | 232 | 11.296 | 0.019 | 11.212 | 0.028
60086.771 | 4.021 | 50731r | 232 | 11.241 | 0.020 | 11.309 | 0.026
60086.836 | 4.086 | 50733r | 207 | 11.278 | 0.021 | 11.233 | 0.026
60086.836 | 4.086 | 50733r | 208 | 11.276 | 0.023 | 11.296 | 0.027
60086.901 | 4.151 | 50735r | 231 | 11.225 | 0.020 | 11.183 | 0.028
60086.966 | 4.216 | 50737r | 207 | 11.247 | 0.019 | 11.220 | 0.031
60087.031 | 4.281 | 50739r | 232 | 11.260 | 0.018 | 11.146 | 0.025
60087.096 | 4.346 | 50741r | 208 | 11.258 | 0.020 | 11.219 | 0.028
60087.160 | 4.410 | 50743r | 157 | 11.208 | 0.018 | 11.198 | 0.026
60087.225 | 4.475 | 50745r | 157 | 11.245 | 0.021 | 11.156 | 0.025
60087.290 | 4.540 | 50747r | 232 | 11.228 | 0.018 | 11.178 | 0.026
60087.355 | 4.605 | 50749r | 208 | 11.220 | 0.020 | 11.196 | 0.026
60087.420 | 4.670 | 50751r | 232 | 11.200 | 0.020 | 11.169 | 0.026
60087.484 | 4.734 | 50753r | 157 | 11.354 | 0.020 | 11.451 | 0.027
60087.485 | 4.735 | 50753r | 158 | 11.208 | 0.020 | 11.180 | 0.032
60087.549 | 4.799 | 50755r | 156 | 11.209 | 0.017 | 11.115 | 0.023
60087.615 | 4.865 | 50757r | 208 | 11.195 | 0.020 | 11.162 | 0.028
60087.680 | 4.930 | 50759r | 232 | 11.181 | 0.018 | 11.139 | 0.023
60087.745 | 4.995 | 50761r | 207 | 11.145 | 0.021 | 11.112 | 0.023
60087.745 | 4.995 | 50761r | 208 | 11.145 | 0.021 | 11.165 | 0.026
60087.874 | 5.124 | 50765r | 207 | 11.172 | 0.018 | 11.158 | 0.024
60088.004 | 5.254 | 50769r | 208 | 11.147 | 0.017 | 11.183 | 0.024
60088.134 | 5.384 | 50773r | 156 | 11.117 | 0.019 | 11.095 | 0.022
60093.586 | 10.836 | 50941r | 211 | 10.719 | 0.019 | 10.725 | 0.021
60093.651 | 10.901 | 50942r | 235 | 10.687 | 0.016 | 10.646 | 0.020
60294.486 | 211.736 | 57146r | 007 | 11.921 | 0.027 | 10.544 | 0.020
60294.616 | 211.866 | 57150r | 042 | 11.920 | 0.021 | 10.505 | 0.018
60294.745 | 211.995 | 57154r | 042 | 11.895 | 0.021 | 10.489 | 0.019
60294.874 | 212.124 | 57158r | 042 | 11.880 | 0.020 | 10.513 | 0.018
60294.938 | 212.188 | 57160r | 017 | 11.849 | 0.020 | 10.513 | 0.018
60295.003 | 212.253 | 57162r | 042 | 11.891 | 0.022 | 10.493 | 0.018
60295.068 | 212.318 | 57164r | 018 | 11.930 | 0.023 | 10.483 | 0.019
60295.197 | 212.447 | 57168r | 018 | 11.839 | 0.020 | 10.510 | 0.019
60295.261 | 212.511 | 57170r | 042 | 11.874 | 0.023 | 10.569 | 0.025
60295.326 | 212.576 | 57172r | 018 | 11.860 | 0.020 | 10.511 | 0.018
60295.390 | 212.640 | 57174r | 042 | 11.929 | 0.024 | 10.508 | 0.021
60295.455 | 212.705 | 57176r | 017 | 11.901 | 0.021 | 10.495 | 0.019
60295.584 | 212.834 | 57180r | 017 | 11.872 | 0.023 | 10.479 | 0.020
60295.649 | 212.899 | 57182r | 042 | 11.911 | 0.025 | 10.517 | 0.021
60295.713 | 212.963 | 57184r | 018 | 11.930 | 0.023 | 10.530 | 0.019
60295.778 | 213.028 | 57186r | 043 | 11.925 | 0.028 | 10.498 | 0.020
60295.842 | 213.092 | 57188r | 018 | 11.905 | 0.024 | 10.527 | 0.020
60295.972 | 213.222 | 57192r | 018 | 11.894 | 0.023 | 10.525 | 0.023
60296.101 | 213.351 | 57196r | 018 | 11.939 | 0.024 | 10.502 | 0.020
60453.625 | 370.875 | 62087r | 131 | 13.141 | 0.038 | 11.795 | 0.036
60453.753 | 371.003 | 62091r | 231 | 13.120 | 0.034 | 11.815 | 0.039
60453.881 | 371.131 | 62095r | 232 | 13.177 | 0.036 | 11.856 | 0.040
60453.945 | 371.195 | 62101r | 206 | 13.194 | 0.035 | 11.741 | 0.039
60454.009 | 371.259 | 62103r | 231 | 13.157 | 0.035 | 11.867 | 0.036
60454.073 | 371.323 | 62105r | 232 | 13.098 | 0.032 | 11.769 | 0.037
60454.137 | 371.387 | 62107r | 134 | 13.101 | 0.032 | 11.817 | 0.043
60454.201 | 371.451 | 62109r | 135 | 13.172 | 0.038 | 11.775 | 0.038
60454.265 | 371.515 | 62111r | 231 | 13.149 | 0.036 | 11.751 | 0.034
60454.329 | 371.579 | 62113r | 206 | 13.265 | 0.038 | 11.836 | 0.037
60454.393 | 371.643 | 62115r | 231 | 13.085 | 0.031 | 11.861 | 0.043
60454.457 | 371.707 | 62117r | 205 | 13.091 | 0.049 | 11.775 | 0.032
60454.458 | 371.708 | 62117r | 206 | 13.134 | 0.036 | 11.844 | 0.038
60454.521 | 371.771 | 62119r | 231 | 13.118 | 0.032 | 11.816 | 0.041
60454.585 | 371.835 | 62121r | 205 | 13.080 | 0.038 | 11.800 | 0.036
60454.650 | 371.900 | 62123r | 231 | 13.168 | 0.032 | 11.815 | 0.033
60454.714 | 371.964 | 62125r | 206 | 13.165 | 0.033 | 11.839 | 0.040
60454.778 | 372.028 | 62127r | 231 | 13.056 | 0.031 | 11.789 | 0.034
60454.842 | 372.092 | 62129r | 206 | 13.254 | 0.036 | 11.705 | 0.038
60454.906 | 372.156 | 62131r | 231 | 13.053 | 0.031 | 11.819 | 0.031
60454.970 | 372.220 | 62133r | 206 | 13.228 | 0.035 | 11.797 | 0.041
60455.097 | 372.347 | 62137r | 125 | 13.120 | 0.034 | 11.768 | 0.036
60455.226 | 372.476 | 62141r | 206 | 13.202 | 0.035 | 11.823 | 0.035

Note. — The columns W1mpro, W1sigmpro, W2mpro, and W2sigmpro are profile-fit
photometry magnitudes and their uncertainties in the Vega system.

Figure 2: NEOWISE-R light curves of SN 2023ixf at 3.4 and 4.6 $\mu$m (bands W1
and W2, respectively). The observed magnitudes shown are in the Vega system
(see Table 1). They have not been corrected for reddening. The left panel is
limited to the early times between 3.631 and 10.901 d, in which the steady
rise in the SN brightness is evident. The right panel shows the entire set of
detections, at both early and late times; for the latter, note the dramatic
reddening in the color of the SN in the two bands at late times, relative to
the early times.

###  2.2 Late-Time Optical Data

####  2.2.1 Konkoly Observatory

Many investigators have continued to follow SN 2023ixf since its discovery. We
(Vinkó, Joó, Pál, Kriskovics, Könyves-Tóth, Szakáts, Vida) obtained optical
photometry with the 0.8 m Ritchey-Chrétien telescope at the Konkoly
Observatory, Hungary (J. Vinkó et al. 2024, in preparation). This includes
late-time Johnson $BV$ and Sloan Digital Sky Survey (SDSS)
$g^{\prime}r^{\prime}i^{\prime}z^{\prime}$ (hereafter $BVgriz$) photometry
from MJD 60297.0 (2023 December 18.5, day 214) and from MJD 60444.93 (2024 May
14.9, day 362.18). These data were processed with standard IRAF222IRAF is
distributed by the National Optical Astronomy Observatories, which are
operated by the Association of Universities for Research in Astronomy, Inc.,
under a cooperative agreement with the National Science Foundation. routines.
Photometric calibration was based on field stars in the Pan-STARRS DR1 (PS1)
catalogue333https://catalogs.mast.stsci.edu/panstarrs/ (Chambers et al.,
2016). In order to obtain reference magnitudes for our $B$ and $V$ frames, the
PS1 magnitudes were transformed into the Johnson-Cousins $BVRI$ system, based
on equations and coefficients provided by Tonry et al. (2012). Finally, the
instrumental magnitudes were transformed into standard $BVgriz$ magnitudes by
applying a linear color term (using $g-i$) and wavelength-dependent zero
points. Since the reference stars were all within a few arcminutes around the
SN, no atmospheric extinction correction was necessary.

####  2.2.2 Lick Observatory

We (Zheng, Brink, Filippenko) performed further follow-up $BVRI$ photometric
observations of SN 2023ixf with both the 0.76 m Katzman Automatic Imaging
Telescope (KAIT) and the 1 m Nickel telescope, as well as spectroscopy with
the Shane 3 m telescope at Lick Observatory; see W. Zheng et al. (2024, in
preparation) for details of the observations and data reduction. Regarding the
photometry, we isolated just the late-time data on MJD 60291.53 and 60303.59
(2023 December 13.53 and 25.59, or days 208.78 and 220.84, respectively),
which bracketed the late-time NEOWISE observations, and interpolated between
these two sets of measurements. As far as spectroscopy, we have included
spectra from Zheng et al. obtained on 60290.53 (2023 December 12.53, day
207.78) and 60445.41 (2024 May 15.41, day 362.66), which are the closest Lick
spectra in time to the late-time NEOWISE observations.

##  3 Analysis

Throughout our analysis we have assumed a total extinction to SN 2023ixf of
$A_{V}=0.12$ mag from Van Dyk et al. (2024). For UV through the near-IR we
adopted the Fitzpatrick (1999) reddening law. The extinction corrections for
the NEOWISE bands are adopted from Wang & Chen (2019).

###  3.1 Early-Time IR Emission

Rather than select every pair of observed NEOWISE W1 and W2 data points to
analyze, for illustrative purposes we have chosen to consider just two sets at
early times, the very first one from MJD 60086.381 (day 3.631) and from
60093.586 (day 10.836). These adequately represent the two periods of early-
time sampling of the light curves in these bands.

In order to put the NEOWISE data in context with the overall SED at these two
epochs, we accumulated published light-curve data at UV, optical, and near-IR
wavelengths corresponding to (or bracketing) the epochs. The data sources were
then an amalgam of Swift UVW2, UVM2, UVW1 from Zimmerman et al. (2024), SDSS
$ugriz$ and Johnson $BVJHK_{s}$ from Teja et al. (2023), Johnson $UBV$, and
SDSS $griz$ from Hiramatsu et al. (2023), and Johnson $JHK$ from Yamanaka et
al. (2023).

Since the NEOWISE measurements are in the Vega system, the entirety of the
dataset presented by Teja et al. (2023), which is in the AB system, had to be
converted to Vega magnitudes. The SDSS magnitudes from Hiramatsu et al. (2023)
also required a similar conversion. No conversion was needed for the Yamanaka
et al. (2023) $JHK$ photometry. For day 3.631 the available UV-optical data
were quasi-contemporaneous with the NEOWISE points; however, $JHK$ required a
linear interpolation between two bracketing epochs (the earlier epoch was at
day 3.4, very close in time to NEOWISE). For day 10.836 none of the
complementary data were contemporaneous, so we were forced to interpolate
between bracketing epochs at all wavelengths.

The resulting SED is shown in Figure 3. In addition to the UV-optical
photometric points we included an FTN-FLOYDS-N spectrum of the SN obtained by
Bostroem et al. (2023) on 2023 May 22, which we downloaded from
WISeREP444https://www.wiserep.org (Yaron & Gal-Yam, 2012). Both the spectrum
and the photometry were first reddening-corrected. This spectrum was further
renormalized to the (dereddened) SN $V$-band brightness. As can be seen in the
figure, the overall agreement is reasonable between the spectrum and the
photometric points across the common wavelength range.

We then attempted to fit a single, simple blackbody to the SED at day 3.631.
We found that a hot, $\sim 26,630$ K blackbody provides a good fit to the
optical data, although a clear excess exists in the UV relative to this fit.
The fit implies that the SN luminosity at that epoch was $\gtrsim 4.1\times
10^{43}$ erg s-1; this is a lower limit, since there is clearly additional
luminosity in the UV. This is consistent with the evidence for strong, early-
time interaction of the SN shock with dense CSM (e.g., Bostroem et al., 2023;
Jacobson-Galán et al., 2023; Teja et al., 2023; Martinez et al., 2024;
Zimmerman et al., 2024); specifically, interaction in SNe II can strengthen
the continuum flux and boost emission lines in the UV simultaneously (Dessart
& Hillier, 2022). The blackbody radius is then $R_{\rm hot}\approx 3.4\times
10^{14}$ cm. The early-phase photometric and spectroscopic UV-optical SN
observations provided evidence for strong interaction of the SN shock with a
dense, confined ($<2\times 10^{15}$ cm) CSM (e.g., Jacobson-Galán et al.,
2023; Smith et al., 2023; Bostroem et al., 2023; Teja et al., 2023). Zimmerman
et al. (2024) further refined the extent of the dense CSM to $R_{\rm
CSM}\approx 2\times 10^{14}$ cm and concluded that it actually delayed shock
breakout (SBO) from hours after explosion to $\sim 3$ d. In other words, the
very first NEOWISE data were likely obtained within just hours after SBO, and
the inferred $R_{\rm hot}$ is consistent with the shock having already overrun
the confinements of the dense CSM. In fact, following Zimmerman et al. (2024)
and assuming a SN expansion velocity $v_{\rm exp}=8,000$ km s-1, on day 3.631
the shock would have been at $\sim 2.5\times 10^{14}$ cm, roughly consistent
with $R_{\rm hot}$ (if $v_{\rm exp}$ instead had been a somewhat higher, $\sim
11,000$ km s-1, the two radii would be in better agreement).

Particularly fascinating here is that an obvious excess in flux, relative to
the hot blackbody, can be seen in Figure 3 at $\gtrsim 1.5$ $\mu$m. We found
that we could account for this IR excess with an additional much cooler
blackbody, at $\sim 1620$ K. Including this extra blackbody provides a
reasonable fit to $JHK_{s}$ and W1, although it does not quite fit W2 as well.
This additional source of IR emission is of a comparatively far smaller
luminosity, $\sim 3.2\times 10^{40}$ erg s-1, than the SN shock (it accounts
for $\lesssim 0.1$% of the total emission). The corresponding blackbody radius
is $R_{\rm IR}\approx 2.6\times 10^{15}$ cm. Kilpatrick et al. (2023) and Van
Dyk et al. (2024), for example, inferred that the RSG progenitor candidate was
surrounded by a dusty shell with an inner radius of $R_{\rm
in}\approx(0.5$–$1.0)\times 10^{15}$ cm. The assumption Van Dyk et al. (2024)
made in their modeling of the star was that the shell extended out to $1000\
\times R_{\rm in}$, with the dust density declining $\propto r^{-2}$.
Furthermore, $\sim 1620$ K is roughly the estimated evaporation temperature of
$\sim 0.01$–0.1 $\mu$m-sized silicate-dominated dust in SN environments (e.g.,
Gall et al., 2014). (Note that Van Dyk et al. 2024 were able to fit the
reddening-corrected SED of the pre-explosion dust shell with a simple $\sim
1761$ K blackbody.) Thus, we speculate that the IR excess was emanating from
the dusty CSM shell, with the SN shock still within $R_{\rm in}$.

This analysis including the NEOWISE data lends credence to the overall picture
of the progenitor star, inferred via the modeling of the observed SED of the
star. The estimated luminosity from the optically-thin dust was still about
two orders-of-magnitude larger than the luminosity of the progenitor candidate
($\sim 9\times 10^{4}\ L_{\odot}$, or $\sim 3.5\times 10^{38}$ erg s-1; Van
Dyk et al. 2024), which implies that the dust shell was likely heated by and
was reprocessing the UV emission from the interaction of the SN shock with the
dense inner CSM. We note that much of the CSM dust was likely destroyed
immediately after explosion by high-energy (extreme UV to $\gamma$-ray)
photons from the blast, through grain sublimation, vaporization, and extreme
grain charging effects (Jones, 2004).

We built a similar SED from the available UV/optical/near-IR/NEOWISE
photometry from day 10.836. Again, we added to this a dereddened and
renormalized FTN-FLOYDS-N SN spectrum from WISEReP from 2023 May 29 (Bostroem
et al. 2023). The resulting SED is shown in Figure 4. We attempted to fit a
warm, $\sim 9050$ K blackbody to the data; however, as can be seen in the
figure, while this fit is very good in the IR, it diverges significantly for
wavelengths $<1\ \mu$m. If we consider the radiation models for SNe IIP by
Dessart et al. (2013; D13), specifically, the m15mlt1 model (with an initial
progenitor mass of $15\ M_{\odot}$ and radius $R_{\star}=1107\ R_{\odot}$)
computed at day 11, it provides a remarkably good comparison, from the UV
through to the mid-IR, with the observed data. This implies that what we were
seeing at that epoch, including with NEOWISE, was SN ejecta-dominated
emission. If we take into account the blackbody fit, the inferred luminosity
is $\gtrsim 5.8\times 10^{42}$ erg s-1, which is a lower limit, since a
significant amount of luminosity is still emerging from the SN at $<1\ \mu$m.
In fact, if we integrate the D13 model (for instance), we obtain a luminosity
of $\sim 1.5\times 10^{43}$ erg s-1. The blackbody radius is $R_{\rm
warm}\approx 1.1\times 10^{15}$ cm. Once again, assuming $v_{\rm exp}=8,000$
km s-1, the shock radius would have been $\sim 7.5\times 10^{14}$ cm, roughly
consistent with $R_{\rm warm}$; similar to day 3.631, a somewhat higher
$v_{\rm exp}=11,000$ km s-1 would result in better agreement between the two
radius estimates.

The blackbody radii and temperatures that we have calculated for both days 3.6
and 10.8 are in a good agreement with the results of Singh et al. (2024).
Note, however, that we have assumed a spherical and homogeneous medium, while
early-phase spectropolarimetric data implied the presence of an aspherical
dense CSM and a clumpy, low-density extended CSM around SN 2023ixf (Vasylyev
et al. 2023; Singh et al. 2024). Nevertheless, our conclusions also fit with
that of Vasylyev et al. (2023), in that they concluded that the expanding SN
ejecta just emerged from the dense CSM region around Day 3.5.

Figure 3: Left: The combined, dereddened UV-optical-IR SED of SN 2023ixf on
day 3.6; see text for sources of the photometry. For comparison, we also show
a dereddened, renormalized FTN/FLOYDS spectrum from day 3.54 (Bostroem et al.
2023; solid red curve). Blackbody SED fitting of only the near-IR and mid-IR
fluxes (filled circles) required two components: One component consisting of a
hot ($\sim 26,630$ K) blackbody (dashed green curve), and the other a cooler
($\sim 1,620$ K) blackbody (“IR”; dashed orange curve). The total fit is shown
as the solid blue curve. The inferred radii of the blackbodies are $R_{\rm
hot}\approx 3.4\times 10^{14}$ and $R_{\rm IR}\approx 2.6\times 10^{15}$ cm.
Right: A zoom-in of just the IR portion of the SED.

Figure 4: Left: The combined, dereddened UV-optical-IR SED of SN 2023ixf on
day 10.8; see text for sources of the photometry. For comparison, we also show
a dereddened, renormalized FTN/FLOYDS spectrum from day 10.73 (Bostroem et al.
2023; red curve). Blackbody fitting of the SED was performed, consisting of
only one component (solid blue curve) at $\sim 9,050$ K, with only the near-IR
and mid-IR fluxes (filled circles) used in the fitting. The fit is poor for
wavelengths $<1\ \mu$m. The addition of a scaled version of the m15mlt1 model
spectrum at day 11 (solid gray curve) from Dessart et al. (2013; D13) provides
a much better representation of the observed data. The inferred radius of the
blackbody is $R_{\rm warm}\approx 1.1\times 10^{15}$ cm. Right: A zoom-in of
just the IR portion of the SED. Note that the D13 model provides an overall
reasonable representation of the SED, even for the NEOWISE bands.

###  3.2 Late-Time Mid-IR Excess

We computed the averages of the various measurements obtained by NEOWISE at
late times in the SN’s evolution, between MJD 60294.5 and 60296.1. SN 2023ixf
was already well on the radioactively-powered exponential tail by this point.
The averages are $11.895\pm 0.023$ mag and $10.509\pm 0.020$ mag in W1 and W2,
respectively, with reference to day 213.

Thanks to the active worldwide follow-up observations of SN 2023ixf, we can
also build the late-time optical-NIR SED for the SN. We assembled the
unpublished optical $BVgriz$ photometry from Konkoly Observatory (from 2023
December 18.5, day 214; J. Vinkó et al. 2024, in preparation) and Lick
Observatory $BVRI$ data (interpolated to the same date), as well as a single
Lick optical spectrum (from 2023 December 12, day 208; W. Zheng et al. 2024,
in preparation); see Section 2. We show the combined optical SED in Figure 5.
Since we are unaware of any available near-IR data close to this late epoch,
we included spectra and fluxes from models of SN IIP explosions for
comparison: The day 207 s15p2 model spectrum courtesy of Luc Dessart, and the
interpolated and distance-scaled day +193 and +242 model fluxes for the Type
IIP SN 2012aw from Pejcha & Prieto (2015). (We compared the Fe ii line
velocities for SN 2023ixf from Zheng et al. 2024, to that of SN 2012aw from
Bose et al. 2013 and found a very good match even at late phases; we also
adopted the SN 2012aw distance of 9.9 Mpc from Bose et al. 2013.) Both the
model spectrum and the set of optical model fluxes appear to compare well with
our late-time measured optical data; thus, we find it to be a reasonable
approach to use the near-IR components of these models as references during
the analysis of the late-time optical-IR SED.

As can be seen in Figure 5, the NEOWISE 3.4 and 4.6 $\mu$m data show a clear
excess relative to the model fluxes. We fit a single blackbody to the optical-
near-IR part of the SED and found a warm component with temperature $\sim
4650$ K and corresponding radius $\sim 5.6\times 10^{14}$ cm. While SN ejecta
are not expected to be realistically represented by a blackbody at late
epochs, we can then use the result of this fit to provide a characterization
of the colder, longer-wavelength excess that is apparent.

As we noted in Section 3.1, we assume that the majority of pre-existing dust
grains, located in the dense, confined circumstellar shell, have been
evaporated at early times after explosion; therefore, these can no longer be
responsible for the excess. Thus, we consider two possible alternate
scenarios: (i) At day 213, newly-formed dust existed either in the inner
(unshocked) ejecta or in the contact discontinuity between the forward and
reverse shocks (the CDS); or (ii) radiation was being emitted by more distant
pre-existing dust grains in the SN environment, heated collisionally or
radiatively by the (forward) SN shock. However, from only the two mid-IR data
points, a detailed investigation of the origin and properties of the assumed
late-time dust is challenging to carry out — we hope that pending JWST
observations of the SN will provide greater insight into these two scenarios.
Furthermore, we note that the large 4.6 $\mu$m flux excess at day 213 can also
be explained by the emergence of the 1–0 vibrational band of carbon monoxide
(CO) at 4.65 $\mu$m, as seen in some SNe IIP with observed mid-IR spectra at a
similar age (e.g., Kotak et al. 2005; 2006; Szalai et al. 2011; Szalai & Vinkó
2013), and also predicted by modeling of exploding RSGs by McLeod et al.
(2024). The presence of a late-time mid-IR excess is in agreement with the
results from Singh et al. (2024); they found that the flattening in the
$K_{s}$-band light curve and the attenuation of the red-edge of the H$\alpha$
line profile $\sim$125 d after explosion is indicative of the onset of
molecular CO and, hence, dust formation in SN 2023ixf. This possibility should
be also taken into account during any analysis.

Thus, we also excluded the W2 flux and fit a two-component blackbody model to
the remainder of the optical-IR SED, holding the parameters from the hot
component fixed. Since a fit to only one mid-IR point would not provide any
physically relevant information, we applied temperature constraints and
assumed two possible scenarios for the “warm” dust component: (i) First, we
assumed the highest theoretical dust temperature possible for amorphous carbon
dust, $T_{\rm IR}=2600$ K (see, e.g., Gall et al. 2014); and, (ii) second, we
assumed a “typical” temperature of “warm” dust ($T_{\rm IR}=700$ K) seen in
SNe IIP at a similar age (see, e.g., Kotak et al. 2009; Szalai et al. 2011;
Szalai & Vinkó 2013). Using these two assumptions, we found a blackbody radius
of $R_{\rm IR}\sim 1.5\times 10^{15}$ cm and $\sim 1.6\times 10^{16}$ cm,
respectively, for the two assumptions; see Figure 6).

Extrapolating the Fe ii velocities (W. Zheng et al. 2024, in preparation) to
day 213, we estimated $\sim 1400$–1500 km s-1 for the ejecta velocity, which
results in (2.6–2.8) $\times 10^{15}$ cm for the ejecta radius. This implies
that, in the case of very high-temperature dust (the first scenario above),
these grains possibly could be within the ejecta. However, assuming a more
realistic dust temperature (the second scenario), the dust would more likely
be outside the ejecta. As Van Dyk et al. (2024) concluded, the dusty shell of
the progenitor likely had an inner radius of $\sim 10^{15}$ cm, with an
$r^{-2}$ density distribution extending outward. Thus, the pre-explosion shell
itself could easily have extended out to $\sim 1.6\times 10^{16}$ cm (well
beyond the confined volume); the dust we infer here could have been located
within the CSM, being either pre-existing or newly-formed in the CDS.

We present in Figure 7 an optical-IR SED comprised of data obtained in the day
370–372 interval. Since at that range of epochs the shape of the measured
optical SED appears to differ significantly from Type IIP atmospheric models
by either Dessart et al. (2023) or Pejcha & Prieto (2015), we are unable to
directly estimate the amount of IR excess in the manner that we did for the
day 211–213 SED. Nevertheless, clear excesses at both 3.3 and 4.6 $\mu$m are
obvious.

Figure 5: One-component blackbody (BB) fit on the Days 211–213 optical SED of
SN 2023ixf (filled circles). NEOWISE-R data (filled gray squares) were
excluded from the fit. Atmospheric models for SNe IIP are also shown for
comparison: The day 207 s15p2 model spectrum (courtesy of Luc Dessart, gray
curve), and the distance-scaled model fluxes interpolated between days 193 and
242, for SN 2012aw from Pejcha & Prieto (2015; open blue triangles); the
distance to SN 2012aw ($D=9.9$ Mpc) was adopted from Bose et al. (2013).

Figure 6: The same as in Fig. 5 but with two-component BB fits on the combined
Days 211–213 optical-IR SED of SN 2023ixf, and without showing atmospheric
models. The NEOWISE-R W2 point was excluded from the fit because of the
potential contamination from CO line emission. Parameters of the hot component
are the same as above. The panels show two scenarios for the “warm” dust
component: one calculated assuming the highest theoretical dust temperature
($T_{\textrm{IR}}=2600$ K, top panel), and another one assuming
$T_{\textrm{IR}}=700$ K (bottom panel); see details in the text. Figure 7:
Optical-IR SED of SN 2023ixf from days 370–372. Atmospheric models for SNe IIP
are also shown for comparison: The day 350 s15p2 model spectrum from Dessart
et al. (2023; D23) and the distance-scaled PP15 models for the same epoch.

###  3.3 Comparison of SN 2023ixf with Other SNe IIP In the Mid-IR

We have considered all of the NEOWISE data for SN 2023ixf, in the context of
the mid-IR emission from other SNe, particularly SNe IIP. To achieve this, we
compared SN 2023ixf with the sample of SNe from Szalai et al. (2019), who
presented photometric data obtained by Spitzer in the IR Array Camera (IRAC;
Fazio et al. 2004) bands at 3.6 and 4.5 $\mu$m during both the cryogenic and
Warm missions; see Figure 8. While the bandpasses for Spitzer IRAC differ
slightly from those of NEOWISE W1 and W2 channels, we can draw some basic
conclusions.

First, as noted above, SN 2023ixf has become the best-observed SN IIP in the
mid-IR during the first several days after explosion. Second, SN 2023ixf is
one of the most luminous SNe IIP in the mid-IR ever seen. This statement is
especially striking, considering the 4.5/4.6 $\mu$m photometric evolution of
SNe IIP (Figure 8; right panel) — at day 213, SN 2023ixf is more luminous than
at early times and than any other SNe IIP detected so far at late times. We
can speculate on why this is the case. The early-time mid-IR luminosity is
consistent with the overall excess observed at other wavelengths and can be
explained by the shock-CSM interaction. Among the Spitzer sample, most of the
SNe were either of low luminosity (e.g., SN 2004dj) or were otherwise normal
(e.g., SN 2007od, SN 2011ja); only SN 2004et was observed to be somewhat
extraordinary (e.g., Maguire et al. 2010; see also Shahbandeh et al. 2023).
The late-time mid-IR excess for SN 2023ixf could be due to post-explosion dust
formation, as we have mentioned above. However, we cannot say much more about
this, based on the NEOWISE data alone; observations with JWST will likely
provide significantly more insight on this.

Figure 8: Mid-IR luminosity evolution of SN 2023ixf (NEOWISE W1 and W2 data,
filled black circles), compared with Spitzer IRAC data for other SNe IIP
(Szalai et al. 2019). Well-sampled objects are highlighted with colored
symbols, while all other published detections are marked with gray symbols and
are not listed in the figure legend. The earliest gray symbol shown is SN
2009js, at 2 days post-explosion (Gandhi et al. 2013; Szalai et al. 2019).

##  4 Conclusions

We have analyzed serendipitous observations of SN 2023ixf made as part of
routine NEOWISE survey scanning operations, starting on day 3.6 through day
10.9 after explosion, and again at late times from days 211 through 213 and
days 370 through 372. For the three epochs in these time ranges that we
analyzed, we combined the NEOWISE observations with data from the UV through
the near-IR, whenever possible. At day 3.6 we approximated the emission in the
optical with a hot, $\sim 26,630$ K blackbody, exhibiting a marked excess in
the UV, likely resulting from strong, early SN shock-CSM interaction. In the
IR, however, a definite excess is also obvious, and we fit that with a cooler,
$\sim 1,620$ K blackbody, with a radius of $\sim 2.6\times 10^{15}$ cm. We
concluded that this is consistent with dust in an inferred circumstellar shell
surrounding the progenitor star having been heated by the UV emission from the
early CSM interaction. On day 10.8 the emission, including that detected with
NEOWISE, was consistent with being SN ejecta-dominated.

At late times we also observed an obvious excess in the NEOWISE bands,
relative to the other wavelengths. This excess could arise either from newly-
formed dust in the inner ejecta or in the contact discontinuity between the
forward and reverse shocks (the CDS), or from more distant pre-existing dust
grains in the SN environment. Furthermore, the observed large excess at 4.6
$\mu$m at late times can also be explained by the emergence of the CO 1–0
vibrational band, seen in other SNe IIP. Observations with JWST are necessary
to confirm detection of the CO band, as well as to better explore the overall
nature of the late-time mid-IR emission.

We found, from comparing to mid-IR data for other SNe IIP, that SN 2023ixf is
the best-observed SN IIP in the mid-IR during the first several days after
explosion and one of the most luminous SNe IIP ever seen in the mid-IR. The
survey operations by the WISE mission, in all its incarnations, are scheduled
to be terminated permanently on 2024 July 31. Together with the
decommissioning of Spitzer over four years ago, the number of available
facilities to gather mid-IR light from nearby SNe will be greatly diminished.
The next avenue will be provided by NEO Surveyor, set for launch no later than
mid-2028 (Mainzer et al. 2023): The mission will be obtaining four detections
at 4.6 and 8 $\mu$m over a six-hour time period, approximately every 13 days,
as part of survey operations, so it may be possible once again to catch SNe
both on the rise and at late times. For future pointed observations it falls
on JWST to be the platform for observing both new and old SNe, to explore
further in detail the nature of dust associated with these spectacular events.

We thank Luc Dessart for providing the day 207 model SN IIP spectrum. This
publication makes use of data products from the Near-Earth Object Wide-field
Infrared Survey Explorer, which is a joint project of the Jet Propulsion
Laboratory/California Institute of Technology and the University of Arizona.
It also uses 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. WISE and NEOWISE are
funded by the National Aeronautics and Space Administration (NASA). This work
has been supported by the GINOP-2-3-2-15-2016-00033 project of the National
Research, Development and Innovation (NRDI) Office of Hungary funded by the
European Union, as well as by NKFIH OTKA FK-134432, KKP-143986, and K-142534
grants, and from the HUN-REN Hungarian Research Network. L.K. and K.V. are
supported by the Bolyai János Research Scholarship of the Hungarian Academy of
Sciences. LK acknowledges the Hungarian National Research, Development and
Innovation Office grant OTKA PD-134784. The authors acknowledge financial
support of the Austrian-Hungarian Action Foundation grants 98öu5, 101öu13,
112öu1. A.V.F.’s research group at UC Berkeley acknowledges financial
assistance from the Christopher R. Redlich Fund, Gary and Cynthia Bengier,
Clark and Sharon Winslow, Alan Eustace (W.Z. is a Bengier-Winslow-Eustace
Specialist in Astronomy), William Draper, Timothy and Melissa Draper, Briggs
and Kathleen Wood, Sanford Robertson (T.G.B. is a Draper-Wood-Robertson
Specialist in Astronomy), and numerous other donors. KAIT and its ongoing
operation at Lick Observatory were made possible by donations from Sun
Microsystems, Inc., the Hewlett- Packard Company, AutoScope Corporation, Lick
Observatory, the U.S. NSF, the University of California, the Sylvia & Jim
Katzman Foundation, and the TABASGO Foundation. 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. Several UC Berkeley undergraduate students helped obtain the 1
m Nickel data. We appreciate the excellent assistance of the staff at Lick
Observatory. Research at Lick Observatory is partially supported by a generous
gift from Google.

##  Appendix A Pre-explosion NEOWISE Non-detections

As pointed out in Section 2.1, NEOWISE obtained pre-explosion observations of
the SN site between 2013 December 18 and 2022 December 18, with the last pair
of single exposures occurring 150.75 d prior to explosion. The progenitor
candidate was not detected in any of these exposures. As Van Dyk et al. (2024)
described, the upper limits on detection were established by isolating in the
NEOWISE-R Single Exposure Source Table, obtained from IRSA, all of the
3$\sigma$ detected objects within 60″ of the SN position for each band. We
show these upper limits in Figure 9. Both the mean and median values in W1 are
$<16.4$ mag; the mean value in W2 is $<14.8$ mag, whereas the median value is
$<14.9$ mag.

Figure 9: NEOWISE-R 3$\sigma$ upper limits on the detection of the SN 2023ixf
progenitor candidate from between 2013 December and 2022 December at 3.4 and
4.6 $\mu$m (bands W1 and W2, respectively). Also shown are the mean values for
the data at both bands (dashed lines). The observed magnitudes shown are in
the Vega system. These have not been reddening-corrected.

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