# Probing the soft X-ray properties and multi-wavelength variability of
SN2023ixf and its progenitor

Sonja Panjkov OzGrav, School of Physics, The University of Melbourne,
Parkville, VIC 3010, Australia  Katie Auchettl OzGrav, School of Physics, The
University of Melbourne, Parkville, VIC 3010, Australia  Department of
Astronomy and Astrophysics, University of California, Santa Cruz, CA, 95064,
USA  Benjamin J. Shappee Institute for Astronomy, University of Hawai‘i, 2680
Woodlawn Drive, Honolulu, HI 96822, USA  Aaron Do Institute for Astronomy,
University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA  Laura A.
Lopez Department of Astronomy, Ohio State University, Columbus, Ohio 43210,
USA  Center for Cosmology and AstroParticle Physics (CCAPP), Ohio State
University, Columbus, Ohio 43210, USA  Flatiron Institute, Center for
Computational Astrophysics, NY 10010, USA  John F. Beacom Department of
Astronomy, Ohio State University, Columbus, Ohio 43210, USA  Center for
Cosmology and AstroParticle Physics (CCAPP), Ohio State University, Columbus,
Ohio 43210, USA  Department of Physics, Ohio State University, Columbus, Ohio
43210, USA Sonja Panjkov<EMAIL_ADDRESS>
###### Abstract

We present a detailed analysis of nearly two decades of optical/UV and X-ray
data to study the multi-wavelength pre-explosion properties and post-explosion
X-ray properties of nearby SN2023ixf located in M101. We find no evidence of
precursor activity in the optical to UV down to a luminosity of $\lesssim
7\times 10^{4}\,\rm L_{\odot}$, while X-ray observations covering nearly 18
years prior to explosion show no evidence of luminous precursor X-ray emission
down to an absorbed 0.3 – 10.0 keV X-ray luminosity of $\sim 6\times 10^{36}$
erg s-1. Extensive Swift observations taken post-explosion did not detect soft
X-ray emission from SN2023ixf within the first $\sim$3.3 days after first
light, which suggests a mass-loss rate for the progenitor of $\lesssim 5\times
10^{-4}\,\rm M_{\odot}$ yr-1 or a radius of $\lesssim 4\times 10^{15}$ cm for
the circumstellar material. Our analysis also suggests that if the progenitor
underwent a mass-loss episode, this had to occur $>$ 0.5 – 1.5 years prior to
explosion, consistent with previous estimates. Swift detected soft X-rays from
SN2023ixf $\sim 4.25$ days after first light, and it rose to a peak luminosity
of $\sim 10^{39}$ erg s-1 after 10 days and has maintained this luminosity for
nearly 50 days post first light. This peak luminosity is lower than expected,
given the evidence that SN2023ixf is interacting with dense material. However,
this might be a natural consequence of an asymmetric circumstellar medium.
X-ray spectra derived from merging all Swift observations over the first 50
days are best described by a two-component bremsstrahlung model consisting of
a heavily absorbed and hotter component similar to that found using NuSTAR,
and a less-absorbed, cooler component. We suggest that this soft component
arises from cooling of the forward shock similar to that found in Type IIn
SN2010jl.

##  1 Introduction

Massive stars ($\gtrsim$ 8 $\rm M_{\odot}$) end their lives as core-collapse
supernovae (SNe) once degeneracy pressure support is overcome by electron
capture and nuclear photodissociation, leading to runaway collapse (e.g.,
Burbidge et al., 1957; Iben, 1974; Woosley et al., 2002; Eldridge & Tout,
2004; Smartt, 2009; Janka, 2012; Ibeling & Heger, 2013; Burrows, 2013).
However, the nature of the explosion is thought to depend on the properties of
the progenitor (e.g., De Donder & Vanbeveren, 1998; Woosley et al., 2002;
Heger et al., 2003; Izzard et al., 2004; Zapartas et al., 2017), as well as
the type and strength of mass loss before core collapse (Smith, 2014). There
have been numerous attempts to identify SN progenitor stars in archival space-
and ground-based images (e.g., Smartt et al., 2015). The majority of
progenitors discovered in pre-explosion images are red supergiants (RSGs) with
masses $\lesssim 20\,\rm M_{\odot}$ which are associated with hydrogen-rich
Type II SNe (Smartt, 2009; Maund et al., 2013), although there have also been
progenitor stars associated with hydrogen-poor Type IIb SNe (Aldering et al.,
1994; Crockett et al., 2008; Maund et al., 2011; Van Dyk et al., 2014) and
hydrogen-stripped Type Ib SNe (Cao et al., 2013; Eldridge et al., 2015;
Kilpatrick et al., 2021).

Prior to core collapse, it is expected that RSGs will lose mass via winds.
However, there is evidence that these winds are not strong enough to explain
the properties of SNe that exhibit strong interaction with dense material
(e.g., Beasor et al., 2020). While historically, massive star evolution was
thought to be dominated by single-star wind mass loss, there is now strong
evidence that $\sim 10$% of massive stars undergo substantial and even
eruptive mass loss a few years prior to core collapse (see e.g., Smith, 2014,
and references therein). This can result in up to 1 $\rm M_{\odot}$ of
material being ejected in the decades to years before explosion (e.g., Smith,
2014; Tinyanont et al., 2019; Margutti et al., 2014; Mauerhan et al., 2013;
Kilpatrick et al., 2021), creating a dense shell of circumstellar material
(CSM) around the progenitor with which the SN shock and ejecta interact (e.g.,
Smith et al., 2017). While the mechanism for such mass loss in single stars is
typically attributed to winds or violent eruptions (e.g., Matsumoto & Metzger,
2022), the frequency and cause of these extreme mass ejections remain
uncertain. However, nuclear burning instabilities or interaction with a binary
companion have been suggested as possible explanations (e.g., Quataert &
Shiode, 2012; Smith, 2014; Smith & Arnett, 2014; Kochanek, 2019; Sun et al.,
2020; Matsuoka & Sawada, 2023).

To shed light on pre-explosion behaviour, numerous studies have attempted to
determine the frequency of these outbursts by searching for evidence of pre-SN
variability in the form of precursor outbursts. These studies include the
detection of precursor emission in individual supernovae such as SN2009ip
(Fraser et al., 2013; Margutti et al., 2014; Smith et al., 2022), SN2010mc
(Ofek et al., 2013), SN2015bh (Elias-Rosa et al., 2016; Jencson et al., 2022),
SN2016bhu (Pastorello et al., 2018), SN2020ltf (Jacobson-Galán et al., 2022a),
and others (e.g., Ofek et al., 2014, 2016; Tartaglia et al., 2016; Kilpatrick
et al., 2018; Strotjohann et al., 2021), and long-term, deep, high-resolution
imaging surveys such as that completed with the Large Binocular Telescope
(e.g., Kochanek et al., 2008; Johnson et al., 2017, 2018; Neustadt et al.,
2023; Rizzo Smith et al., 2023). Johnson et al. (2018) found that the
progenitors of a sample of Type II-P/L SNe exhibited up to 10% variability in
the decade prior to explosion, with no more than 37% of these exhibiting
outburst prior to explosion. Strotjohann et al. (2021) suggested that $\sim
25\%$ of Type IIn SNe experienced precursor outbursts in the three months
prior to explosion with luminosities $>5\times 10^{40}$ erg s-1, the detection
of which can be used to constrain the mass-loss rate and mechanism (Matsumoto
& Metzger, 2022).

Apart from looking for precursor emission, core-collapse SNe exhibit
photometric and spectroscopic evidence of enhanced mass loss (Smith et al.,
2011; Shivvers et al., 2015). For example, the presence of dense CSM from end-
of-life mass loss can appear as narrow emission lines in the early ($\sim$
days to hours after shock breakout) optical SN spectra (e.g., Gal-Yam et al.,
2014; Kiewe et al., 2012; Jacobson-Galán et al., 2022a; Terreran et al., 2021;
Tinyanont et al., 2022). These emission lines, commonly referred to as ‘flash’
ionisation features, allow the composition and density of the CSM to be
probed, and thus provide insight into the progenitor and its mass-loss history
up to radii of $\sim 10^{15}$ cm (e.g., Khazov et al., 2016; Yaron et al.,
2017; Bruch et al., 2021; Jacobson-Galán et al., 2022a; Boian & Groh, 2020).
For narrow emission lines from CSM interaction to be observed and not buried
within the signal from the SN photosphere, mass loss rates of $>\,10^{-2}$ –
$10^{-3}\,\rm M_{\odot}$ are generally required (Smith, 2014; Fransson &
Jerkstrand, 2015).

The shock heating of the CSM not only provides insight into the mass loss
history of a progenitor during its final stages of evolution, but also
produces significant X-ray, UV, and radio emission (Chevalier & Fransson,
2006). As the X-ray emission depends on the CSM density and the evolutionary
parameters of the SN and its progenitor, one can then use the X-ray properties
to constrain the wind density and progenitor’s mass-loss history (e.g.,
Chevalier, 1982; Chevalier & Fransson, 2006; Dwarkadas & Gruszko, 2012;
Chandra, 2018).

X-rays have been detected from a growing number of interacting transients (see
Smith, 2017, Dwarkadas & Gruszko 2012 and Chandra 2018 for reviews) including
the Type IIn SNe 2005kl, 2006jd, and 2010jl (e.g., Chandra et al., 2015;
Katsuda et al., 2016), the interacting Type Ib SN2014c (Margutti et al., 2017;
Brethauer et al., 2022), and the peculiar fast, blue optical transient
AT2018cow (Xiang et al., 2021; Margutti et al., 2018a; Kuin et al., 2019;
Rivera Sandoval & Maccarone, 2018; Savchenko et al., 2018; Miller et al.,
2018). Since these X-rays are indicative of interaction with dense material
surrounding the progenitor, their detection can help to reveal the precise
structure of the CSM. For example, the onset of X-rays from SN2014c after
$\sim 20$ days indicated the presence of a low-density cavity surrounding the
progenitor which extended out to a radius of $R\sim(0.8$ – $2)\times 10^{16}$
cm. From this, Margutti et al. (2017) inferred that significant mass loss did
not occur within the last 7 years of the progenitor’s life, assuming a wind
velocity of 1000 $\rm km\,s^{-1}$. In addition, Katsuda et al. (2016)
suggested an aspherical, torus-shaped CSM for a sample of Type IIn SNe based
on the spectral evolution of the X-ray emission from a single-component,
heavily-absorbed model to a moderately- and heavily-absorbed two-component,
thermal model.

In this paper, we present a comprehensive study of both the multi-wavelength
emission observed prior to the explosion of SN2023ixf, and the X-ray emission
detected after first light by the Neil Gehrels Swift Gamma-ray Burst Mission
(Gehrels et al., 2004, hereafter Swift). This builds on previous studies that
have examined the infrared (e.g., Szalai & Dyk, 2023; Kilpatrick et al., 2023;
Jencson et al., 2023) and optical variability (e.g., Neustadt et al., 2023;
Dong et al., 2023), and complements those that have focused on the higher
energy post-explosion X-ray emission (e.g., Grefenstette et al., 2023) by
making use of soft X-ray observations from Swift. In Section 2, we summarise
the current knowledge of SN2023ixf at the time of writing. In Section 3, we
present our observations, while in Section 4, we present our analysis of the
pre-explosion properties and post-explosion X-rays. In Section 5, we present
our discussion before ending with our summary and conclusions in Section 6.

##  2 SN2023ixf

In this section, we provide an overview of the properties of SN2023ixf and its
progenitor based on the current literature and available data.

SN2023ixf was discovered on the 19 May 2023 17:27 UTC (Itagaki, 2023) and is
one of the closest and brightest core-collapse SNe of the last decade,
reaching peak absolute $u$\- and $g$-band magnitudes of $-18.6$ and $-18.4$,
respectively (Jacobson-Galan et al., 2023). Located in the host galaxy M101
(RA = 14:03:38.580, DEC = +54:18:42.1) at a distance of 6.4$\pm$0.2 Mpc
(Shappee & Stanek, 2011) and at a redshift of $z=0.000804$ (Perley et al.,
2023), it presents a valuable opportunity to study the evolution of a core-
collapse SN in detail.

SN2023ixf’s evolution has been extensively monitored, with numerous
facilities, and professional and amateur astronomers reporting early
photometric (e.g., Villafane et al., 2023; Kendurkar & Balam, 2023a, b;
Brothers et al., 2023; Pessev et al., 2023; Vannini, 2023; Desrosiers et al.,
2023; Fowler et al., 2023; D’Avanzo et al., 2023; Silva, 2023a; Vannini &
Julio, 2023a, b; Balam & Kendurkar, 2023; Maund et al., 2023; Singh et al.,
2023; Koltenbah, 2023; Mayya et al., 2023; Kendurkar & Balam, 2023c; Chen et
al., 2023; Silva, 2023b; Daglas, 2023; Sgro et al., 2023) and spectroscopic
(e.g., Sutaria & Ray, 2023; Sutaria et al., 2023; Zhang et al., 2023b;
González-Carballo et al., 2023; Stritzinger et al., 2023; BenZvi et al.,
2023a, b; Lundquist et al., 2023) observations.

Analysis of pre-discovery data from the Zwicky Transient Facility (ZTF)
(Perley & Irani, 2023), Asteroid Terrestrial-impact Last Alert System (ATLAS)
(Fulton et al., 2023), and many other telescopes constrained the explosion
time to between 19:30 and 20:30 UTC on 18 May 2023 (Yaron et al., 2023;
Chufarin et al., 2023; Zhang et al., 2023a; Limeburner, 2023; Mao et al.,
2023; Hamann, 2023; Filippenko et al., 2023). In our analysis and similar to
Jacobson-Galan et al. (2023), we adopt the time of first light as MJD 60082.83
$\pm$ 0.02 (Mao et al., 2023).

Figure 1: A summary of the mass-loss rates derived from both this study and
other studies presented in the literature.

Shortly following its detection, SN2023ixf was classified as a Type II SN
based on the strong flash ionisation features of H, He, C, and N in its
spectrum, as well as the subsequent emergence of broad P-cygni features from H
and He (Perley et al., 2023; Perley & Gal-Yam, 2023; Perley, 2023; Jacobson-
Galan et al., 2023; Teja et al., 2023; Hiramatsu et al., 2023; Bianciardi et
al., 2023). In addition, a strong blue continuum with Balmer emission and
features of He II, N IV, and C IV, as well as similarities to other Type II
SNe including SN2014G, SN2017ahn, and SN2020pni (Bostroem et al., 2023), led
Yamanaka et al. (2023) to suggest that SN2023ixf is a high-luminosity Type II
SN embedded in a nitrogen- and helium-rich CSM. Optical spectropolarimetry of
SN2023ixf revealed a high-continuum polarisation of $\sim 1\%$ up to day 2.5
post-explosion, which decreased to $\sim 0.5\%$ on day 3.5 and persisted until
day 14.5. This decline coincided with the disappearance of the flash
ionisation features from its spectrum (Vasylyev et al., 2023). Vasylyev et al.
(2023) attributed the temporal evolution of the polarisation to an aspherical
explosion in a highly-asymmetric CSM that was carved out by pre-explosion mass
loss in the progenitor.

Comparisons to light curve and spectral models from the non-LTE radiative
transfer code CMFGEN and the radiation hydrodynamics code HERACLES, along with
the disappearance of the narrow emission lines from SN2023ixf’s spectrum a few
days after the explosion (Smith et al., 2023), indicate that the progenitor
had a dense, solar-metallicity CSM that extended out to a radius of $(0.5$ –
$1)\times 10^{15}$ cm (Jacobson-Galan et al., 2023), which is consistent with
a wind with a density that decreases following $r^{-2}$ (Kochanek, 2019).
Jacobson-Galan et al. (2023) also determined an enhanced progenitor mass-loss
rate of $10^{-2}$ $\rm M_{\odot}\,yr^{-1}$ in the 3 – 6 years prior to
explosion, with Bostroem et al. (2023) finding a value of $10^{-3}$ –
$10^{-2}$ $\rm M_{\odot}\,yr^{-1}$ when comparing to additional CMFGEN models.
This is in contrast to Jencson et al. (2023), who estimated a lower pre-
explosion mass-loss rate between $3\times 10^{-5}$ and $3\times 10^{-4}$ $\rm
M_{\odot}\,yr^{-1}$ in the final 3 – 19 years before explosion using near- and
mid-infrared (IR) spectral energy distribution (SED) modelling, while the SED
fits to Spizter and HST data by Niu et al. (2023) yielded a value of $1\times
10^{-5}$ $\rm M_{\odot}\,yr^{-1}$.

In addition, Jencson et al. (2023) found no evidence of pre-explosion
outbursts in Spitzer data, and instead favoured a scenario where a steady,
enhanced wind ejected material for $>$ 13 years out to a radius of $>4\times
10^{14}$ cm. This is consistent with the results of Neustadt et al. (2023),
who found no evidence of outbursts in the progenitor’s final 15 years and a
mass-loss rate of $\sim 10^{-5}$ $\rm M_{\odot}\,yr^{-1}$. Using the empirical
mass-loss rate prescription from Goldman et al. (2017), Soraisam et al.
(2023c) estimated a mass-loss rate of $(2$ – $4)\times 10^{-4}$ $\rm
M_{\odot}\,yr^{-1}$, in closer agreement with the results of Jencson et al.
(2023) and Neustadt et al. (2023) than those from Jacobson-Galan et al. (2023)
and Bostroem et al. (2023). The current mass-loss rate estimates for SN2023ixf
are summarised in Figure 1.

From the emergence of a multi-peaked emission line profile of H$\alpha$ at
$t\sim 16$ days, Teja et al. (2023) proposed a shell-shaped CSM with inner and
outer radii of $\sim 8.5\times 10^{14}$ cm and $\sim 20.9\times 10^{14}$ cm
respectively, corresponding to enhanced mass loss $\sim$ 35 – 65 years before
the explosion. The flash ionisation features and U–V colour of SN2023ixf led
Hiramatsu et al. (2023) to suggest a delayed shock breakout due to a dense CSM
with a radial extent of $\sim$ (3 – 7) $\times 10^{14}$ cm. Under the
assumption of continuous mass loss, Hiramatsu et al. (2023) determined a mass-
loss rate of 0.1 – 1 $\rm M_{\odot}\,yr^{-1}$ in the final 1 – 2 years before
explosion, with the rate decreasing to 0.01 – 0.1 $\rm M_{\odot}\,yr^{-1}$ in
the $\sim$ 0.7 – 0.4 years prior to explosion. Assuming an eruptive mass-loss
mechanism, they proposed these eruptions released 0.3 – 1 $\rm M_{\odot}$
roughly one year prior to explosion.

Pre-explosion HST and Spitzer images have since revealed a point source at the
SN position that is consistent with a RSG progenitor shrouded by a large
amount (0.1 – 1 $\rm M_{\odot}$, Hiramatsu et al. 2023) of CSM (Soraisam et
al., 2023b, a; Mayya, 2023; Jencson et al., 2023; Soraisam et al., 2023c;
Neustadt et al., 2023). However, no UV nor X-ray counterpart can be seen in
AstroSat-UVIT (Basu et al., 2023), Chandra (Kong, 2023), or XMM-Newton images
(Matsunaga et al., 2023), consistent with our study.

From the pre-explosion data, progenitor masses of $11\pm 2$ $\rm M_{\odot}$
(Kilpatrick et al., 2023), $\sim 12$ $\rm M_{\odot}$ (Pledger & Shara, 2023),
$\sim$ $15$ $\rm M_{\odot}$ (Szalai & Dyk, 2023), $17\pm 4$ $\rm M_{\odot}$
(Jencson et al., 2023), $20\pm 4$ $\rm M_{\odot}$ (Soraisam et al., 2023c),
16.2 – 17.4 $\rm M_{\odot}$ (Niu et al., 2023), and $\sim 9.3$ $\rm M_{\odot}$
to $\sim 13.6$ $\rm M_{\odot}$ (Neustadt et al., 2023) have been suggested. In
addition, the star formation histories constructed by Niu et al. (2023) using
the surrounding resolved stellar populations indicated a higher-mass, Type II
SN progenitor in the 17 – 19 $\rm M_{\odot}$ range. Similarly, shock cooling
emission models of the light curve indicated a progenitor radius of $410\pm
10$ $\rm R_{\odot}$, consistent with a RSG (Hosseinzadeh et al., 2023) and in
agreement with RSG designations based on HR evolutionary tracks (Kilpatrick et
al., 2023; Jencson et al., 2023), the progenitor’s IR colour and pulsations
(Soraisam et al., 2023c), and SED modelling of the progenitor (Neustadt et
al., 2023).

Szalai & Dyk (2023) identify no significant flux variability in pre-explosion
Spitzer images collected between 2004 and 2019 that might indicate eruptive
mass-loss activity. Likewise, no variability was identified during the
progenitor’s final 15 years using optical pre-explosion data from the Large
Binocular Telescope (Neustadt et al., 2023) or 15 – 20 years prior to
explosion in the UV using GALEX observations (Flinner et al., 2023).
Performing a more detailed analysis of the Spitzer data, Kilpatrick et al.
(2023) identified a 2.8 year timescale variability in Spitzer 3.6 $\mu$m and
4.6 $\mu$m imaging, which was subsequently confirmed by Jencson et al. (2023)
and Soraisam et al. (2023c). These authors attributed this variability to
pulsations as opposed to an eruptive outburst.

Searches for pre-explosion optical outbursts in Distance Less Than 40 MPc
survey (DLT40), ATLAS, and ZTF data by Dong et al. (2023) found a low
probability of significant outbursts in the five years prior to the explosion.
They also inferred a maximum ejected CSM mass of $\sim\,0.015\,\rm M_{\odot}$,
leading them to suggest that if the dense CSM surrounding SN2023ixf is the
result of one or more precursor outbursts, they were likely faint, of short
duration ($\sim$ days to months), or occurred more than 5 years before
explosion. Based on their analysis, Dong et al. (2023) proposed that more than
one physical mechanism may be responsible for the dense CSM observed in
SN2023ixf, such as the interaction of stellar winds from binary companions
(e.g., Kochanek, 2019).

No emission was initially detected at radio frequencies (Berger et al., 2023b;
Matthews et al., 2023b; Chandra et al., 2023a; Matthews et al., 2023c);
however statistically significant emission with a flux density of $41\pm
8\,\mu$Jy was detected 29 days after discovery (Matthews et al., 2023a). Using
1.3 mm (230 GHz) observations, Berger et al. (2023a) determined upper limits
of $8.6\times 10^{25}\rm\,erg\,s^{-1}$ at 2.7 and 7.7 days, and $3.4\times
10^{25}\rm\,erg\,s^{-1}$ at 18.6 days. Searches for neutrinos (Thwaites et
al., 2023; Nakahata & Super-Kamiokande Collaboration, 2023; Guetta et al.,
2023) and gamma rays (Marti-Devesa, 2023) from SN2023ixf are consistent with
background. Using the derived upper limits from the Fermi-LAT gamma-ray flux
and the IceCube neutrino flux, in addition to the shock and CSM properties,
Sarmah (2023) placed a limit of $\sim 10^{-11}\rm\,erg\,cm^{-2}\,s^{-1}$ on
the gamma-ray flux and $\sim 10^{-3}\rm\,GeV\,cm^{-2}$ on the neutrino fluence
for emission produced via the proton-proton chain.

However, X-ray data reveal a point source at the location of SN2023ixf post
explosion. NuSTAR observations in the 3 – 20 keV range show a highly-absorbed
continuum with a strong emission line at $\sim 6.4$ keV, likely attributable
to Fe. The extrapolated broadband flux in the 0.3 – 30 keV range of $2.3\times
10^{-12}$ erg cm-2 s-1 yields an intrinsic, absorbed X-ray luminosity of
$1.1\times 10^{40}$ erg s-1 (Grefenstette, 2023). A subsequent detailed
analysis of NuSTAR observations yielded an absorbed X-ray luminosity of
$2.5\times 10^{40}$ erg s-1 in the 0.3 – 79 keV range, assuming a hot,
thermal-bremsstrahlung continuum ($T>25$ keV), from which a progenitor mass-
loss rate of $\dot{M}\sim 3\times 10^{-4}$ $\rm M_{\odot}\,yr^{-1}$ was
calculated (Grefenstette et al., 2023).

The best-fit model to Chandra observations was found to be a $\sim$ 10 keV
plasma with a column density of $\sim 3.2\times 10^{22}$ cm-2, consistent with
a normal stellar wind (Kochanek, 2019). From this, Chandra et al. (2023b)
estimated an unabsorbed flux in the 0.3 – 10 keV band of $(1.6\pm 0.1)\times
10^{-12}$ erg cm-2 s-1, corresponding to a luminosity of $8\times 10^{39}$ erg
s-1 at a distance of 6.4 Mpc.

Hard X-rays from ART-XC onboard the SRG Observatory were instead found to be
best described by an absorbed power-law model with a narrow Gaussian at 6.4
keV and a column density fixed to $2\times 10^{23}$ cm-2. In addition, the
ART-XC data showed no variability across a timescale of $\sim 10$ ks in the 4
– 12 keV band (Mereminskiy et al., 2023).

Follow-up Swift target-of-opportunity observations identified $8.8\pm 3.7$
background-subtracted counts in a 20-arcsecond region around SN2023ixf’s
reported position. Assuming an absorbed power law with an index of 1.3 and a
column density of $2\times 10^{23}$ cm-2, Kong (2023) found an absorbed flux
in the 0.03 – 30 keV range of $7.7\times 10^{-13}$ erg cm-2 s-1, corresponding
to a luminosity of $3.8\times 10^{39}$ erg s-1 at 6.4 Mpc.

In summary, SN2023ixf is a nearby, interacting, low-luminosity Type II SN from
a RSG progenitor that shows signatures of enhanced mass loss during its final
years.

##  3 Observations

SN2023ixf and its progenitor have been extensively observed at multiple
wavelengths. In this section, we provide an overview of the UV, optical and
X-ray facilities relevant to this study and describe our data reduction and
preparation techniques.

###  3.1 Optical/UV photometry

In Figure 2, we show the pre-explosion UV/optical light curves of SN2023ixf
from the All-Sky Automated Survey for Supernovae (ASAS-SN), the Zwicky
Transient Facility (ZTF), the Asteroid Terrestrial-impact Last Alert System
(ATLAS), and Swift. In Figure 3, we show the post-explosion optical/UV light
curves of SN2023ixf.

####  3.1.1 ASAS-SN

ASAS-SN (Shappee et al., 2014; Kochanek et al., 2017; Hart et al., 2023) is an
automated optical transient survey that monitors the visible sky every $\sim$
20 hours to a depth of $\sim$ 18.5 mag in the $g$-band. Starting in late 2011,
ASAS-SN began observing the northern sky in the $V$-band which had a depth of
$\sim$ 17.5 mag. In 2017 – 2018, the survey switched to the $g$-band to gain a
magnitude of depth and added additional units for cadence. The survey now
consists of 20 individual telescopes with 14-cm aperture Nikon telephoto
lenses with $\sim 8$ arcsecond pixels that are grouped together into five
4-telescope units. The five telescope units are located at Haleakala
Observatory, McDonald Observatory, the South African Astrophysical
Observatory, with the remaining two at the Cerro Tololo Inter-American
Observatory.

We used the Sky Patrol V1111https://asas-sn.osu.edu/ (Kochanek et al., 2017)
to obtain image-subtracted light curves for the location of SN2023ixf in
addition to a grid of 13 points adjacent to the SN location. The Sky Patrol
uses images reduced by the ASAS-SN fully automated pipeline which includes the
ISIS image subtraction package (Alard & Lupton, 1998; Alard, 2000). When
serving image-subtracted light curves, the Sky Patrol first co-adds the
subtracted images from the 3 dithers taken at each pointing during survey
operations. To derive the $g$\- and $V$-band photometry from these images, Sky
Patrol V1 uses the IRAF apphot package with a 2-pixel (or $\sim 16$ arcsecond)
radius aperture to perform aperture photometry on each subtracted image,
generating a differential light curve. The photometry is calibrated using the
AAVSO Photometric All-Sky Survey (Henden et al., 2015). Images with a full
width at half maximum of 1.7 pixels or greater and images with a shallow depth
(3$\sigma$ detection limit of $<18.4$ mag) were discarded. ASAS-SN observed
M101 665 times from Jan 2012 to Nov 2018 in the $V$-band and 864 times from
Nov 2017 through May 2023 in the $g$-band.

Figure 2: Pre-explosion UV/optical light curves of SN2023ixf as seen by Swift,
ASAS-SN, ZTF, and ATLAS. Here, solid data points correspond to fluxes that are
$\geq 3\sigma$ above the reference flux in that band, while the shaded
triangles indicate that the emission is consistent with the reference flux in
that band.

####  3.1.2 ATLAS

ATLAS currently consists of a quadruple 0.5-m telescope system that has two
units in Hawaii (Haleakala and Mauna Loa), one in Chile (El Sauce), and one in
South Africa (Sutherland). The ATLAS observing strategy is to obtain a
sequence of four 30-second exposures spread out over an hour (Smith et al.,
2020), using either a cyan $(c)$ filter [4420 – 650 nm] or an orange $(o)$
filter [560 – 820 nm], depending on the Moon phase (Tonry et al., 2018a).

ATLAS data are accessible through the ATLAS Forced Photometry
Server222https://fallingstar-data.com/forcedphot/ (Shingles et al., 2021). The
photometric routine used, called tphot, is based on algorithms described in
Tonry (2011) and Sonnett et al. (2013) and can be deployed on either reduced
or difference images. Both types of images have been calibrated
astrometrically and photometrically using the ATLAS All-Sky Stellar Reference
Catalog (Refcat, Tonry et al., 2018b), and the difference images also use a
modified version of the image subtraction algorithm HOTPANTS (Becker, 2015) to
subtract a reference sky frame.

Counting each 30-second exposure individually, ATLAS has observed the location
of SN2023ixf 2491 times between 30 July 2015 and 2 July 2023. We do not use
photometry from images where the location of SN 2023ixf is within 40 pixels of
a chip edge (10 images), where the best-fit axis ratio of a source at that
location is above 1.5 (16 images), where the $5\sigma$ limiting magnitude is
less than 16 (56 images), or where the ATLAS pipeline presents an error flag
(no images). This leaves us with 1844 $o$-band and 548 $c$-band images.

We then co-added nightly observations, discarding observations with flux
values over three times their uncertainty from the median flux, weighting the
remaining observations by their inverse variance in flux, and recording the
interpolated 50th percentile values for epoch, flux, and uncertainty. The
final light curve has 438 $o$-band entries and 123 $c$-band entries.

####  3.1.3 Zwicky Transient Facility

ZTF $g$-, $r$-, and $i$-band photometry covering the full ZTF survey from 17
March 2018 ($\rm JD=2458194.5$) until 08 July 2023 ($\rm JD=2460133.8$) were
obtained using the ZTF forced photometry service (Masci et al., 2019). This
covers a phase of $\sim 1850$ days prior to discovery until $\sim 50$ days
after discovery of SN2023ixf. Following the procedure outlined in the ZTF
forced photometry manual
v2.3333https://irsa.ipac.caltech.edu/data/ZTF/docs/ztf_forced_photometry.pdf,
we apply the standard baseline corrections to the data and use a signal-to-
noise threshold of 3 for all available data.

####  3.1.4 Swift UVOT

Due to its proximity and the discovery of SN2011fe in 2011, M101 has been
extensively monitored by Swift. Prior to the discovery of SN2023ixf, there had
been 218 observations overlapping the location of this event. These
observations were carried out between 29 August 2006 ($\rm MJD=53976.48613$)
and 08 December 2019 $\rm(MJD=58825.72311$) and have Swift target IDs of
35892, 30896, 32081, 32088, 32094, 32101, 33635, 11002, and 32481. These
observations were conducted using both the UltraViolet and Optical Telescope
(Roming et al., 2005, UVOT) and the X-ray Telescope (Burrows et al., 2005,
XRT). The total exposure time of these observations is $\sim 431$ ks.

Since the discovery of SN2023ixf, Swift has so far observed the source 58
times. These observations began on 20 May 2023 ($\rm MJD=60084.26901$) and
have Swift target IDs of 16038, 16043, 32481, and 89625. The total combined
exposure of these observations is $\sim 68$ ks. For the majority of Swift
observations, Swift observed either SN2023ixf or the location of the SN using
at least one or more of the six UVOT filters (Poole et al. 2008: $V$ (5425.3
Å), $B$ (4349.6 Å), $U$ (3467.1 Å), $UVW1$ (2580.8 Å), $UVM2$ (2246.4 Å), and
$UVW2$ (2054.6 Å)).

To derive the UVOT photometry both prior to and during the rise of SN2023ixf,
we used the HEASARC uvotsource package. To extract the UV and optical counts,
we used a circular region with a 5 arcsecond radius centered on the position
of SN2023ixf and source-free background regions with a radius of 20 arcseconds
located at ($\alpha$,$\delta$) = (14:03:42.5088, +54:18:12.172) and
($\alpha$,$\delta$) = (14:03:31.6839, +54:16:03.572) for the pre- and post-
discovery data, respectively. The UVOT count rates are converted into AB
magnitudes and flux densities using the most recent calibrations (Poole et
al., 2008; Breeveld et al., 2010). We do not correct the photometry for
Galactic extinction.

Figure 3: Post-explosion optical (top panel) and UV (bottom panel) light
curves of SN2023ixf as seen by Swift, ASAS-SN, ATLAS, and ZTF. Here, only data
detected with $\geq 3\sigma$ detection significance is shown.

###  3.2 X-ray observations

####  3.2.1 Swift XRT

In addition to the UVOT, Swift simultaneously observed the location and rise
of SN2023ixf using the XRT in photon-counting mode. Following the Swift XRT
reduction guide, we reduced all observations using the standard filter and
screening criteria, and the most up-to-date calibration files. Using the task
XRTPIPELINE, we reprocessed all level one XRT data, producing cleaned event
files and exposure maps for all observations.

To increase the signal-to-noise ratio of our observations, we combined our
individual images using XSELECT version 2.5b. Here, we combined the
observations taken prior to the discovery of SN2023ixf into 33 time bins,
spanning $\sim 6000$ days since the first serendipitous Swift observation of
this location. For the observations taken post-discovery, we combined these
observations into 19 time bins spanning $\sim 46$ days after first light. We
also merged all observations taken prior to and after the discovery of
SN2023ixf to produce a deep Swift XRT observation for both before and after
the discovery of SN2023ixf (see Figure 4 middle and right panel).

Background-subtracted count rates were derived from each of these merged
observations using a 20 arcsecond source region centered on the position of
SN2023ixf for observations taken after discovery and a 15 arcsecond source
region for observations taken prior to discovery. A smaller source region was
used for the prior observations due to the presence of an X-ray bright source
near the location of SN2023ixf (see Figure 4 left and middle panel). For all
observations we used the same source free background region with a radius of
100 arcseconds centered at $(\alpha,\delta)$ = (14:03:27.7541, +54:14:16.237).
All extracted count rates were corrected for the encircled energy fraction
(Moretti et al., 2004).

Using our merged event files, we extracted a spectrum from the position of the
source using the Swift analysis tool XRTPRODUCTS and the source and background
regions defined above. Ancillary response files were generated using the task
XRTMKARF and the individual exposure maps from xrtpipeline that were merged
using the HEASARC analysis tool ximage version 4.5.1. The response matrix
files were obtained from the most recent calibration database. The resulting
spectrum was grouped to have a minimum of 15 counts per energy bin using the
ftools command grppha.

####  3.2.2 Chandra

The location of SN2023ixf has been serendipitously observed using the Chandra
X-ray Observatory 21 times since 2000, with the most recent archival
observation taken in 2017. These observations were taken in VFAINT or FAINT
mode under the observation IDs: 934, 6175, 6170, 6169, 6152, 6118, 6115, 6114,
5323, 5322, 5309, 5300, 5297, 5296, 4737, 4736, 4735, 4733, 4732, 4731, 2065,
and 19304. These observations were conducted using the ACIS-S detector and
have a total combined exposure time of $\sim$ 966.2 ks.

All Chandra data were reduced using version 4.15 of the Chandra analysis
software CIAO. We reprocessed the level one data using the CIAO command
chandra_repro and the most up-to-date calibration database. To improve the
absolute astrometry of these observations, we used the CIAO tool wcs_match and
cross-matched X-ray sources found within these observations using the tool
wavdetect, with the USNO-A2.0
catalog444https://cxc.cfa.harvard.edu/ciao/threads/reproject_aspect/index.html#usno.
We then used reproject_obs to reproject these event files to a common tangent
point based on the updated world coordinate system (WCS) information of the
earliest and deepest Chandra observation in our dataset (ObsID: 934). This
command also takes the reprojected event files and merges them together to
form a single event file. We then used flux_obs to combine the reprojected
observations to create an exposure-corrected image in the broad (see Figure 4
left panel), soft, medium, and hard energy bands (see Figure 5).

To place constraints on pre-explosion X-ray emission, we used a circular
region centered on SN2023ixf with a radius of 2 arcseconds and a source-free
background region located at $(\alpha,\delta)$ = (14:03:27.4629,
+54:17:34.929) with a radius of 20 arcseconds. A region of this size encloses
95% of all source photons at 1.496 keV, and as such, the extracted count rates
were corrected for encircled energy
fraction.555http://cxc.harvard.edu/proposer/POG/html/chap4.html

Figure 4:  (Left Panel): The merged, broadband, pre-explosion Chandra
observation of the location of SN2023ixf. The two arcsecond radius green
circle shows the location of SN2023ixf, and the black, cyan, and magenta
crosses mark the locations of the high-mass X-ray binaries (HMXBs) CXO
J140341.1+541903, CXO J140336.1+541924, and [CHP2004] J140339.3+541827,
respectively. (Middle Panel): The merged, broadband Swift observation obtained
using all available pre-explosion observations. The green circle here has a
radius of 15 arcseconds and is centered on SN2023ixf used to derive count
rates. (Right panel:) The merged, broadband Swift observation created using
all available post-explosion observations. Here the green circle has a 20
arcsecond radius and is centered on SN2023ixf. Significant X-ray emission
arises from the location of the source. Note that the images are all aligned
to a common reference frame. Figure 5: A merged and exposure-corrected Chandra
X-ray image of the location of SN2023ixf (green diamond). The black, cyan, and
magenta crosses mark the locations of HMXBs CXO J140341.1+541903, CXO
J140336.1+541924, and [CHP2004] J140339.3+541827, respectively. Here, the 0.5
– 1.2 keV (soft) emission is in red, the 1.2 – 2.0 keV (medium) emission is in
green, and the 2.0 – 7.0 keV (hard) emission is in blue.

####  3.2.3 XMM-Newton

XMM-Newton observed the location of SN2023ixf four times prior to its
explosion. The first observation was taken in 2002, with the most recent from
2018. These observations have ObsIDs 0104260101, 0164560701, 0212480201, and
0824450501, totaling $\sim 220\,$ks prior to filtering. To analyse these
observations, we used the XMM-Newton Science System (SAS) version 20.0.0 and
the most up-to-date calibration file. Before extracting count rates, we
produced cleaned event files by removing time intervals which were
contaminated by a high background, or those during which proton flares were
identified when generating count rate histograms for energies between 10.0 –
12.0 keV. We used the standard screening criteria as suggested by the current
SAS analysis threads and XMM-Newton Users Handbook. For the MOS detectors, we
used single to quadruple patterned events (PATTERN $\leq$ 12), while for the
PN detectors only single and double patterned events (PATTERN $\leq$ 4) were
selected. The standard canned screening set of FLAGS for both the MOS
(#XMMEA_EM) and PN (#XMMEA_EP) detector were also selected.

Count rates were extracted from a circular region with a radius of 20
arcseconds centered on the location of SN2023ixf and a source-free background
region of radius 90 arcseconds centered at $(\alpha,\delta)$ = (14:03:40.1033,
+54:21:13.639). As a region of this size encloses only $\sim 80$% of all
source photons, all extracted counts were corrected for this aperture. Before
extracting our counts, we combined both the MOS and PN detectors for each
observation using the SAS tools command merge.

##  4 Analysis

In this section, we infer the properties of the progenitor of SN2023ixf by
searching for evidence of pre-explosion variability and mass loss using both
pre- and post-explosion observations.

###  4.1 Properties of the explosion

To constrain the pre-explosion mass loss using post-explosion X-ray
observations (see Section 5.2.2), we require an estimate of the bolometric
luminosity. As such, we use the SUPERBOL pipeline (Nicholl, 2018) to calculate
the bolometric light curve using the Swift UVOT light curves extracted post-
explosion and the $u$/$U$, $B$, $g$, $V$, $r$/$R$, $i$/$I$, and $z$ photometry
published in Figure 1 of Jacobson-Galan et al. (2023). To show all light
curves across the same timescale, we used the $i$-band as the reference filter
and interpolated/extrapolated each light curve using a polynomial of order
four between MJD 60082 and MJD 60100. After correcting for Galactic extinction
(E$(B-V)=0.033$ mag, Kilpatrick et al. 2023), we then fit the resulting SED
with a blackbody model to derive the luminosity, radius, and temperature. In
Figure 6, we present our derived bolometric luminosity for SN2023ixf, along
with the bolometric luminosities of other Type IIn SNe and the
pseudobolometric luminosity for SN2023ixf from Hiramatsu et al. (2023).

Figure 6: The bolometric light curve of SN2023ixf (yellow stars) compared to
the pseudobolometric light curve ($UBVRI$, blue stars) from Hiramatsu et al.
(2023) and a sample of other Type IIn SNe. Data Sources: SN2006gy (Smith et
al., 2010); SN2010jl (Chandra et al., 2015); SN2014c (Margutti et al., 2017);
SN2015da (Tartaglia et al., 2020); SN2017hcc (Prieto et al., 2017); SNe
2018bwr, 2018gwa, 2018kag, 2018lnb, 2019cac, 2019cmy, 2019ctt, 2019dde,
2019dnz, 2019qt (Soumagnac et al., 2020); and all remaining SNe (Taddia et
al., 2013).

Our analysis in Section 5.2.2 also requires an estimate of the ejecta mass.
From the analytical light curve model of Hiramatsu et al. (2023) that
attributes the SN emission to shock interaction between the ejecta and the
CSM, we use $\nu=\sqrt{\frac{2(5-\delta)(n-5)E_{SN}}{(3-\delta)(n-3)M_{SN}}}$
to estimate the ejecta mass. Here, $M_{SN}$ is the ejecta mass, $E_{SN}$ is
the explosion energy, the exponents of the broken power law that describes the
density of the unshocked SN ejecta are $\delta=0$ and $n=12$ (as is standard
for RSG progenitors), and $\nu$ is the characteristic velocity of the ejecta
which corresponds to the photospheric velocity at maximum light (see Equation
4 from Hiramatsu et al., 2023). For $\nu$, we use the lower limit on the SN
shock velocity from Jacobson-Galan et al. (2023), who determined a value of
8500 km s-1 using the blue edge of the H$\alpha$ absorption profile. Assuming
$E_{SN}=(0.5-2)\times 10^{51}$ erg s-1, we get an ejecta mass of $0.9-3.6\,\rm
M_{\odot}$, with an energy of $E_{SN}=10^{51}$ erg s-1 corresponding to an
ejecta mass of $1.8\,\rm M_{\odot}$.

Figure 7: The peak-to-peak luminosity changes of the pre-SN differential
luminosity ($\Delta\lambda L_{\lambda}$) of the SN2023ixf progenitor as
observed in the Swift UVOT filters (solid coloured circles). The solid
horizontal lines correspond to the root mean square of the peak-to-peak
luminosity of our pre-explosion light curves, while the dotted lines
correspond to the 1$\sigma$ error. The grey squares correspond to the mean of
the peak-to-peak luminosity changes of our comparison sample, while the shaded
grey regions correspond to the standard deviation of this mean. The observed
scatter in the luminosity of SN2023ixf’s progenitor is consistent with the
comparison sample, indicating no pre-SN variability of SN2023ixf at these
wavelengths. Figure 8: The peak-to-peak luminosity changes of the pre-SN
differential luminosity ($\Delta\lambda L_{\lambda}$) of the SN2023ixf
progenitor as observed in the ZTF filters. See Figure 7 for more details.

Figure 9: The peak-to-peak luminosity changes of the pre-SN differential
luminosity ($\Delta\lambda L_{\lambda}$) of the SN2023ixf progenitor as
observed in the ATLAS filters. See Figure 7 for more details.

Figure 10: Differential luminosity of the SN2023ixf progenitor as observed in
the ASAS-SN filters. See Figure 7 for more details.

###  4.2 Variability

To constrain the presence of variability prior to explosion, we take advantage
of the methods presented in Johnson et al. (2017, 2018) and Neustadt et al.
(2023). Here, we examine the variability of the progenitor by calculating the
peak-to-peak luminosity changes of the pre-SN differential light curves
($\Delta\lambda L_{\lambda}$). We then compare this variability to both the
root mean square (RMS) of our data and to the RMS scatter of the comparison
sample about the mean of their peak-to-peak luminosity changes. Our comparison
sample consists of regions nearby the position of the SN from which these
comparison light curves were extracted. This was done to better understand any
systematic errors in the light curves. For our Swift and ZTF sample, we used a
total of 12 control light curves, while for ATLAS and ASAS-SN we used 7 and
13, respectively. These sample points were chosen to avoid obvious nearby
sources such as the optical/UV bright HMXBs shown in Figure 4 and Figure 5.

In Figures 7 – 10, we present our resulting peak-to-peak luminosity
variability analysis for both the progenitor of SN2023ixf and our control
light curves.

Our plots suggest that for the majority of wavelengths we analysed, the
observed scatter in the luminosity at the location of SN2023ixf is consistent
with the comparison sample. This suggests that there is no evidence of pre-SN
variability in these bands, consistent with the results of Neustadt et al.
(2023) and Dong et al. (2023). We do see some evidence of possible dimming
detected in the Swift W2 band nearly 6000 days prior to explosion. However,
when one considers the uncertainties in our analysis, these data points lie no
more than 1.5$\sigma$ from the mean of our controls, suggesting that it may
not be significant. Similarly, for the ATLAS $o$-band, ASAS-SN $g$-band, and
ASAS-SN $V$-band, the few data points detected above the mean RMS and standard
deviation of the controls and RMS of the data have large uncertainties and are
thus consistent with the background. This suggests that there is no evidence
of stochastic variability in these bands. Using the mean RMS of these light
curves (see Table 1), we adopt an upper limit on the variability in these
bands of $\lesssim 7\times 10^{4}\rm\,L_{\odot}$.

To further investigate any long-term trends in the luminosity and search for
coherent variability, we also fit the changes in the band luminosities with a
simple linear function $L(t)=At+B$. For completeness, we also perform the same
fit to the comparison sample. We find that slopes are both positive and
negative across the bands and have a mean slope of $\sim 2\times
10^{2}\rm\,L_{\odot}$ yr-1. All bands have a slope that is consistent with
zero and/or comparable to the mean slope of the control samples. This suggests
that there is no evidence of a long-term variability during the final years
before explosion down to a luminosity of $\lesssim 2\,\times
10^{2}\rm\,L_{\odot}\,$yr-1 in these bands. This is consistent with that found
by Neustadt et al. (2023) and Dong et al. (2023).

###  4.3 X-ray properties

Due to the proximity of M101, Chandra, XMM-Newton, and Swift have all observed
SN2023ixf and the surrounding field in detail. As such, this provides us with
the opportunity to place constraints not only on the X-ray emission associated
with SN2023ixf as it evolves, but also allows us to place constraints on the
pre-explosion properties of the progenitor.

In Figure 11, we show the X-ray light curve from the individual Chandra and
XMM-Newton observations, as well as from the combined Swift observations. For
the Swift observations, we also show hardness ratios for the emission detected
after discovery.

In Figure 12, we show our Swift X-ray spectrum. To estimate the X-ray
luminosity, we convert our extracted count rate into a flux using
WebPIMMS666https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl and
assume an absorbed bremsstrahlung model redshifted to M101 that has a
temperature of $\Gamma=35$ keV and a column density of $N_{H}\sim 3.9\times
10^{22}$ cm-2 (see Section 4.3.2 for more discussion about the spectral
properties).

####  4.3.1 Pre-explosion

We find no significant evidence of X-ray emission arising from the location of
SN2023ixf up to 8500 days prior to discovery (see Figure 11 upper panel). The
most constraining limit comes from the available Chandra X-ray observations,
which are nearly an order of magnitude deeper than the XMM-Newton and Swift
limits presented in Figure 11. As such, to place the deepest constraint on the
X-ray emission at the location of SN2023ixf, we merge all Chandra data
together and determine a 3$\sigma$ count rate upper limit of $8\times 10^{-5}$
counts/sec in the 0.3–10.0 keV energy range. Assuming our best-fit absorbed
bremsstrahlung model (see Section 4.3.2), this gives us an absorbed flux
(0.3–10.0 keV) of $2.1\times 10^{-15}$ erg cm-2 s-1, which corresponds to an
absorbed (unabsorbed) X-ray luminosity (0.3–10.0 keV) of $3.3\times 10^{36}$
erg s-1 ($5.7\times 10^{36}$ erg s-1). If we use the more recent, but
shallower Swift observations that cover up to $6000$ days prior to explosion,
we obtain a 3$\sigma$ count rate upper limit of $2\times 10^{-4}$ counts/s in
the 0.3–10.0 keV energy range, which corresponds to an unabsorbed X-ray
luminosity (0.3–10.0 keV) of $5.8\times 10^{37}$ erg s-1 assuming our best-fit
absorbed bremsstrahlung model.

In the left panel of Figure 4, we show the pre-explosion image from our merged
Chandra observations, while in the centre panel of Figure 4, we have the pre-
explosion image created by merging all available pre-explosion Swift data.

We do note that after we merged all Swift observations taken prior to
discovery (Figure 4 middle panel), our analysis suggests the presence of
possible weak ($\sim 3\sigma$) X-ray emission coincident with the location of
the SN. This is apparent for the observations found between 3200 and 4300 days
prior to discovery (see Figure 11 upper panel). Using the same model as above,
this emission has an absorbed X-ray luminosity (0.3–10 keV) between $4.8\times
10^{37}$ $\rm erg\,s^{-1}$ and $3.7\times 10^{38}$ $\rm erg\,s^{-1}$. However,
we note that close to the location of SN2023ixf, our merged Chandra
observations (Figure 4 left panel) show the HMXBs CXO J140341.1+541903, CXO
J140336.1+541924, and [CHP2004] J140339.3+541827 (black, cyan, and magenta
crosses in Figure 4, Evans et al., 2010; Mineo et al., 2012). Unfortunately,
due to the spatial resolution of the Swift XRT, it is difficult to disentangle
the emission from SN2023ixf and that associated with these nearby binary
systems (see Figure 4 middle panel). As such, we believe it likely that this
emission is associated with these sources rather than pre-explosion X-ray
emission from the SN itself.

Figure 11: Upper: Broadband ($0.3-10.0$ keV) X-ray light curve of SN2023ixf
prior to explosion as seen by Chandra, XMM-Newton, and Swift. The down arrows
correspond to $3\sigma$ (or $3.5\sigma$) upper limits and the lightly shaded
data points correspond to $3\sigma$ X-ray detections which we argue are most
likely associated with a nearby HMXB. Lower left: The broadband X-ray light
curve of SN2023ixf post-explosion emission as seen by Swift. Lower right: The
hardness ratios (as derived using $\rm HR=(Soft\ Counts-Hard\ Counts)/(\rm
Soft\ Counts+Hard\ Counts)$) of the post-explosion X-ray emission, with up
arrows indicating the 3$\sigma$ lower limits. The emission is relatively hard,
consistent with a high-temperature thermal component.

####  4.3.2 Post-explosion

Swift began monitoring the evolution of SN2023ixf $\sim$ 0.67 days after
discovery (MJD = 60083.73), or $\sim$ 1.57 days after first light (MJD =
60082.833). Swift did not detect soft X-ray (0.3–10.0 keV) emission until 4.25
days after first light (or 3.36 days after discovery). This is in contrast to
the discovery of hard X-rays ($>3$ keV) at $\sim 3.9$ days after first light
using NuSTAR (Grefenstette et al., 2023). However, this is likely not
surprising since the significantly enhanced column density measured by
Grefenstette et al. (2023) at this time implies the soft X-rays would be
absorbed. To place the strongest constraints on the emission during the first
$\sim 4$ days after first light, we merged the first 13 post-discovery Swift
observations of SN2023ixf and from this calculated a 3$\sigma$ upper limit to
the count rate of $5\times 10^{-4}$ counts/s in the 0.3 – 10.0 keV energy
range.

At 4.25 days after first light, Swift detected X-ray emission with a
significance of $>4.5\sigma$ (see Figure 4 lower left panel). This emission
has continued to rise nearly an order of magnitude to its current peak count
rate of $0.0094\pm 0.002$ counts/s, corresponding to an absorbed X-ray (0.3–10
keV) luminosity of $(1.6\pm 0.4)\times 10^{39}$ erg s-1 assuming our best-fit
bremsstrahlung model discussed below.

Unfortunately, due to the relatively short exposures of the Swift
observations, we are unable to extract a spectrum from each individual
observation. As such, we merged all available post-explosion Swift
observations and extracted a spectrum to constrain the nature of this
emission. This spectrum describes the soft X-ray emission arising from
SN2023ixf in the first $\sim 46$ days since first light and its best-fit
spectral model discussed below is shown in Figure 12.

To study the nature of this emission, we fit the 0.3 – 10.0 keV spectrum using
an absorbed power law redshifted to the host galaxy (XSPEC model:
tbabs*zashift*powerlaw), similar to that done by Grefenstette et al. (2023).
However, due to the emission seen at energies $\lesssim 2$ keV, we find that
our best-fit model with a column density of $N_{H}=(3.4_{-1.1}^{+1.5}\times
10^{22}$) cm-2 and a photon index of $\Gamma=1.6\pm 0.5$ fails to fit the
emission seen at these lower energies. However, if we instead fit the spectrum
at energies $\gtrsim$ 2 keV, our fit is consistent with that of Grefenstette
(2023) using NuSTAR data.

Assuming that this power law results from either X-ray synchrotron emission or
inverse Compton (IC) scattering as is typical for X-ray emitting SNe (e.g.,
Chevalier & Fransson, 2006), then the observed X-ray flux would be
proportional to $\nu^{1-\Gamma}$, where $\nu$ is the frequency and $\Gamma$ is
the photon index derived from our best-fit power law. As such, a value of
$\Gamma=1.6$ as derived above, would indicate a hard X-ray spectrum, which is
consistent with that seen from other X-ray bright SNe (e.g., Chandra, 2018).

However, for SNe that are interacting with a dense CSM, their fast shocks
result in hard X-ray emission that is dominated by thermal bremsstrahlung
emission with temperatures $>1$ keV (e.g., as seen in SN2014C, SN2010jl, and
SN2006jd: Chandra et al., 2012, 2015; Katsuda et al., 2016; Margutti et al.,
2017; Thomas et al., 2022; Brethauer et al., 2022). Since these high-
temperature thermal models can mimic hard power laws, we also fit the spectrum
using an absorbed, thermal-bremsstrahlung model (XSPEC model:
tbabs*zashift*bremss). As with our best-fit power law model, our best-fit
thermal model with a column density of $N_{H}=(2.4_{-0.5}^{+0.7}\times
10^{22}$) cm-2 and a temperature of $kT=33_{-30}^{+50}$ keV fails to fit the
emission seen at lower energies (see Figure 12). Nonetheless, the thermal
model is a better fit than the single power law and is consistent with that
found by Chandra et al. (2023b) using Chandra and by Grefenstette et al.
(2023) using a second epoch of NuSTAR observations taken $\sim 10$ days after
first light.

In an attempt to explain this excess seen at lower energies, we tried adding
either an absorbed bremsstrahlung model or an absorbed power law model to our
best-fit bremsstrahlung model. This is similar to what was done for SN2010jl
(Fransson et al., 2014; Chandra et al., 2015), whose Chandra and Swift
observations showed the presence of an additional soft component $\sim 40-600$
days after discovery, which the authors attributed to the cooling of the shock
front. We find that an additional bremsstrahlung model significantly improves
the fit. With this additional component, the spectrum is best fit (reduced
$\chi^{2}$ = 1.02) with a column density of
$N_{H,soft}=(0.9_{-0.3}^{+0.5}\times 10^{22}$) cm-2 and a temperature of
$kT_{soft}=0.3\pm 0.1$ keV for the low-energy (soft) component, and a column
density of $N_{H,hard}=(3.9_{-1.7}^{+1.2}\times 10^{22}$) cm-2 and a
temperature of $kT_{hard}=35_{-27}^{+110}$ keV for the high-energy (hard)
component. This fit and its residuals are plotted in magenta in Figure 12.

To derive the fluxes and luminosities required for our analysis in Section 5,
we use our best-fit column density ($N_{H,hard}=3.9\times 10^{22}$ cm-2) and
temperature ($kT_{hard}=35$ keV) associated with the high-temperature
component of our best-fit, two-component, bremsstrahlung model. This high
temperature is thought to result from the expanding forward shock, meaning we
can use this temperature to place constraints on the shock velocity and
compare our results to those determined from optical spectra. To calculate the
shock velocity, we use $\nu_{s}=[(16k_{B}T_{sh})/(3\mu m_{H})]^{1/2}$, where
$\nu_{s}$ is the shock velocity, $k_{B}T_{sh}$ is our shock temperature,
$\mu=0.604$ is the mean atomic weight, $k_{B}$ is Boltzmann’s constant, and
$m_{H}$ is the mass of hydrogen (e.g., Auchettl et al., 2014). Using this
equation and our temperature from the high-energy component of our best-fit,
two-component bremsstrahlung model, we find a shock velocity of
$5440_{-2840}^{+5630}$ $\rm km\,s^{-1}$ which is consistent with that found by
Jacobson-Galan et al. (2023) using H$\alpha$ absorption profiles and
Grefenstette et al. (2023) using NuSTAR data.

Figure 12: Swift XRT X-ray spectrum obtained by merging all observations taken
within the first $\sim 46$ days of first light (black data points). Shown as
the magenta dot-dashed line is the best-fit, two-component, absorbed
bremsstrahlung model. The teal dashed line is our best-fit, single-component,
absorbed bremsstrahlung model, while the solid blue line is our best-fit,
single-component, absorbed power law model. We have also plotted as an orange
dotted line the best-fit model from Grefenstette et al. (2023) using their
second epoch NuSTAR observations. Shown in the bottom panel are the residuals
of our best-fit models.

##  5 Discussion

###  5.1 Lack of precursor variability from optical to X-rays

One way of constraining pre-explosion mass loss is to search for variable
precursor emission from the progenitor. Evidence of end-of-life mass loss may
also be seen in the form of precursor outbursts that are detected in the weeks
to months before the main SN explosion (Ofek et al., 2014). These outbursts
are thought to be associated with the different stages of nuclear burning
(e.g., Fuller, 2017; Shiode & Quataert, 2014; Wu & Fuller, 2021),
instabilities in nuclear shell burning (e.g., Woosley et al., 2002), or
interaction with a binary companion (e.g., Matsuoka & Sawada, 2023).

Such an analysis has been performed for SN2023ixf in both the mid- and near-IR
using Spitzer and ground-based observations, respectively (Kilpatrick et al.,
2023; Jencson et al., 2023; Soraisam et al., 2023b), in some optical bands
(Neustadt et al., 2023; Dong et al., 2023), and in the UV with GALEX (Flinner
et al., 2023). Kilpatrick et al. (2023); Jencson et al. (2023) and Soraisam et
al. (2023b) showed that the progenitor exhibited significant variability with
a period of $\sim$ 1000 – 1200 days and an amplitude similar to the more
luminous population of pulsating, dusty RSGs. However, Jencson et al. (2023)
found no evidence for re-brightening due to eruptive, pre-SN outbursts
predicted from early, post-SN observations (e.g., Jacobson-Galan et al.,
2023), nor due to those expected from instabilities on the timescales of the
final nuclear burning stages (e.g., Woosley et al., 2002). Similarly, Neustadt
et al. (2023) and Dong et al. (2023) found no evidence of variability in the
optical, with Neustadt et al. (2023) deriving a limit of $<100$ $\rm
L_{\odot}$ yr-1, suggesting that the progenitor did not undergo a luminous
outburst within the 15 years prior to discovery or it would have led to
changes in the optical depth, making it detectable.

We search for evidence of possible pre-explosion variability/emission not only
in the bands similar to that observed by Neustadt et al. (2023) and Dong et
al. (2023), but also using X-ray data from Swift, and UV/optical data from the
Swift UVOT, as well as that observed using ATLAS, ZTF, and ASAS-SN. Here, we
consider not only emission during the last year prior to discovery, but also
the possibility of pre-explosion variability spanning nearly 2 decades of
observations.

Following the analysis presented in Johnson et al. (2017, 2018) and Neustadt
et al. (2023) (see Section 4.2), we found no evidence of variability in
optical to UV bands as detected by Swift, ASAS-SN, ZTF, and ATLAS in the $\sim
20$ years prior to explosion. This is consistent with that found by Neustadt
et al. (2023) and Dong et al. (2023). We derived a limit on the stochastic
variability of $\lesssim 7\times 10^{4}\rm\,L_{\odot}$ and on the long-term
variability of $<\,2\times 10^{2}\,\rm L_{\odot}$ yr-1. These values are
consistent with that obtained by Neustadt et al. (2023). In X-rays, we find no
evidence of pre-explosion emission down to a limit of $5.8\times 10^{37}$ erg
s-1 in the 16 years ($\sim 6000$ days) prior to explosion.

Neustadt et al. (2023) suggested that due to the heavy obscuration associated
with the source, any short-lived outburst with a peak luminosity
$\gtrsim\,5\times 10^{5}\,\rm L_{\odot}$ should lead to a detectable signature
in the decade-long light curves, unless it lined up with a seasonal gap. Due
to the extensive coverage by Swift, ASAS-SN, ATLAS, and ZTF prior to
explosion, our results suggest that there was no bright, short-lived, pre-SN
outburst within the $\sim$5 years prior to explosion, similar to that found by
Dong et al. (2023). In addition, our results for Swift, ASAS-SN, ATLAS, and
ZTF in Table 1 require that any outburst had a luminosity $\lesssim 7\times
10^{4}\rm\,L_{\odot}$. This is consistent with the suggestion by Neustadt et
al. (2023), Dong et al. (2023), and Jencson et al. (2023) that there was no
luminous, short-lived outburst within the 15 years prior to explosion, but in
contrast to the prediction by Jacobson-Galan et al. (2023) that the dense CSM
inferred from early spectra could have arisen if the progenitor experienced
periods of enhanced mass loss during the final 3 – 6 years before core
collapse.

However, the lack of variability seen in these wavelengths may not be so
surprising. Currently, there have been nearly 30 SNe that have exhibited pre-
SN activity (e.g., Jacobson-Galán et al., 2022a; Strotjohann et al., 2021),
but current surveys searching for precursors and SN progenitors (e.g., Johnson
et al., 2017, 2018) suggest that not all SNe exhibit luminous precursors and
at most, $\sim$ 2.5% of all core-collapse SNe or $\sim$ 25% of Type IIn SNe
produce luminous, eruptive outbursts prior to explosion (Strotjohann et al.,
2021). As such, it is possible that the formation of SN2023ixf’s CSM results
not from episodic mass loss, but rather from binary interactions (Matsuoka &
Sawada, 2023) that gave rise to its observed asymmetry (Smith et al., 2023;
Vasylyev et al., 2023). Here, the collision of the winds from the two stars in
the binary system would produce a high-density (and possibly asymmetric) CSM
around the progenitor (e.g., Kochanek, 2019).

###  5.2 Pre-explosion mass loss

Using pre- and post-explosion X-ray observations, it is possible to place
constraints on the density of the CSM. This is because the mass that is
ejected from the progenitor star during the pre-SN phase becomes the CSM with
which the SN ejecta interacts following the explosion.

####  5.2.1 Pre-explosion X-ray constraints on the mass loss

The interaction of the shock with the CSM can power the resulting SN and
produce both non-thermal X-rays and radio emission (e.g., Margutti et al.,
2014, 2017). However, evidence of mass loss may also be seen as precursor
outbursts in X-rays and at other wavelengths. Due to its proximity, the
location of SN2023ixf has been extensively observed using Chandra, Swift, and
XMM-Newton (see Figure 4). Similar to our analysis of the pre-explosion
optical and UV light curves (see Section 4.2), our pre-explosion X-ray
analysis also finds no evidence of luminous precursor emission at higher
energies.

Using our deep pre-explosion Chandra observation, we can place constraints on
the pre-explosion mass loss. Since we find no evidence of outbursts that would
be consistent with an eruption scenario, we assume that any precursor emission
would have been generated by a wind with a mass loss rate $\dot{M}$ and wind
velocity $v_{w}$. Equation 30 from Matsumoto & Metzger (2022) allows us to
estimate the mass-loss rate using the progenitor mass ($M_{\star}$), wind
velocity ($v_{w}$), and precursor luminosity ($L_{pre}$) according to
$\dot{M}\simeq
1.5\left(\frac{M_{\star}}{10\,M_{\odot}}\right)^{-2}\left(\frac{v_{\rm
w}}{10^{3}{\,\rm km\,s^{-1}}}\right)^{-2}\left(\frac{L_{\rm
pre}}{10^{40}{\,\rm erg\,s^{-1}}}\right)^{3}\ \rm M_{\odot}\,yr^{-1}$.

Adopting a progenitor mass between 11 and 20 $\rm M_{\odot}$ (Kilpatrick et
al., 2023; Jencson et al., 2023; Soraisam et al., 2023b), a wind velocity of
50 km s-1, and using for $L_{pre}$ our unabsorbed 3$\sigma$ upper limit to the
X-ray emission from Chandra observations that span $>\,6000$ days ($>$ 18
years) prior to explosion ($L_{X}<5.7\times 10^{36}$ erg s-1), we get
$\dot{M}=(3-9)\times 10^{-8}\rm\,M_{\odot}$ yr-1. If we use the more recent
Swift upper limit derived from merging all available Swift pre-explosion
observations that cover $<6000$ days ($<$ 16 years) prior to explosion
($L_{X}<5.8\times 10^{37}$ erg s-1), we get $\dot{M}=(3-10)\times
10^{-5}\rm\,M_{\odot}$ yr-1.

The mass-loss rate derived using the more recent Swift constraint (covering
$<$ 16 years prior to explosion) is consistent with the mass loss expected for
normal RSGs (Beasor et al., 2020) and that derived by Neustadt et al. (2023)
and Jencson et al. (2023), who modelled the IR SED of the progenitor assuming
it is surrounded by a dusty CSM. However, it is lower than that derived by
Jacobson-Galan et al. (2023) from early spectra and Soraisam et al. (2023c)
who used the mass-loss prescription of Goldman et al. (2017) and a SED-
independent luminosity for the progenitor. As our mass-loss rate from deep
Chandra observations spanning $>18$ years prior to explosion is even more
constraining, this could suggest that the mass loss may have increased over
the two decades prior to explosion. This is consistent with the results of
Jencson et al. (2023), whose findings suggest that the progenitor had a steady
but enhanced wind that developed over the final decade or more prior to
explosion.

####  5.2.2 Post-Explosion X-ray Constraints on the Mass Loss

Post-explosion X-ray observations can also be used to place deep constraints
on the properties of the CSM for Type Ia (e.g., Margutti et al., 2012), Type
Ibc (e.g., Drout et al., 2016), and Type Ib/IIn SNe such as SN2014c (e.g.,
Margutti et al., 2014). In these papers, they follow the formalism that within
the first $\sim$ month of evolution, the X-ray emission is dominated by IC
scattering that results in photons from the photosphere being upscattered to
X-ray energies by relativistic electrons from the expanding SN shock front
(Björnsson & Fransson, 2004; Chevalier & Fransson, 2006; Margutti et al.,
2017, 2012).

If the progenitor lost material at a constant rate $\dot{M}$, which seems to
be consistent with the results presented in Section 5.2 and those derived by
Jencson et al. (2023), we can use the formalism presented in Equations A7 and
A8 of Margutti et al. (2012) and Margutti et al. (2018b) to calculate the wind
density ($\dot{M}/v_{w}$). We can also use the fact that Swift observations of
SN2023ixf taken within the first $\sim 3.3$ days after first light showed no
soft X-rays. Merging these observations, we derive a $3\sigma$ upper limit to
the unabsorbed luminosity in the 0.3 – 10.0 keV energy band of $7.2\times
10^{38}$ erg s-1.

To calculate the corresponding IC emission, we assume the following: that the
ejecta has a density profile that follows $\rho\propto R^{-n}$ with $n\sim 9$
as appropriate for compact progenitors (Matzner & McKee, 1999), that the
shock-accelerated electrons are best described by a power-law distribution
with an index of 3, that 1% of the post-shock energy density goes in
relativistic electrons (Reynolds et al., 2021), that the shock velocity
depends on the wind velocity, explosion energy, ejecta mass, and CSM density
which is described by $\rho_{CSM}=\dot{M}/4\pi\nu_{w}R^{2}$, and that
$L_{X-ray}\propto L_{bol}$, where $L_{bol}$ is the bolometric luminosity of
the source.

Using an explosion energy of $10^{51}$ erg and an ejecta mass of
$M_{ej}=1.8\,\rm M{{}_{\odot}}$ as estimated in Section 4.1, we find that the
lack of soft X-rays during the first $\sim 3.3$ days after first light implies
a mass-loss rate of $\dot{M}\lesssim 5\times 10^{-4}\,\rm M_{\odot}\,\rm
yr^{-1}$, assuming a wind velocity of 50 km s-1. Using our derived mass-loss
rate, and Equations A3 and A7 from Margutti et al. (2018b), we estimate that
this low-density environment occurs out to a distance of $R<3.7\times 10^{15}$
cm.

As NuSTAR observations showed that the source was X-ray bright in the hard
X-ray band (Grefenstette et al., 2023), it is possible that SN2023ixf was
emitting X-rays much earlier than the first Swift detection at 4.26 days after
first light, but that the exposure time and sensitivity of the Swift
observations were not sufficient to detect the highly-absorbed, high-
temperature emission from the source. As such, if we perform the calculation
above but instead use the time of the first Swift observation (1.57 days after
first light) and its corresponding bolometric luminosity, we get
$\dot{M}<3.1\times 10^{-4}\,\rm M_{\odot}\,yr^{-1}$ and $R<1.8\times 10^{15}$
cm. These results and those derived above are consistent with that calculated
using our pre-explosion constraints and those published in the literature,
including the mass-loss rate from Grefenstette et al. (2023) using NuSTAR data
and the shock radii derived by Grefenstette et al. (2023), Smith et al.
(2023), and Jacobson-Galan et al. (2023).

We can also use our best-fit hydrogen column density
($N_{H,hard}=(3.9_{-1.7}^{+1.2}\times 10^{22}$) cm-2) to place a constraint on
the mass loss, which yields a value consistent with Grefenstette et al.
(2023). Equation 4.1 from Fransson et al. (1996) gives $N_{H}=(2.1\times
10^{22})(1/(s-1))(v_{w}/10\,$km
s${}^{-1})^{-1}(\dot{M}/10^{-5}M_{\odot})(v_{s}/10^{4}$ km
s${}^{-1})^{1-s}(t/8.90$ days$)^{1-s}$ cm-2, where $N_{H}$ is the column
density derived in Section 4.3, $t$ is the time at which this column density
was measured, $\dot{M}$ is the mass-loss rate, $v_{s}$ is the shock velocity
as derived in Section 4.3, $v_{w}=50$ km s-1 is the wind velocity, and $s=2$
is the index of the density profile. We find that the mass-loss rate of the
progenitor was $(1.6_{-1.0}^{+0.9})\times 10^{-4}\rm M_{\odot}$ yr-1, which is
similar to both Grefenstette et al. (2023) and what we derived above.

Our derived shock radius suggests that the CSM of SN2023ixf is relatively
compact. This is consistent with Grefenstette et al. (2023), who measured a
rapidly decreasing column density between their observations, but also with
the disappearance of CSM interaction signatures within 8 days of discovery
(e.g., Jacobson-Galan et al., 2023; Smith et al., 2023) and the non-detection
at millimeter wavelengths (Berger et al., 2023a).

Assuming a constant shock velocity, we can use the ratio of the shock velocity
($\nu_{shock}$) to the CSM velocity (or the wind velocity, $\nu_{wind}$) and
the time of observation ($t_{obs}$) to place constraints on the pre-SN
ejection time of the CSM ($t_{ejection}$) according to
$t_{ejection}>t_{obs}(\nu_{shock}/\nu_{wind})$ from Dickinson et al. (2023).
Using our derived mass-loss rate, the shock velocity derived in Section 4.3
and a wind velocity of 50 kms-1, we find that SN2023ixf exhibited a mass loss
episode at least $>0.5-1.5\,(v_{w}/50$ km s${}^{-1})$ years prior to
explosion. This is consistent with Jacobson-Galan et al. (2023), Kilpatrick et
al. (2023), Jencson et al. (2022), and Smith et al. (2023), who suggested that
the progenitor experienced enhanced mass loss $>$1 year prior to explosion.

###  5.3 X-ray constraints on the CSM

In Figure 12, we present our merged Swift X-ray spectrum with the best-fit
models discussed in Section 4.3 and a model using the best-fit parameters
($N_{H}=5.6\times 10^{22}$ cm-2 and $kT=34$ keV) from epoch two ($\sim 11$
days after first light) associated with the early NuSTAR data from
Grefenstette et al. (2023). While both our single-component models and the
NuSTAR model describe the data equally well for energies $>3$ keV, these
models underestimate the emission at energies $<2$ keV. We find that an
additional low-temperature, absorbed bremsstrahlung component is able to
reproduce the emission seen at energies $<2$ keV.

Similar to the well-studied Type IIn SN2010jl (Fransson et al., 2014; Chandra
et al., 2015), we attribute this soft component to the cooling of the forward
shock. Using the mass-loss constraint derived in Section 5.2.2 and the shock
velocity derived from our high-temperature, bremsstrahlung component (see
Section 4.3.2), we can calculate the cooling time. To do this we use Equation
6 from Fransson et al. 2014: $t_{\rm cool}=26.6\left({\dot{M}\over
0.1\,M_{\odot}\rm yr^{-1}}\right)^{-1}\left({v_{\rm w}\over 100\,km\rm
s^{-1}}\right)\left({V_{\rm s}\over 3000\,\rm km\rm
s^{-1}}\right)^{3}\left({t\over{\rm years}}\right)^{1.46}\rm days$, where
$\dot{M}$ is the mass-loss rate, $v_{w}$ is the wind velocity, $V_{s}$ is the
shock velocity, and $t$ is the time at which the mass-loss rate was measured.
Assuming $v_{w}=50\rm\,\,kms^{-1}$, $\dot{M}=5\times 10^{-4}\,\rm M_{\odot}$
yr-1, $V_{s}=5441\,\,\rm kms^{-1}$, and $t\sim 3.3$ days, we obtained a
cooling time of $t_{cool}\sim 17$ days. Thus, it is possible that the forward
shock is indeed cooling, giving rise to this additional component.

If we assume that the shock is propagating through a wind characterised by
$\rho\propto R^{-2}$, we would expect to see a decrease in the measured column
density as the shock evolves, provided that the vast majority of the measured
column density arises from CSM. As the Galactic absorption is much less than
the measured column density derived in our analysis ($N_{H,gal}=7.9\times
10^{20}$ cm-2, HI4PI Collaboration et al. 2016), this is not an unreasonable
assumption. In fact, the decrease in column density has been seen by both
Grefenstette et al. (2023) and Chandra et al. (2023b), who found a column
density of $N_{H}=26\times 10^{22}$ cm-2 $\sim 4$ days after first light,
which decreased to $N_{H}=5.6\times 10^{22}$ cm-2 on day $\sim 11$ and then
$N_{H}\sim 3.2\times 10^{22}$ cm-2 on day $\sim 14$. Combined with our
constraint of $3.9\times 10^{22}$ cm-2 by $\sim 46$ days, we find that the
column density is decreasing following a simple power law with an index of
$\sim 1.5$.

This gradient is much steeper than expected assuming a steady, spherically-
symmetric wind (i.e., an index of 2). This could suggest that the source
underwent either variable mass loss from the progenitor before explosion or
the progenitor is surrounded by an intrinsically asymmetric CSM geometry
(Fransson et al., 1996) such as that seen in other interacting SN (e.g.,
SN2006jd, SN2010jl, and SN1998S; Chandra et al., 2012; Fransson et al., 2014;
Leonard et al., 2000). The lack of pre-explosion outbursts (see Section 4.2,
Neustadt et al. 2023, Jencson et al. 2023, Dong et al. 2023), and the
evolution of the spectral features found in the high resolution spectra of
SN2023ixf during its first week suggest that the CSM is likely asymmetric
(e.g., Smith et al., 2023), supporting the case that the observed decay in
column density is largely due to the geometry of the CSM.

We can also derive the mass of the CSM swept up by the forward shock using the
following (Chandra, 2018; Margalit et al., 2022):
$M_{CSM}=\dot{M}R_{s}(v_{w}/10\,$km s${}^{-1})^{-1}$. During the first $\sim
46$ days, the shock had swept up $\sim 0.04-0.07\,\rm M_{\odot}$ of CSM. This
is consistent with that estimated using optical spectra (Jacobson-Galan et
al., 2023; Bostroem et al., 2023).

Figure 13: Unabsorbed X-ray luminosities (in units of $10^{41}\,\rm
erg\,s^{-1}$) for SN2023ixf assuming a thermal bremsstrahlung model (0.3 – 10
keV, yellow stars) and other SNe detected in X-rays as a function of the time
since first light (MJD=60082.83). Stars represent Type IIn SNe, while arrows
indicate the measurement is an upper limit. For those SNe that are not Type
IIn, the marker colour gives the SN type. Data Source and energy band: SN1970G
(Immler & Kuntz, 2005, (0.3 – 2 keV)), SN1978K (Chandra, 2018, (0.3 – 8 keV)),
SN1980K (Schlegel, 2001, (2 keV)), SN1986J (Houck et al., 1998, (0.5 – 2.2
keV)), SN1988Z (Schlegel & Petre, 2006, (0.2 – 2 keV)), SN1993J (Chandra et
al., 2009, (0.3 – 8 keV)), SN1994W (Chandra, 2018, (0.3 – 8 keV)), SN1995N
(Chandra et al., 2005, (0.1 – 10 keV)), SN1996cr (Chandra, 2018, (0.5 - 2
keV)), SN1998bw (Kouveliotou et al., 2004, (0.3 – 10 keV)), SN1998S (Pooley et
al., 2002, (2 – 10 keV)), SN1999em (Schlegel, 2001, (2 keV)), SN1999gi
(Schlegel, 2001, (2 keV)), SN2003bg (Soderberg et al., 2006, (0.5 – 8 keV)),
SN2004et (Misra et al., 2007, (0.5 – 8 keV)), SN2005kd (Chandra, 2018, (0.3 –
8 keV)), SN2005ip (Katsuda et al., 2014, (0.2 – 10 keV)), SN2006bp (Immler et
al., 2007, (0.2 – 10 keV)), SN2006gy (Chandra, 2018, (0.5 – 2 keV)), SN2006jc
(Immler et al., 2008, (0.2 – 10 keV)), SN2006jd (Chandra, 2018, (0.3 – 8
keV)), SCP06F6 (Levan et al., 2013, (0.2 – 10 keV)), SN2008ax (Roming et al.,
2009, (0.2 – 10 keV)), SN2010jl (Chandra et al., 2015, (0.2 – 10 keV)),
SN2011dh (Soderberg et al., 2012, (0.3 – 8 keV)), SN2011fe (Margutti et al.,
2012, (0.5 – 8 keV)), SN2014c (Brethauer et al., 2022, (0.3 – 100 keV)),
PTF12dam (Margutti et al., 2018a, (0.3 – 10 keV)), SN2017hcc (Chandra et al.,
2022, (0.3 – 10 keV)),SN2018gk (Bose et al., 2021, (0.3 – 10 keV)), SN2019ehk
(Jacobson-Galán et al., 2020, (0.3 – 10 keV)), SN2021gno (Jacobson-Galán et
al., 2022b, (0.3 – 10 keV)).

###  5.4 Comparison with other X-ray bright SNe

SNe that explode in dense CSM are expected to be dominated by thermal X-ray
emission from the forward shock with temperatures on the order of 10 to a few
10s of keV (e.g., see review by Chandra, 2018). The increase in X-ray emission
seen from SN2023ixf indicates that its shock is interacting with a dense,
hydrogen-rich CSM that was recently ejected by the progenitor prior to its
death. This is consistent with our results presented in Section 4.3.2 and the
growing body of evidence that pre-explosion mass loss occurs in the
progenitors of Type IIn-like SNe through either binary interaction or episodic
mass loss (e.g., see review by Smith, 2014).

In Figure 13, we plot our X-ray luminosity as a function of time for SN2023ixf
assuming a thermal bremsstrahlung model and compare it to a sample of other
X-ray bright SNe. The rise in the X-ray emission of SN2023ixf (Figure 11,
lower left panel) has been seen in a number of interacting SNe including Type
IIn SN2010jl (Chandra et al., 2015) and SN2006jd (Chandra, 2018), and the Type
Ib SN2014c that transitioned into a strongly interacting Type IIn (Margutti et
al., 2017; Thomas et al., 2022). However, the timescales to peak X-ray
brightness for these other events are much longer ($\sim 100$ days) compared
to the $\sim 10$ days it took for the soft X-rays from SN2023ixf to attain
their current peak. This difference in rise time is likely due to the location
of the CSM, with the CSM surrounding SN2023ixf likely more compact compared to
the other SNe that show interaction or which emit X-rays (Figure 13). This
rise in X-ray brightness is also likely a result of the decrease in shock
temperature as it sweeps and interacts with more material over time. This
causes the X-ray emission to shift from higher to lower X-rays energies. Such
behaviour is consistent with the decrease in shock temperature seen by NuSTAR
(Grefenstette et al., 2023). The flattening in the X-ray light curve seen
after $\sim 10$ days (See Figure 11 lower left panel) suggests that the X-ray
emission is currently powered by CSM interaction. This behaviour has been seen
in both SN2014c and other Type IIn SNe with well-sampled X-ray light curves
(e.g., SN2010jl Chandra et al. 2015, SN2006jd Chandra et al. 2012 and see
Figure 13).

Compared to other Type IIn SNe, both the bolometric luminosity (see Figure 6)
and the X-ray luminosity of SN2023ixf (see Figure 13) are currently a few
orders of magnitude less than that seen for strongly interacting SNe such as
SN2010jl (Chandra et al., 2015), SN2006jd (Chandra et al., 2012), and SN2005kd
(Dwarkadas et al., 2016), and is more consistent in luminosity with that seen
from Type IIn SN2006gy (Smith et al., 2007), SN1998S (Pooley et al., 2002),
SN2017hcc (Chandra et al., 2022), or some Type IIP, Type IIb, or Type Ib SNe.
The low X-ray luminosity of SN2023ixf is rather peculiar given: that Type IIn
SNe tend to be much brighter in X-rays than other core-collapse SNe (e.g.,
Chandra, 2018), that the shock appears to be interacting with dense material
based on flash ionisation features (e.g., Jacobson-Galan et al., 2023; Smith
et al., 2023; Jencson et al., 2023), the constraints on the CSM both in this
work and others in the literature, the enhanced mass-loss rates derived in
this and in other works, and the large column densities relative to the
Galactic column density measured here and by NuSTAR (Grefenstette et al.,
2023) and Chandra (Chandra et al., 2023b).

Similar to the Type IIn SN SN2017hcc, which exhibited weak X-ray and radio
emission but bright IR emission, it is possible that SN2023ixf’s low-
luminosity X-ray emission arises from an asymmetric CSM. In this case, the
conversion of kinetic energy is inefficient in some directions, causing a
lower X-ray flux. Another possibility is that the X-ray emission is suppressed
due to instabilities at the shock front or due to the hot post-shock gas
driving weak shocks into the colder surrounding material, transferring energy
before it can be radiated (e.g., Steinberg & Metzger, 2018). As a number of
studies in the literature suggest that the CSM is asymmetric (e.g., Smith et
al., 2023; Berger et al., 2023b; Vasylyev et al., 2023), it is likely the
lower X-ray luminosity is a signature of the CSM distribution.

We can expect that once the X-ray luminosity begins to fade, it will decay
following $t^{-1}$ if the CSM was formed via a steady wind whose density
decreases as $r^{-2}$ (e.g., Chevalier, 1998; Dwarkadas & Gruszko, 2012).
However, it has been shown that a number of SNe that are known to interact
with a dense CSM deviate from this behaviour (see e.g., Dwarkadas & Gruszko,
2012; Ross & Dwarkadas, 2017; Chandra, 2018). This could result from either a
CSM that does not follow $r^{-2}$ (Salamanca, 2003; Dwarkadas, 2011; Dwarkadas
& Gruszko, 2012), or from the fact that our X-ray instruments observe in a
narrow X-ray band (Dwarkadas & Gruszko, 2012) and thus our bolometric
corrections are not capturing the full emission. As our analysis suggests that
the CSM was formed from low-luminosity, episodic mass loss or from binary
interaction, we expect that the emission will decay following a power law that
deviates from this behaviour. As such, it will be critical to continue
monitoring the X-ray evolution of SN2023ixf as it evolves to further probe the
mass-loss history of its stellar progenitor.

##  6 Summary and conclusions

We have presented a comprehensive study of the pre-explosion UV, optical, and
X-ray properties, and post-explosion X-ray emission of the nearby Type IIn SN
SN2023ixf as detected by Swift, ASAS-SN, ATLAS, ZTF, Chandra, and XMM-Newton.
Here, we analysed data from nearly two decades prior to explosion and $\sim
50$ days post explosion, focusing on the pre-explosion variability, presence
of X-ray signatures prior to explosion and the evolution of the luminous soft
X-ray emission. In summary:

  1. 1.

Using nearly two decades of data, we showed that the progenitor of SN2023ixf
exhibited no significant evidence of pre-explosive/precursor activity in the
optical, UV, and X-rays as detected by Swift, ASAS-SN, ZTF, ATLAS, Chandra,
and XMM-Newton. This is consistent with that found by Neustadt et al. (2023)
and Dong et al. (2023). Our analysis suggests that any evidence of precursor
activity in optical to UV was characterised by a luminosity $\lesssim 7\times
10^{4}\,\rm L_{\odot}$, consistent with Neustadt et al. (2023), Jencson et al.
(2023), and Dong et al. (2023) who suggested that there was no bright, short-
lived outburst within the last $\sim$ decades prior to explosion.

  2. 2.

Extensive, serendipitous monitoring of the location of SN2023ixf by Chandra,
Swift, and XMM-Newton prior to explosion shows no evidence of luminous
precursor emission down to 5.7$\times 10^{36}$ erg s-1 based on Chandra
observations spanning more than 18 years prior to the explosion. Using Swift
observations from the progenitor’s final 16 years, we instead obtain a value
of less than 5.8$\times 10^{37}$ erg s-1 and derive a mass-loss rate of
$(3-10)\times 10^{-5}\rm\,M_{\odot}$ yr-1, which is consistent with those
rates derived by Neustadt et al. (2023) and Jencson et al. (2023).

  3. 3.

Using the extensive set of Swift XRT observations of SN2023ixf that have been
taken since its discovery, we find that Swift did not detect any soft X-ray
emission down to an unabsorbed luminosity of $7.2\times 10^{38}$ erg s-1
within the first $\sim 3.3$ days after first light. Assuming that the emission
is dominated by IC scattering, similar to that done for other SNe (e.g.,
Margutti et al., 2012, 2014), we derive a mass-loss rate of $\lesssim 5\times
10^{-4}\rm\,M_{\odot}$ yr-1 and a radius of $R<3.7\times 10^{15}$ cm for the
CSM. Our analysis suggests that the progenitor underwent a mass-loss episode
at least $>$ 0.5 – 1.5 $(v_{w}/50$ km s${}^{-1})$ years prior to explosion.

  4. 4.

By merging the available Swift XRT observations, we find that the emission
over the first $\sim 50$ days is best described by an absorbed, two-
temperature component bremsstrahlung model. Here, the hard component has a
temperature of $\sim 35$ keV and a column density of $3.9\times 10^{22}$ cm-2,
consistent with that found using NuSTAR (Grefenstette et al., 2023). The soft
component has a temperature of $\sim 0.3$ keV and a column density of
$0.9\times 10^{22}$ cm-2, which we suggest results from the forward shock
cooling, similar to that seen in SN2010jl. We also derive a swept up mass of
0.04 – 0.07 $\rm M_{\odot}$ for the CSM.

  5. 5.

Similar to other interacting SNe, we find that the X-ray emission of SN2023ixf
has risen to peak brightness and is now plateauing. However, the rise to peak
was much faster than other interacting SNe, while the peak luminosity is
nearly a few orders of magnitude less than that seen for strongly interacting
SNe such as SN2010jl or SN2006jd. This is peculiar considering the fact that
SN2023ixf showed evidence of flash ionisation features and enhanced mass-loss
rates. We suggest that the low X-ray luminosity may be a natural consequence
of an asymmetric CSM, similar to that of the Type IIn SN 2017hcc and
consistent with the findings of Smith et al. (2023), Berger et al. (2023b),
and Vasylyev et al. (2023).

Due to its proximity and extensive pre- and post-explosion multiwavelength
coverage, SN2023ixf provides a unique opportunity to not only understand the
time-dependent mass loss, variability and CSM formation associated with the
final stages of red supergiant evolution, but also the possible effects of
binarity on these systems. The continued monitoring of the electromagnetic
emission (in addition to neutrinos and gravitational waves) from SN2023ixf
will provide some of the strongest constraints on the mechanism associated
with stellar collapse of a massive progenitor seen in the last few decades.

Parts of this research were conducted by the Australian Research Council
Centre of Excellence for Gravitational Wave Discovery (OzGrav), through
project number CE170100004. LAL acknowledges support by the Simons Foundation
and the Heising-Simons Foundation. This work was performed in part at the
Simons Foundation Flatiron Institute’s Center for Computational Astrophysics
during LAL’s tenure as an IDEA Scholar. JFB was supported by National Science
Foundation grant No. PHY-2310018.

##  7 Appendix

In Table 1, we present the results of our variability analysis as outlined in
Section 4.2.

Table 1: Variability Limits for SN2023ixf in various bands.  |  | Variability [$10^{4}\,\rm L_{\odot}$]
---|---|---
Instrument | Band | Progenitor | Control Sample | Control Sample
|  | RMS | Mean RMS | $\sigma$
Swift | V | 26.23 | 4.67 | 12.72
Swift | B | 17.76 | 3.51 | 10.68
Swift | U | 10.93 | 8.03 | 12.92
Swift | W1 | 8.45 | 4.35 | 8.00
Swift | M2 | 8.44 | 2.11 | 7.06
Swift | W2 | 10.72 | 2.31 | 7.24
ZTF | r | 0.75 | 2.29 | 9.33
ZTF | g | 1.02 | 3.39 | 13.26
ZTF | i | 0.92 | 2.28 | 6.42
ASASSN | g | 0.02 | 0.01 | 0.03
ASASSN | V | 0.03 | 0.01 | 0.03
ATLAS | c | 0.00 | 0.01 | 0.01
ATLAS | o | 0.01 | 0.01 | 0.01

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