SN2023ixf: Closest Type II Supernova in M101
- SN2023ixf is a hydrogen-rich Type II core-collapse supernova in M101, notable for its proximity and early multiwavelength observational coverage from hours post-explosion to nebular phases.
- Flash spectroscopy and polarimetric analyses revealed rapid radiative acceleration and asymmetric, dense circumstellar material affecting early spectral features.
- Hydrodynamical and radiative-transfer models constrained the progenitor’s mass to 10–15 M⊙ and explosion energies around (0.7–1.2)×10^51 erg, highlighting a complex pre-explosion mass-loss history.
SN 2023ixf is a hydrogen-rich Type II core-collapse supernova in Messier 101 (M101), discovered on 2023-05-19, with first light constrained to MJD 60082.757 ± 0.097 or MJD 60082.788 and a host distance of about 6.85–6.9 Mpc. Its proximity made it the closest core-collapse supernova of the decade and enabled observations from hours after explosion through the nebular phase and into the second and third post-explosion years across the optical, ultraviolet, infrared, radio, X-ray, spectropolarimetric, and neutrino domains. SN 2023ixf is therefore a central case for delayed shock breakout in dense circumstellar material (CSM), radiative acceleration of a slow red-supergiant wind, asymmetric CSM and ejecta geometry, and the transition from early circumstellar reprocessing to late-time dust formation and interaction-powered emission (Jacobson-Galán, 10 Jul 2025).
1. Discovery, host galaxy, and observational setting
SN 2023ixf was discovered in M101 on 2023-05-19 17:27:15 UTC at clear-band magnitude . Several studies adopt an explosion epoch near MJD 60082.75–60082.79. The host distance is variously given as Mpc, Mpc, or Mpc, and the host redshift was either adopted as or measured from narrow host Na I D and Ca II H&K absorption as (Dickinson et al., 2024).
The event was followed almost immediately. Optical spectroscopy began at d; spectropolarimetry began at d; high-resolution WIYN/NEID echelle spectroscopy at began at 1.51 d; Swift/UVOT near-UV spectroscopy was obtained at 0 d; HST/STIS NUV spectroscopy at 1 d; AstroSat/UVIT far-UV spectroscopy at 2 d; NuSTAR observed at 3 d and 4 d; and IceCube performed targeted follow-up in 4-day and 32-day windows (Jacobson-Galán, 10 Jul 2025).
This cadence was unusually diagnostic because the most CSM-sensitive phases evolved on hour-to-day timescales. Multiple studies explicitly emphasize that the first 48 hours were decisive for measuring the pristine wind speed, capturing the onset of radiative acceleration, and separating flash-ionized narrow-line emission from later shocked and ejecta-dominated components (Dickinson et al., 2024).
2. Early flash spectroscopy and delayed shock breakout
The earliest spectra showed the classic flash-ionization or flash-interaction phenomenology of dense, close-in CSM. At 1.51 d, the high-resolution spectrum displayed narrow emission features with widths 5 km s6 from H I, He I, He II, C III, N III, C IV, and N IV, with most peaks blueshifted by 7 km s8 and with suppressed red wings. On that same epoch, He I 5876 Å was the narrowest line with 9 km s0, H I/He II/N III/C III had 1–80 km s2, and the highest-ionization lines C IV/N IV had 3–250 km s4 (Dickinson et al., 2024).
The line forest evolved extremely rapidly. He I was visible at 1.51 d but disappeared by 2.62 d. The narrow H5 component equivalent width dropped by a factor of 6 between 1.51 and 2.62 d; its 7 broadened from 8 km s9 to 0 km s1 over the first four days; and its centroid drifted to 2 km s3. Lower-resolution early-time spectroscopy extended the same phenomenology to a broader electron-scattering context: narrow cores embedded in broad, symmetric Lorentzian wings persisted for roughly 8 days after first light before the spectra transitioned to ejecta-dominated Type II P-Cygni profiles (Jacobson-Galan et al., 2023).
The early continuum evolution was also extreme. One photometric analysis found that the bolometric luminosity rose from 4 erg s5 at 1.1 d to 6 erg s7 at 8 d, while the color temperature increased from 9 K to 0 K over 1 d with the blackbody radius remaining nearly constant at 2 cm (Singh et al., 2024). Another spectroscopic analysis found that the continuum temperature rose from 3 K at day 1.4 to a peak 4 K at 5 d, then declined to 6 K by day 14 (Zheng et al., 18 Mar 2025). Both are consistent with delayed shock breakout in dense CSM.
The kinematic interpretation was framed explicitly in terms of radiative acceleration:
7
and, for single-scattering Thomson acceleration,
8
Using the bolometric light curve and an adopted shock velocity 9 km s0, the maximum radiative acceleration at the shock location was estimated as 1 km s2 by day 1.51, 3 km s4 by day 2.62, and 5 km s6 by day 3.62, while the outer edge of the dense CSM had 7 km s8 at day 1.51 (Dickinson et al., 2024).
A key result of this monitoring was that the narrowest He I line gave a direct upper bound on the pristine wind speed in that zone:
9
squarely in the expected range for red-supergiant winds. The subsequent broadening to 0 km s1 over 2–3 d and then to post-shock speeds of order 3 km s4 established that early narrow-line widths were not static wind tracers but were being modified in real time by shock-powered radiation and then by shock engulfment (Dickinson et al., 2024).
3. Circumstellar material: structure, kinematics, and mass-loss history
A consistent theme across the literature is that SN 2023ixf exploded inside dense, confined, and asymmetric CSM. In steady-wind form, the density is written as
5
and early optical/UV modeling with CMFGEN and HERACLES required a dense solar-metallicity CSM concentrated at
6
with a sharp reduction in density beyond 7 cm. For an assumed red-supergiant wind speed 8 km s9, the preferred mass-loss rate was
0
sustained for 1–6 yr before explosion, implying
2
in the dense inner region (Jacobson-Galan et al., 2023).
The WIYN/NEID analysis found a somewhat smaller characteristic scale for the densest line-forming region,
3
and inferred that the enhanced mass-loss phase began at least
4
yr before core collapse for a pre-SN outflow speed of 5 km s6 (Dickinson et al., 2024). This difference in radius does not erase the broader agreement that the most optically important CSM was compact and formed shortly before explosion.
A two-zone CSM was inferred in one STELLA-based hydrodynamical analysis. That work favored a confined dense CSM extending up to 7 cm with a mass-loss rate of 8, plus an extended CSM from 9 cm to at least 0 cm with a mass-loss rate of 1, both assuming a wind velocity of 10 km s2 (Singh et al., 2024). This suggests a stratified mass-loss history rather than a single steady state.
Independent hard X-ray observations supported a dense but confined line-of-sight CSM. NuSTAR spectra at 3 d and 4 d were fit by hot thermal bremsstrahlung with 5 keV at Epoch I and 6 keV at Epoch II, absorbed by an intrinsic neutral column declining from
7
to
8
The same spectra showed a prominent Fe K9 line and an intrinsic 0.3–79 keV luminosity of order
0
implying 1 at 2 cm and
3
for 4 km s5 (Grefenstette et al., 2023). Those values are lower than the flash-spectroscopy inferences, but they probe a different combination of geometry, radius, and density and are explicitly discussed in the literature as compatible with a multi-component or asymmetric CSM.
The geometry was not spherical. Predominantly blueshifted narrow-line profiles, muted red wings, the absence of narrow P-Cygni absorption at early times, and strong early polarization together indicate that the dense CSM was aspherical and likely concentrated out of the line of sight, for example as a toroidal or equatorial overdensity (Dickinson et al., 2024). Polarized radiative-transfer modeling of the first week favored a pole-to-equator density contrast 6 (Vasylyev et al., 6 May 2025).
Late-time spectroscopy showed that the mass-loss history was not limited to the final years. A nebular spectrum at 445 d displayed a boxy H7 component with sharp edges near 8 km s9, interpreted as ejecta interaction with a CSM shell at 00 cm. For a typical red-supergiant wind speed of 01 km s02, that shell would correspond to mass loss occurring up until 03–1000 yr before explosion (Folatelli et al., 14 Feb 2025). Taken together, the published constraints suggest a multi-zone CSM built over timescales from years to centuries.
4. Progenitor star and pre-explosion environment
Pre-explosion imaging identified a dust-enshrouded, variable red-supergiant progenitor candidate at the supernova position. One HST-based study found a single red source in F814W with 04 mag and no convincing detection in F435W or F555W, implying an extremely red color and a preferred progenitor mass of 05–10 06, while explicitly noting that additional circumstellar dust could bias the mass low (Pledger et al., 2023). A later multiwavelength registration to HST, Spitzer, and ground-based near-IR data identified a single point source consistent with the SN position across optical to 4.5 07m and derived a chance-coincidence probability of 08 for the favored source (Kilpatrick et al., 2023).
The pre-explosion local environment from HETDEX showed relatively low extinction and low recent star formation in the immediate vicinity. Within 09 arcsec, the median values were
10
and
11
while the metal-rich branch of 12 gave approximately solar metallicity with 13 (Liu et al., 2023). That same work found no significant change in extinction before and after explosion.
| Approach | Representative progenitor result | Reference |
|---|---|---|
| HST-only optical counterpart | Very red source; preferred initial mass 14–10 15 | (Pledger et al., 2023) |
| HST+Spitzer+NIR SED with DUSTY | 16; 17 | (Kilpatrick et al., 2023) |
| Time-dependent dusty RSG SED | 18 K, 19, 20 | (Qin et al., 2023) |
| CFHTLS progenitor plus HETDEX environment | 21 mag source; progenitor 22 | (Liu et al., 2023) |
Despite the spread in inferred initial mass, the studies agree on several qualitative points. The progenitor was a red supergiant; it was heavily affected by circumstellar dust; and it was variable in the years before explosion. One SED analysis found a dusty shell with
23
and
24
with a mass-loss rate
25
for 26 km s27 (Kilpatrick et al., 2023). Another, more extinction-heavy time-dependent fit instead obtained
28
29
and
30
with a pulsation period
31
Mid-infrared variability was pronounced. Over 29 Spitzer epochs spanning 9 yr, the source varied by 32 in both 3.6 and 4.5 33m, with a quasi-sinusoidal timescale of 34 yr (Kilpatrick et al., 2023). A different time-dependent analysis interpreted the variability as a 0.11 dex luminosity modulation coupled to variable dust optical depth and/or 35, consistent with pulsation-enhanced mass loss and dust sublimation/condensation cycles (Qin et al., 2023).
The main controversy is therefore not whether the progenitor was a dusty red supergiant, but how much of the pre-explosion luminosity was hidden or reprocessed by circumstellar dust and how strongly blending or geometry biased single-band optical estimates. That is why published progenitor masses span from the low-mass end of the SN II red-supergiant population to the high-luminosity end of the so-called red-supergiant problem.
5. Explosion properties and evolution from plateau to nebular phase
Photometrically, SN 2023ixf was bright and relatively fast-declining for a Type II event. A global citizen-science campaign measured a peak
36
at 2023-05-25 21:37 UTC (Sgro et al., 2023). Another long-baseline study found
37
classified the event as a fast decliner in the IIL subclass, and reported a short “plateau” shorter than 38 d with 39 mag (Zheng et al., 18 Mar 2025). Other analyses placed the plateau duration closer to 40–85 d (Bersten et al., 2023, Kozyreva et al., 2024). The earliest spectra were briefly IIn-like because of flash-ionized CSM, but the event did not evolve like a genuine long-lived Type IIn; within 41–3 weeks it followed a normal Type II spectroscopic sequence (Zheng et al., 18 Mar 2025).
One-dimensional radiation-hydrodynamical modeling of the bolometric light curve and Fe II 42 velocity evolution found a preferred solution with
43
a pre-SN radius of
44
a pre-SN mass of 10.9 45, and an ejecta mass of
46
That analysis concluded that SN 2023ixf was a normal Type II event and that the modeling robustly favored
47
because 15 48 models could not simultaneously match plateau luminosity and duration (Bersten et al., 2023).
A different STELLA-based interpretation proposed a lower explosion energy and a more strongly reduced hydrogen envelope:
49
with a wind-like CSM of 50 extending to 51 cm. That work further argued that the first 52 hr after the last non-detection could not be explained by shock–CSM interaction alone and therefore modeled a distinct precursor shell with
53
and
54
launched a fraction of a day before the main shock breakout (Kozyreva et al., 2024). This remains a model-dependent alternative rather than a universally adopted interpretation.
More recent non-LTE radiative-transfer modeling from 55 to 56 d favored an enhanced-mass-loss 15 57 red-supergiant progenitor yielding ejecta of 7–8 58, kinetic energy
59
and
60
with prolonged interaction mediated by a cold dense shell (CDS) at 61 km s62 and no evidence for material faster than the CDS during 20–120 d (Dessart et al., 6 Apr 2026). The same ejecta model, extended to 63 d, required persistent CSM interaction at all epochs, dominating the UV at all times and the optical after 64 d (Dessart et al., 13 May 2026).
Nebular spectroscopy has generally pulled the mass scale back toward the lower or middle part of the pre-explosion range. One 445-d analysis used the stability of [O I] and [Ca II] against interaction contamination and their flux ratio,
65
to favor
66
(Folatelli et al., 14 Feb 2025). Another day 1–442 synthesis found that nebular spectra between 200–400 d matched best with a 15 67 progenitor model (Zheng et al., 18 Mar 2025). The broad implication is that, although pre-explosion SED-based masses remain model-sensitive, hydrodynamical and nebular analyses often converge on 68–15 69.
6. Polarization, asymmetry, and dust formation
Polarimetry showed that asymmetry was not a secondary detail but a defining property of SN 2023ixf. At +2.33 d, the intrinsic R-band continuum polarization was
70
well above the Serkowski-based ISP upper limit
71
During the photospheric phase the continuum polarization dropped toward the ISP level, reaching 72–0.11% by +26–31 d, before rising again to
73
at +73.19 d and
74
at +76.19 d as the light curve left the plateau (Shrestha et al., 2024). A common ambiguity in SN polarimetry is whether the signal is dominated by ISP; for SN 2023ixf, the early amplitude, the rapid time evolution, and the late return toward the ISP envelope all argue strongly for intrinsic polarization.
A denser spectropolarimetric campaign from +1.4 to +120 d found the same three-stage evolution in more detail: continuum polarization initially near 75, falling steeply within days to 76 as the ejecta swept up the optically thick CSM, then staying low at 77 before rising again to 78 near the transition to nebular conditions (Vasylyev et al., 6 May 2025). Two-dimensional polarized radiative-transfer modeling attributed the early phase to an aspherical CSM with pole-to-equator density contrast 79 and the late-time surge to an asymmetric distribution of 80Ni deep in the ejecta (Vasylyev et al., 6 May 2025).
A complementary polarimetric analysis described three distinct peaks in the polarization evolution at 1.4 d, 6.4 d, and 79.2 d. It associated them, respectively, with an asymmetric dense CSM, an aspherical shock front plus clumpiness in the low-density extended CSM, and an aspherical inner ejecta or He core. The same study identified two dominant axes, one associated with the CSM and outer ejecta and the other with the inner ejecta or He core, and linked high-velocity broad Balmer absorption to an aspherical shock front (Singh et al., 2024). The convergence of these studies is that the early and late asymmetries had distinct physical origins.
Dust formation and infrared reprocessing were likewise time-resolved. Optical/NIR photometry, spectroscopy, and JWST data from 150 to 750 d showed an early IR excess by 1.8 d, a broad secondary NIR rebrightening over about 89 to 175 d, progressive attenuation of the red wing of H81 from about 132 d, and CO emission detected by about 217 d. The early 1.8–33.6 d excess was interpreted as a radiative “flash” IR echo from pre-existing circumstellar dust, while the 89–175 d rebrightening was more naturally explained by an extended echo from structured wind material at 82 cm (Singh et al., 14 Mar 2026).
The first direct signature of internal dust formation was the onset of H83 red-wing attenuation at about 132 d. Modeling of the H84 profile between 141 and 418 d yielded an internal silicate-equivalent dust mass rising from about
85
to
86
JWST SED decomposition further required a cold silicate-bearing component that grew from
87
at 253 d to
88
by 601–723 d, alongside a cold graphite component of order 89 that tracked lingering echo emission (Singh et al., 14 Mar 2026).
Late-time non-LTE UV-to-IR radiative-transfer modeling to 998 d independently required dust formation beginning at 90 d, first in the CDS and later in the inner ejecta, with internal dust plausibly reaching
91
by 92 d, plus an external cold dust component to match the mid-IR (Dessart et al., 13 May 2026). By 445 d, the spectrum had developed a complex H93 profile consisting of a boxy interaction component plus a central ejecta component, indicating that ejecta–CSM interaction and dust-related asymmetry had become inseparable parts of the late-time evolution (Folatelli et al., 14 Feb 2025).
7. High-energy and multimessenger constraints
SN 2023ixf produced the earliest hard X-ray detection of a non-relativistic stellar explosion then known. NuSTAR detected it up to 94 keV at 95 d and 96 d. The spectra were consistent with hot thermal bremsstrahlung absorbed by a thick neutral medium, with a clear neutral Fe K97 emission line and with the intrinsic soft X-ray brightening driven mainly by declining absorption rather than by a large increase in intrinsic luminosity. The preferred interpretation was forward-shock thermal emission in dense, confined CSM at radii 98 cm (Grefenstette et al., 2023).
These X-rays provided an independent shock diagnostic. Using
99
the measured temperatures implied a forward-shock velocity of at least several 00 km s01, with the caveat that incomplete electron–ion equilibration could make the true shock speed closer to 02 km s03 (Grefenstette et al., 2023). The same data gave a Thomson depth
04
of 05 at Epoch I and 06 at Epoch II, so the absorber was Compton-thin. Early radio/mm non-detections were then naturally attributed to very large free–free optical depth in the same dense CSM (Grefenstette et al., 2023).
The neutrino follow-up produced a non-detection. IceCube analyzed a 4-day fast-response window and a 32-day optimized window under an 07 spectrum. The 4-day analysis gave
08
and the 32-day analysis had unblinded
09
both consistent with the background-only hypothesis. The reported 90% CL upper limits on time-integrated energy flux were
10
for the 4-day window and
11
for the 32-day window, corresponding to isotropic-equivalent source-frame energies of
12
and
13
respectively, under the convention used in that work (Mand et al., 9 Jul 2025).
These upper limits do not rule out shock acceleration in SN 2023ixf, but they do constrain very optimistic interaction-powered neutrino scenarios. In the same IceCube study, folding the Kheirandish & Murase model flux with IceCube’s effective area yielded an expected detection probability of only 14 neutrinos in 30 days, so the non-detection was itself consistent with model expectations (Mand et al., 9 Jul 2025). The high-energy record of SN 2023ixf therefore complements the optical and infrared evidence: the event clearly hosted strong shock–CSM interaction, but not at a level that produced a detectable TeV neutrino burst in IceCube.