Type Ibn/Icn Supernova SN 2023xgo

• Type Ibn/Icn Supernova SN 2023xgo is defined by the interaction of stripped-envelope core-collapse ejecta with a hydrogen-poor, dual-regime CSM showing both carbon and helium spectral features.
• Light-curve modeling indicates a fast rise with dual-regime CSM properties, pointing to variable mass-loss rates and complex progenitor dynamics.
• The spectral evolution from an early carbon flash to later helium-dominated emission underscores its benchmark role in understanding diverse supernova progenitor scenarios.

A Type Ibn/Icn supernova, such as SN 2023xgo, is defined by the interaction of core-collapse supernova ejecta from a stripped-envelope progenitor with a hydrogen-poor, dense circumstellar medium (CSM). The spectral and photometric evolution of SN 2023xgo reveals critical insights into progenitor mass loss, explosion physics, and the diversity of the resulting transients. SN 2023xgo exhibits a unique transitional behaviour between the classical Ibn class (characterised by strong helium emission from CSM interaction) and the Icn class (marked by early carbon features arising from C/O-rich CSM). Observational signatures, together with light-curve and spectral modelling, establish SN 2023xgo as a benchmark event for understanding the continuum between supernovae that explode in helium-rich versus carbon-rich environments.

1. Spectroscopic Evolution and Classification

SN 2023xgo was initially identified by its blue, flash-ionized spectrum, exhibiting a strong, narrow C III λ5696 emission at very early phases (−2.35 days relative to r-band peak). This "carbon flash" feature, typical of Type Icn supernovae, diminished as the supernova evolved. Within days, the spectrum transitioned to classical Ibn features—narrow and later intermediate-width He I (λ5876, λ6678, λ7065) P-Cygni profiles with velocities initially near 1800 km s⁻¹, broadening to 10,000 km s⁻¹ post-maximum (Gangopadhyay et al., 12 Jun 2025). The pseudo-equivalent widths of He I are among the largest in the known Ibn/Icn class.

The spectral sequence, from carbon-dominated to helium-dominated lines, provides evidence for the complex composition and stratification of the CSM—a signature that SN 2023xgo occupies a transitional regime between SNe Ibn and Icn (Pursiainen et al., 2023). Further, the detection and subsequent fading of flash-ionization features, alongside the later emergence of strong He I lines, demonstrates the segregation of ionized CSM layers and the evolution of the interaction zone as the shock propagates outward.

2. Photometric Properties and Light Curve Modelling

The early-time light curve of SN 2023xgo is characterised by a fast rise to peak (≈5.1±2.3 d), followed by a decline rate of 0.14 mag d⁻¹ for the first 30 days—slower than most fast-evolving transients, but well within the Ibn/Icn range. The r-band absolute magnitude at peak is −17.65 ± 0.04 mag, making SN 2023xgo the faintest member in the Ibn sample considered (Gangopadhyay et al., 12 Jun 2025). The colour evolution remains typical for the class, and host galaxy properties appear unremarkable.

Semi-analytical light curve models constrain the CSM and ejecta properties. The early photometric phase, modelled as pure shock-powered CSM interaction, requires a compact shell (R = 10¹²–10¹³ cm) with mass M_CSM ≈ 0.22 M☉, ejecta mass M_ej ≈ 0.12 M☉, and a mass-loss rate of 10⁻⁴–10⁻³ M☉ yr⁻¹. Post-maximum, more extended CSM (R_out ≈ 10¹⁵ cm) with a stratified density profile (power-law index s ≈ 2.9) and mass-loss rates from 0.1 up to 2.7 M☉ yr⁻¹ are favoured, implicating a variable or episodic mass ejection history (Gangopadhyay et al., 12 Jun 2025). A dual-regime CSM structure—with both compact and extended components—explains the observed light-curve features and colours.

3. Progenitor Scenarios and Mass-Loss History

Spectral and photometric modelling support a low-mass (≈3 M☉) helium star progenitor, possibly interacting with a binary companion or via a massive single-star channel. The dual CSM regime (early compact, later extended) is interpreted as evidence of a radially-structured outflow, shaped by asymmetry or temporal changes in progenitor mass-loss. This inference is consistent with both binary interaction scenarios—in which angular momentum transfer causes rapidly variable mass-loss—and eruptive single-star behaviour, such as wave-driven or silicon-burning induced ejections (Gangopadhyay et al., 12 Jun 2025).

Key equations used to quantify progenitor mass-loss include

$$\dot{M} = 4\pi R_{\mathrm{CSM}}^2\, \rho_{\mathrm{CSM}}\, v_{w}$$

with observed wind velocities near 1800 km s⁻¹.

Observations of pre-supernova NIR echoes (Yamanaka et al., 25 Aug 2025) imply the existence of a massive carbon-rich dust shell at R ≈ 10¹⁶ cm, originating from a pre-explosion outburst with a high mass-loss rate (~0.1 M☉ yr⁻¹). This behaviour is reported in both binary-dominated systems (as in SN 2023fyq (Dong et al., 7 May 2024)) and carbon-rich Wolf–Rayet stars. A plausible implication is that SN 2023xgo underwent at least one major eruptive episode before core collapse, populating the CSM with carbon-rich, dust-producing material.

4. Circumstellar Interaction Physics

The transition from early Icn-like carbon features to later Ibn-like helium emission is interpreted as a function of the ionization and the stratification of the CSM. The inner regions are highly carbon-enriched, flash-ionized by the initial shock breakout, while the outer layers are helium-rich, producing the characteristic Ibn spectra as the ejecta–CSM shock expands.

Light-curve modelling favours interaction scenarios where the shock luminosity evolves as

$$L_{\mathrm{CSM}}(t) \propto \frac{E_{\rm shock}}{t_{\rm diff}}$$

with CSM diffusion times governed by opacity ($\kappa$), mass, and radius.

In addition to optical diagnostics, X-ray modelling provides direct access to CSM composition and density. Soft X-ray flux is sensitive to envelope stripping signatures (He vs. C/O) via photoelectric absorption; hard X-ray emission tracks kinetic energy and shock velocity, robustly constraining explosion energy and mass-loss regimes (Inoue et al., 12 Dec 2024). A bright initial soft X-ray phase is expected in the first few days due to an ionized CSM. For SN 2023xgo, rapid soft and hard X-ray follow-up can resolve degeneracies in progenitor configuration and envelope stripping depth.

5. Implications for the Ibn/Icn Supernova Continuum

SN 2023xgo exemplifies the continuum between SNe Ibn (helium-dominated CSM interaction) and Icn (carbon-dominated, H/He-poor CSM). The early flash-ionized C III emission, fade to Ibn He I lines, and low initial nickel mass (from light-curve fits) support both binary channel ultra-stripped scenarios (Moriya et al., 7 Jul 2025) and possible single-star carbon-flash models. The presence of pre-existing carbon dust at distances ~10¹⁶ cm (Yamanaka et al., 25 Aug 2025) draws further correspondence with pre-explosion Wolf–Rayet eruptions or binary-induced mass ejection.

A comparative table of key observational and modelling parameters is presented below:

Property	Early (Icn-like)	Late (Ibn-like)
Dominant lines	C III λ5696	He I λ5876, λ6678
Velocity (km s⁻¹)	~1800	up to 10000
CSM regime	Compact (~10¹³ cm)	Extended (~10¹⁵ cm)
Mass-loss rate (M☉ yr⁻¹)	10⁻³	0.1–2.7
Ejecta mass (M☉)	~0.12	~0.12
CSM mass (M☉)	~0.22	~0.22
NIR excess/circumstellar dust	Yes (carbon-rich)	Yes

This table highlights the temporal, spectral, and spatial evolution of SN 2023xgo as it transitions between Icn and Ibn phenomenology.

6. Theoretical and Observational Significance

SN 2023xgo provides constraints on mass-loss history, explosion physics, and the final fate of massive stars. The dual CSM regime and evolving spectral signatures require models that account for asymmetric or time-varying mass ejection—consistent with binary interaction, ultra-stripped progenitors, and Wolf–Rayet eruptions (Moriya et al., 7 Jul 2025, Dong et al., 7 May 2024, Yamanaka et al., 25 Aug 2025). The event demonstrates that some SNe Ibn/Icn arise from low-mass, binary-stripped helium stars following an ultra-stripped SN channel, while others may represent massive WR stars undergoing sudden mass-loss eruptions.

From an observational strategy perspective, SN 2023xgo underscores the importance of rapid multi-wavelength (optical, NIR, X-ray) follow-up, early spectral coverage, and detailed light-curve modelling. These approaches are essential to discriminate between progenitor scenarios and to unravel the physical processes governing CSM formation and pre-supernova mass loss.

7. Current Debates and Future Directions

SN 2023xgo has stimulated new discussion about the continuum between SNe Ibn and Icn, the possible overlap in progenitor systems, and the role of flash-ionized CSM in shaping observational diversity. Key controversies include:

• The respective roles of binary interaction and single-star evolution in creating radially structured, carbon- and helium-rich CSM.
• The extent to which ultra-stripped core-collapse SNe contribute to the Ibn population.
• The interpretation of NIR echoes: distinguishing newly formed dust in the cool dense shell from pre-existing circumstellar dust generated by pre-SN eruptions.
• The accuracy and degeneracy of modelling approaches, particularly in constraining ejecta/CSM masses and mass-loss rates during episodic outbursts.

Future studies will benefit from targeted X-ray diagnostics (Inoue et al., 12 Dec 2024), multi-band polarimetry, full SED evolutionary tracks, and comparative population analyses using new transient surveys. SN 2023xgo remains a crucial object for calibrating these models and for refining the physical interpretation of the Ibn/Icn class.

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SN 2023xgo, as a transitional Ibn/Icn supernova, demonstrates the importance of multi-stage mass loss, stratified CSM composition, and complex binary/single-star evolutionary pathways in shaping the heterogeneity observed across hydrogen-poor, rapid-interacting supernovae. Its observational and modelling signatures will continue to inform both theoretical and empirical investigations into massive star death and circumstellar interaction-driven transients.