An infrared echo from a circumstellar disk in the hydrogen- and helium-poor SN 2024aecx

We present near-infrared (NIR) spectroscopy of the hydrogen- and helium-poor (Type Ic) supernova (SN) 2024aecx that displays a strong NIR excess emerging 32 days post peak. SN 2024aecx is a peculiar SN Ic that exhibited luminous shock-cooling emission at early times, suggestive of close-in circumstellar medium (CSM), unexpected for this class of SNe. Its early NIR spectra are typical for a SN Ic but with strong CI absorption features. By ~32 days post peak, the spectra show a strong NIR excess, while maintaining normal optical colors, unprecedented for SNe Ic. We find that the NIR excess is well fit with a single-temperature, optically thin dust model with declining temperature, increasing mass, and roughly constant luminosity over time. The NIR excess appears too promptly for dust to have formed in the SN ejecta, indicating an IR echo from pre-existing dust in the CSM. The IR echo is likely powered by the relatively slowly evolving SN peak light, and not the brief shock cooling emission, as the latter requires unrealistically high CSM densities to explain the observed dust mass. We consider different potential CSM geometries and find that a thick face-on disk with an inner edge around $5\times 10^{16}$ cm can best explain the dust mass and temperature evolution. In this scenario, the SN shock should start interacting with this CSM $440\pm200$ days post explosion. CSM around SN Ic is rare, and follow-up observations of SN 2024aecx will probe the mass-loss process responsible for removing hydrogen and helium from their progenitor star.

💡 Research Summary

This paper presents a comprehensive study of the hydrogen‑ and helium‑poor (Type Ic) supernova SN 2024aecx, focusing on the discovery and interpretation of a strong near‑infrared (NIR) excess that appears about 32 days after optical maximum. SN 2024aecx was discovered early (within a day of explosion) in NGC 3521 by the ATLAS survey, and initial photometry showed a rapid, blue decline consistent with shock‑cooling emission—an unusual feature for a stripped‑envelope supernova. Early optical spectra resembled normal Ic events such as SN 1994I, with prominent C I absorption, but lacked hydrogen or helium lines.

The authors obtained an extensive set of NIR spectra using multiple facilities (Keck/NIRES, Gemini‑North/GNIRS, MMT/MMIRS, Keck/MOSFIRE, Gemini‑South/F2, GTC/EMIR, and IRTF/SpeX) covering 0.7–2.4 µm from –18 days to +60 days relative to peak. Data reduction employed standard pipelines (pypeit, spextool, xtellcor) and flux calibration was anchored to contemporaneous K‑band photometry. Optical photometry from the Swope telescope provided uB V g r i light curves, confirming that the optical colors remain typical of a normal Ic supernova throughout the monitoring period.

Host‑galaxy extinction was estimated from Milky Way foreground (E(B–V)=0.05 mag) and a saturated Na I D absorption component at the host redshift, yielding an additional E(B–V)≈0.3 mag. This modest extinction does not significantly affect the NIR analysis.

Spectroscopically, the NIR continuum is ordinary until about 12 days post‑peak, after which a pronounced excess emerges, peaking near 2 µm while the optical spectrum shows no corresponding reddening. The authors model this excess with an optically thin, single‑temperature dust component. The best‑fit dust temperature declines from ~1900 K to ~1300 K over the observed interval, while the inferred dust mass grows from ~10⁻⁴ M⊙ to ~10⁻³ M⊙. The total NIR luminosity remains roughly constant at ~10⁴² erg s⁻¹, indicating that the heating source is the slowly declining SN peak luminosity rather than the brief shock‑cooling flash.

To discriminate between newly formed ejecta dust and pre‑existing circumstellar dust, the authors consider formation timescales, required densities, and the fact that the excess appears too early for ejecta condensation. They therefore favor an infrared echo: pre‑existing dust in the circumstellar medium (CSM) is heated by the SN radiation and re‑radiates in the NIR. The authors explore three CSM geometries—spherical shell, thin spherical shell, and a thick, face‑on disk—using radiative‑transfer calculations that match the observed temperature and mass evolution. A spherical configuration would demand unrealistically high dust masses and densities at large radii, while a thin shell cannot reproduce the relatively flat NIR luminosity. The thick disk model, with an inner radius of ~5×10¹⁶ cm, a vertical thickness of ~1×10¹⁶ cm, and an inclination of ~30°, successfully reproduces the data. In this scenario, the dust resides in a dense equatorial structure likely formed by non‑conservative mass loss shortly before core collapse.

The early shock‑cooling emission suggests the presence of a very close‑in CSM component (∼10¹⁴ cm), but modeling shows that invoking this material to power the NIR echo would require CSM densities orders of magnitude higher than plausible for a stripped‑envelope progenitor. Thus, the authors conclude that the shock‑cooling flash and the IR echo are powered by distinct CSM components: the former by a tenuous inner shell, the latter by the more massive outer disk.

A key prediction of the disk model is that the SN forward shock, expanding at ~10 000 km s⁻¹, will encounter the inner edge of the disk roughly 440 ± 200 days after explosion. This interaction should manifest as a resurgence of optical emission lines (e.g., intermediate‑width H α if any residual hydrogen is present), enhanced radio synchrotron emission, and X‑ray brightening. The authors propose a coordinated multi‑wavelength follow‑up campaign, including JWST NIRSpec/MIRI spectroscopy to probe dust composition, VLA radio monitoring, and Chandra/XMM‑Newton X‑ray observations to capture the shock‑CSM interaction.

The discovery of an IR echo in a Type Ic supernova is unprecedented; most previous IR echoes have been associated with Type IIn, Ibn, or II‑P events where dense CSM is expected. This work therefore provides a rare window into the mass‑loss history of a progenitor that has already shed its hydrogen and helium envelopes yet retained a substantial equatorial dust reservoir. It challenges the conventional view that binary mass transfer alone can explain the stripping of massive stars, suggesting that additional, possibly eruptive, mass‑loss episodes occur in the final years before collapse. The paper concludes that SN 2024aecx offers a unique laboratory for studying the geometry, composition, and timing of CSM around stripped‑envelope supernovae, and that continued observations will be essential to refine models of massive star evolution and explosion mechanisms.