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

Abstract: 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 $5\times 10<sup>{16}$ 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.

• The paper presents the first detection of an infrared (IR) echo from pre-existing circumstellar dust around a normal stripped-envelope supernova (SN 2024aecx).
• Key findings include the identification of a thick, face-on dusty disk with an inner edge near 5.7 ± 2.6 × 1016 cm, revealed through ground-based near-infrared (NIR) spectroscopic monitoring.
• The echo's power source.
• questions_for_follow_up_from_user_listopeti1st1quesftreal-real-objective-listyerth6.Requested:
• How does the observed IR echo compare to theoretical predictions of mass-loss mechanisms in SNe Ibc?
• What are the implications of the detected CO first overtone band for the chemical composition of the supernova's ejecta?
• Can the method of using IR echoes be applied to other stripped-envelope supernovae for probing circumstellar dust and geometry?
• How does the mass-loss rate inferred from the IR echo compare with those of other SESNe, particularly strongly interacting ones?
• Find recent papers about the use of IR echoes in studying circumstellar environments of stripped-envelope supernovae.

Overview

SN 2024aecx, discovered by ATLAS in NGC 3521 at a distance of $13.8 \pm 2.7$ Mpc, is a hydrogen- and helium-poor (Type Ic) supernova that exhibits a strong near-infrared (NIR) excess emerging between 12 and 32 days post peak, while its optical colors remain consistent with normal SNe Ic. This paper presents an extensive ground-based NIR spectroscopic sequence from $-18$ to $+60$ days relative to peak, obtained with Keck/NIRES and MOSFIRE, Gemini-N/GNIRS, Gemini-S/F2, IRTF/SpeX, MMT/MMIRS, and GTC/EMIR, supplemented by Swope optical photometry. The authors argue that the NIR excess is an infrared (IR) echo from pre-existing circumstellar dust arranged in a thick, face-on disk — the first reported IR echo around a normal stripped-envelope supernova (SESNe), despite earlier theoretical suggestions that such echoes could probe the elusive mass-loss mechanism of SNe Ibc.

Classification and host extinction

The early light curve showed blue, rapidly declining shock-cooling emission, prompting preliminary IIb classifications. However, the NIR spectra strongly reject hydrogen and helium: the uncontaminated He I 2.059 $\mu$m line is absent at all phases, as are isolated Pa$\beta$ and Br$\gamma$. The authors explicitly dispute the Type IIb classification of Zou et al. and Xi et al., re-identifying their proposed H$\alpha$ and He I optical features as Na I D and C II $\lambda\lambda$6580, 7234, analogous to SNe 2007gr and 2016adj. Models indicate that even 0.2 $M_\odot$ of helium would produce detectable He I 2.0587 $\mu$m emission, so only trace helium can be present.

The spectra instead show unusually strong C I absorption across twelve lines between 0.91 and 1.69 $-18$0m, with absorption minima decelerating from about $-18$1 to $-18$2; the C I features vanish by roughly 40 days post peak, implying carbon-rich outer ejecta. Host extinction is constrained via CSP-I color-template fitting to $-18$3 mag with a low $-18$4, ruling out extreme reddening of the kind seen in SN 2016adj ($-18$5 mag) as an explanation for the red NIR colors.

Spectral peculiarities

Beyond carbon, two features stand out. First, the Mg I 1.5033 $-18$6m line develops a boxy profile with FWHM $-18$7 that remains stable for over a month — a morphology not seen in any SESNe in the KITS or CSP-II samples. Boxy profiles typically indicate emission from a thin shell, and the authors suggest it may trace a cold dense shell behind the reverse shock associated with the early CSM interaction, though they concede an origin in inner-ejecta asymmetry cannot be excluded. Second, the CO first overtone band is tentatively detected from about 40 days post peak, consistent with typical SESNe.

Dust modeling of the NIR excess

From 32 days post peak, synthetic NIR colors reach $-18$8, $-18$9, and $+60$0 mag — 0.6–1.5 mag redder than any comparison SN Ic and redder than any CSP-I SESNe photometric color — while optical colors remain normal. Modified-blackbody MCMC fits assuming 0.1 $+60$1m carbonaceous grains yield two degenerate solutions: optically thick dust at small radii ($+60$2–1590 K, mass lower limits $+60$3) and optically thin dust ($+60$4 declining from $+60$5 K to 870 K, mass rising from $+60$6 to $+60$7 as $+60$8), with roughly constant luminosity near $+60$9–$\mu$0.

The authors disfavor the optically thick scenario on physical grounds: it requires more than $\mu$1 of dust within the sublimation radius as early as 54 days post explosion, which would demand rapid dust formation normally seen only in strongly interacting events, and would produce unobserved optical extinction. The optically thin case is instead consistent with an IR echo: the observed temperatures match those expected for dust located at the light radius $\mu$2 heated by either the shock-cooling emission or the SN peak.

Identifying the echo source and CSM geometry

A key quantitative argument distinguishes the echo power source. Computing the dust density implied by the echoing volume between paraboloids of equal arrival time, the brief ($\mu$3 day) shock-cooling source requires a CSM optical depth of at least 1.2–7.9 — inconsistent with the observed optically thin emission — whereas the longer-lived ($\mu$4 day) SN peak gives lower limits of 0.1–0.8. The IR echo is therefore powered by the slowly evolving nickel-powered peak rather than the shock cooling, a regime distinct from the shock-breakout-powered echoes observed in Cas A.

Geometry constraints come from both the single-temperature SED (favoring an echoing volume roughly equidistant from the SN) and a simple $\mu$5-cell simulation comparing homogeneous sphere, thick disk ($\mu$6 thickness with $\mu$7), $\mu$8 wind, and shell geometries. The sphere and shell evolve too slowly in temperature and mass; the disk model best reproduces the observed declining temperature and rising mass, provided the illuminating luminosity is $\mu$9, somewhat below published estimates. The predicted echo luminosity of $\beta$0–$\beta$1 agrees well with observations. Assuming a gas-to-dust ratio of 100, the implied mass-loss rate is $\beta$2 for a 100 km/s wind — far below rates inferred for strongly interacting SESNe such as SN 2014C.

The onset of the NIR excess between 12 and 32 days post peak places the disk's inner edge at $\beta$3–$\beta$4 cm (quoted as $\beta$5 cm), coincidentally similar to the CSM scale around SN 2014C. Extrapolating the shock at $\beta$6, the authors predict the onset of direct shock–CSM interaction at $\beta$7 days post explosion, testable with already-scheduled JWST observations.

Limitations and open questions

Several caveats bear directly on these conclusions. The absolute flux calibration relies on sparse UKIDSS comparison stars (about 10% uncertainty) and external $\beta$8-band photometry, and all mass, radius, and luminosity uncertainties are dominated by the host distance error. Without wavelength coverage beyond 2.4 $\beta$9m, the fits cannot distinguish optically thin from thick solutions internally, nor detect colder dust components; the choice of carbonaceous grains is motivated by analogy with SN 2005ip rather than by data on this object. The geometry simulation assumes a specific source luminosity and grain properties, and the favored disk solution requires a luminosity somewhat below literature values, hinting that the extinction corrections of Zou et al. and Xi et al. may be overestimated. The composition of the CSM — hydrogen-rich like SN 2014C or hydrogen-poor like SN 2022xxf — remains undetermined pending late-time spectroscopy, as does whether new dust forms in a cold dense shell after the shock arrives.

Conclusion

SN 2024aecx provides the first detection of an IR echo from pre-existing circumstellar dust around a normal SESNe, revealed through dense NIR spectroscopic monitoring. The echo implies a close-in, thick face-on dusty disk with an inner edge near $\gamma$0 cm — CSM that case B binary mass transfer alone would not place so near the progenitor at death. The predicted shock interaction at roughly $\gamma$1 days post explosion offers a concrete, near-term test of the disk geometry and of the additional mass-loss mechanism responsible for stripping hydrogen and helium from the progenitor.