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

TL;DR

This paper reports the discovery of an infrared echo from pre-existing circumstellar dust around a peculiar Type Ic supernova, revealing insights into its progenitor's mass-loss history through near-infrared spectroscopy and dust modeling.

Contribution

It presents the first detection of an IR echo in a hydrogen- and helium-poor supernova, constraining the CSM geometry and the timing of shock interaction.

Findings

NIR excess appears 32 days post peak with a declining dust temperature.

The IR echo is best explained by a thick face-on disk CSM geometry.

Shock interaction with the CSM is expected around 440 days post explosion.

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 $5\times 10^{16}$ 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.