2025ApJ...995...54G

Type Ia-CSM supernovae (SNe) are a rare and peculiar subclass of thermonuclear SNe characterized by emission lines of hydrogen or helium, indicative of high-density circumstellar medium (CSM). Their implied mass-loss rates of ∼10^–4-10^–1M_⊙ yr^–1 (assuming ∼100 km s^–1 winds) from optical observations are generally in excess of values observed in realistic SN Ia progenitors. In this paper, we present an independent study of CSM densities around a sample of 29 archival Ia-CSM SNe using radio observations with the Karl G. Jansky Very Large Array at 6 GHz. Motivated by the late (∼2 yr) radio detection of the Ia-CSM SN 2020eyj, we observed old (>1 yr) SNe, when we are more likely to see the emergent synchrotron emission that may have been suppressed earlier by free-free absorption by the CSM. We do not detect radio emission down to 3σ limits of ∼35 μJy in our sample. The only radio-detected candidate in our sample, SN 2022esa, was likely misclassified as a Ia-CSM with early spectra, and appears more consistent with a peculiar Ic based on later epochs. Assuming wind-like CSM with temperatures between 2 × 10⁴ K and 10⁵ K, and a magnetic field-to-shock energy fraction ϵ_B = 0.01 - 0.1, the radio upper limits rule out mass-loss rates between ∼10^–4 and 10^–2M_⊙ yr^–1 (100 km s^–1)^–1. This is somewhat in tension with the estimates from optical observations, and may indicate that more complex CSM geometries and/or lower values of ϵ_B may be present.