A Late-time Radio Survey of Type Ia-CSM Supernovae with the Very Large Array

Type Ia-CSM supernovae (SNe) are a rare and peculiar subclass of thermonuclear SNe characterized by emission lines of hydrogen or helium, indicative of a high-density circumstellar medium (CSM). Their implied mass-loss rates of ~ 10⁻⁴-10⁻¹ M_ 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 Very Large Array at 6 GHz. Motivated by the late (~2 yr) radio detection of the Ia-CSM SN 2020eyj, we observed old ($>$1 yr) SNe where we are more likely to see the emergent synchrotron emission that may have been suppressed earlier by free-free absorption by the CSM. We do not detect radio emission down to 3$σ$ limits of ~35 $μ$Jy in our sample. The only radio-detected candidate in our sample, SN 2022esa, was likely mis-classified as a Ia-CSM with early spectra, and appears more consistent with a peculiar Ic based on later-epochs. Assuming a wind-like CSM with temperatures between 2 × 10⁴ K and 10⁵ K, and magnetic field-to-shock energy fraction (εB) = $0.01-0.1$, the radio upper limits rule out mass-loss rates between ~ 10⁻⁴-10⁻² M_ 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 εB may be present.