The Two Most Recent Thermonuclear Supernovae in the Local Group: Radio Constraints on their Progenitors and Evolution

Young supernova remnants (SNRs) provide a unique perspective on supernova (SN) progenitors and connect the late evolution of SNe with the onset of the SNR phase. Here we study SN 1885A and G1.9+0.3, the most recent thermonuclear SNe in the Local Group (with ages ∼100 yr) with radio data, which provide a sensitive probe of the SN environment and energetics. We reduce and co-add 4-8 GHz observations from the Karl G. Jansky Very Large Array (VLA) to produce the deepest radio image of the central region of M31 (rms noise of 1.3 μJy beam^-1 at 6.2 GHz). We detect some diffuse emission near SN 1885A at 2.6σ, but the association with SN 1885A is uncertain because diffuse radio emission pervades M31's central region. Accounting for this diffuse emission, we get a 3σ upper limit of 11 μJy for SN 1885A, about four times less luminous than G1.9+0.3 at 6.2 GHz. Together with Hubble Space Telescope data on SN 1885A, we estimate its ambient density, n ₀ < 0.04 cm^-3 (±0.03 cm^-3 due to systematics), and constrain the joint kinetic energy-ejecta mass parameter space. For G1.9+0.3, archival VLA observations yielded n ₀ = 0.18 cm^-3. We synthesize the multi-wavelength data on SN 1885A to argue that it is likely the product of a sub-Chandrasekhar explosion, while G1.9+0.3 appears consistent with both Chandrasekhar and sub-Chandrasekhar SN Ia models.