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

Abstract

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 μ Jy beam −1 at 6.2 GHz). We detect some diffuse emission near SN 1885A at 2.6 σ , 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 3 σ upper limit of 11 μ Jy 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.