The Probable Detection of SN 1923A: The Oldest Radio Supernova?

Based on the results of VLA observations, we report the detection of two unresolved radio sources that are coincident with the reported optical position of SN 1923A in M83. For the source closest to the SN position, the flux density was determined to be 0.30 ± 0.05 mJy at 20 cm and 0.093 ± 0.028 mJy at 6 cm. The flux density of the second nearby source was determined to be 0.29 ± 0.05 mJy at 20 cm and 0.13 ± 0.028 mJy at 6 cm. Both sources are nonthermal, with spectral indices of α = -1.0 ± 0.30 and -0.69 ± 0.24, respectively. SN 1923A has been designated a Type II-P. No Type II-P (other than SN 1987A) has been detected previously in the radio. The radio emission from both sources appears to be fading with time. At an age of approximately 68 yr when we observed it, this would be the oldest radio SN (of known age) yet detected.