Detection of optical emission from the supernova remnant G7.7-3.7

We present the first optical study of the supernova remnant (SNR) G7.7–3.7, with the aim of determining its evolutionary phase since it has been suggested to be the remnant of SN 386 AD. We obtained narrow-band images in the filters H α + [N II], H β, [O III], [S II] that revealed faint optical emission in the southern region of the SNR consisting of two filaments elongated in the east–west direction aligned with the X-ray emitting region of the remnant. The filaments were seen in H α + [N II], [O III] images and marginally in the [S II] images, with a non-detection in H β. Long-slit spectroscopy of the three regions along one filament revealed large ratios of [S II]/H α = (1.6–2.5), consistent with that expected for a shock-heated SNR. The [S II] doublet ratio observed in two of the regions implies an upper limit for the electron density of the gas, with estimates falling below 400 cm⁻³ and 600 cm⁻³ in the respective areas. We discuss potential physical mechanisms that formed the observed optical filaments and we suggest that most likely they resulted by a collision of the SNR with a dense circumstellar shell lying at the southern region of the remnant.