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

TL;DR

This study presents the first optical detection of supernova remnant G7.7-3.7, revealing faint filaments aligned with X-ray emissions, and suggests they result from the remnant colliding with a dense circumstellar shell.

Contribution

First optical imaging and spectroscopy of G7.7-3.7, identifying shock-heated filaments and providing insights into its interaction with surrounding material.

Findings

Detected faint optical filaments aligned with X-ray emission.

Spectroscopy indicates shock-heated gas with high [SII]/Hα ratios.

Estimated electron densities below 600 cm$^{-3}$.

Abstract

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$\alpha$ + [NII], H$\beta$, [OIII], [SII] 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$\alpha$ + [NII], [OIII] images and marginally in the [SII] images, with a non-detection in H$\beta$. Long-slit spectroscopy of three regions along one filament revealed large ratios of [SII] / H$\alpha$ = (1.6-2.5), consistent with that expected for a shock-heated SNR. The [SII] 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.