2025MNRAS.543..761A

Radio observations have revealed that G152.4-2.1 and G203.1 + 6.6 are shell-type supernova remnants (SNRs), both characterized by their large angular extents. To investigate the nature of these remnants, as well as the physical conditions within their associated shocks and surrounding ambient medium, we conducted optical observations of SNRs G152.4-2.1 and G203.1 + 6.6. We obtained long-slit spectra at several positions across these SNRs using the 1.5-m Russian-Turkish Telescope (RTT150). For both SNRs, we measured [S II]/H$\alpha$ $\ge 0.4$, which is a characteristic of shock-heated gas. Electron density measurement in G152.4-2.1, derived from the [S II] $\lambda \lambda$6716, 6731 lines, indicates an electron density of approximately 630 cm$^{-3}$, corresponding to a pre-shock density of $\sim$22 cm$^{-3}$. For G203.1 + 6.6, we find that the electron density reaches up to $\sim$2800 cm$^{-3}$, while the corresponding pre-shock density is estimated to be $\sim$97 cm$^{-3}$. The observed H$\alpha$/H$\beta$ ratio variations in the spectra of both remnants suggest considerable reddening $E(B-V)$ differences across the regions. Shock velocities in both SNRs are estimated to be approximately 80 km s$^{-1}$, as inferred from the [O III]$\lambda$5007/H$\beta$ line ratio. Finally, we interpret the evolutionary stage of each SNR.