Discovery of an Exceptional Optical Nebulosity in the Suspected Galactic SN Iax Remnant Pa 30 Linked to the Historical Guest Star of 1181 CE

A newly recognized young Galactic supernova (SN) remnant, Pa 30 (G123.1+4.6), centered on a hot central star with a ∼16,000 km s−1 wind velocity, has recently been proposed to be the result of a double-degenerate merger leading to an SN Iax event associated with the guest star of 1181 CE. Here we present deep optical [S ii] λλ6716,6731 images of Pa 30 that reveal an extraordinary and highly structured nebula 170'' in diameter with dozens of long (5''–20''), radially aligned filaments with a convergence point near the hot central star. Optical spectra of filaments indicate a peak expansion velocity ≃1100 km s−1 with electron densities of ≤100–700 cm−3 and a thick shell-like structure resembling its appearance in 22 μm WISE images. No Hα emission was seen ([S ii] λ6716/Hα >5), with the only other line emission detected being faint [Ar iii] λ7136, suggesting a S- and Ar-rich but H-poor remnant. The nebula's angular size, estimated 2.3 kpc distance, and 1100 km s−1 expansion velocity are consistent with an explosion date around 1181 CE. The remnant's unusual appearance may be due to the photoionization of wind-driven ejecta due to clump–wind interactions caused by the central star's high-luminosity wind.