Discovery of an Apparent High Latitude Galactic Supernova Remnant

TL;DR

This paper reports the discovery of a high-latitude Galactic supernova remnant characterized by optical, UV, and X-ray emissions, with unusual spectral features that challenge typical SNR classifications.

Contribution

It presents the identification and multi-wavelength analysis of a new high-latitude SNR with atypical optical spectral line ratios.

Findings

Detected faint Hα filaments with Balmer and weak forbidden line emissions.

Coincident UV and X-ray emissions support SNR identification.

Observed unusual optical line ratios, prompting discussion on spectral diversity.

Abstract

Deep H$\alpha$ images of a faint emission complex 4.0 x 5.5 degrees in angular extent and located far off the Galactic plane at l = 70.0 degrees, b=-21.5 degrees reveal numerous thin filaments suggestive of a supernova remnant's shock emission. Low dispersion optical spectra covering the wavelength range 4500 - 7500 A show only Balmer line emissions for one filament while three others show a Balmer dominated spectrum along with weak [N I] 5198, 5200 A, [O I] 6300, 6364 A, [N II] 6583 A, [S II] 6716, 6731 A and in one case [O III] 5007 A line emission. Many of the brighter H$\alpha$ filaments are visible in near UV GALEX images presumably due to C III] 1909 A line emission. ROSAT All Sky Survey images of this region show a faint crescent shaped X-ray emission nebula coincident with the portion of the H$\alpha$ nebulosity closest to the Galactic plane. The presence of long, thin Balmer dominated emission filaments with associated UV emission and coincident X-ray emission suggests this nebula is a high latitude Galactic supernova remnant despite a lack of known associated nonthermal radio emission. Relative line intensities of the optical lines in some filaments differ from commonly observed [S II]/H$\alpha$ > 0.4 radiative shocked filaments and typical Balmer filaments in supernova remnants. We discuss possible causes for the unusual optical SNR spectra.