SRG/eROSITA discovery of a large circular SNR candidate G116.6−26.1: SN Ia explosion probing the gas of the Milky Way halo?

We report a discovery of a new X-ray-selected supernova remnant (SNR) candidate SRGe J0023+3625 = G116.6−26.1 found in the SRG/eROSITA all-sky survey. The source features a large angular extent (∼4° in diameter), nearly circular shape, and X-ray spectrum dominated by emission lines of helium- and hydrogen-like oxygen. It lacks bright counterparts of similar extent at other wavelengths which could be unequivocally associated with it. Given the relatively high Galactic latitude of the source, b ≈ −26°, we interpret these observational properties as an indication of the off-disc location of this SNR candidate. Namely, we propose that this object originated from a Type Ia supernotva which exploded some 40 000 yr ago in the low density ( $\sim 10^{-3}\, {\rm cm^{-3}}$ ) and hot ( $\sim (1{-}2)\times 10^6\, {\rm K}$ ) gas of the Milky Way halo at a distance of ∼3 kpc from the Sun. The low density of the halo gas implies that the cooling and collisional ionization equilibrium (CEI) time-scales downstream of the forward shock are much longer than the age of the SNR. This results in a relatively soft spectrum, reflecting pre-shock ionization state of the gas, and strong boost in the plasma emissivity (compared to CEI) due to enhanced collisional excitation through the increased electron temperature. If confirmed, such a rare object would provide us with a unique ‘in situ’ probe of physical conditions (density, temperature and metallicity) near the interface between the Milky Way's disc and the halo.