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 ($\sim 4$ deg 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, $b\approx-26$ deg, we
interpret these observational properties as an indication of the off-disk
location of this SNR candidate. Namely, we propose that this object originated
from a Type Ia supernova which exploded some 40 000 yr ago in the low density
($\sim 10^{-3}\,{\rm cm^{-3}}$) and hot ($\sim (1-2)\times10^6\,{\rm K}$) gas
of the Milky Way halo at a distance of $\sim 3\,{\rm kpc}$ from the Sun. The
low density of the halo gas implies that the cooling and collisional ionization
equilibrium (CEI) timescales 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 disk and the halo.
