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

TL;DR

This paper reports the discovery of a large, off-disk supernova remnant candidate in the Milky Way halo, providing insights into the halo's gas properties and the aftermath of a Type Ia supernova explosion.

Contribution

It presents the identification and analysis of a new X-ray supernova remnant candidate in the Milky Way halo, suggesting its origin from a Type Ia supernova and exploring its implications for halo gas conditions.

Findings

Large angular extent (~4 deg) SNR candidate with X-ray emission lines.

Likely originated from a Type Ia supernova ~40,000 years ago.

Probes physical conditions of the Milky Way halo gas.

Abstract

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.