SRG/eROSITA discovery of a large circular SNR candidate G116.6−26.1: SN Ia explosion probing the gas of the Milky Way halo?. Issue 1 (24th August 2021)

• Record Type: Journal Article
• Authors: Churazov, E M; Khabibullin, I I; Bykov, A M; Chugai, N N; Sunyaev, R A; Zinchenko, I I
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 507:Issue 1(2021)
• Page Start: 971
• Page End: 982
• Publication Date: 2021-08-24
• Subjects: radiation mechanisms: thermal -- ISM: supernova remnants -- Galaxy: halo -- X-rays: general; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stab2125
• Languages: English
• ISSNs: 0035-8711

Abstract

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 (∼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.