SRG/eROSITA discovery of a radio-faint X-ray candidate supernova remnant SRGe J003602.3+605421 = G121.1-1.9

Abstract

We report the discovery of a candidate X-ray supernova remnant SRGe J003602.3+605421 = G121.1-1.9 in the course of the SRG/eROSITA all-sky survey. The object is located at (l, b) = (121.1°, -1.9°), is ≈36 arcmin in angular size, and has a nearly circular shape. Clear variations in the spectral shape of the X-ray emission across the object are detected, with the emission from the inner (within 9 arcmin) and outer (9-18 arcmin) parts dominated by iron and oxygen/neon lines, respectively. The non-equilibrium plasma emission model is capable of describing the spectrum of the outer part with an initial gas temperature 0.1 keV, final temperature 0.5 keV, and ionization age ~2 × 10¹⁰ cm^-3 s. The observed spectrum of the inner region is more complicated (plausibly due to the contribution of the outer shell) and requires a substantial overabundance of iron for all models that we have tried. The derived X-ray absorption is equal to (4-6) × 10²¹ cm^-2, locating the object at a distance beyond 1.5 kpc, and implying its age ~(5-30) × 1000 yr. No bright radio, infrared, H α, or gamma-ray counterpart of this object has been found in the publicly available archival data. A model invoking a canonical 10⁵¹ erg explosion (either SN Ia or core collapse) in the hot and tenuous medium in the outer region of the Galaxy ~9 kpc away might explain the bulk of the observed features. This scenario can be tested with future deep X-ray and radio observations.