LOFAR detection of faint radio emission from the supernova remnant SRGe J0023+3625 = G116.6−26.1: probing the Milky Way synchrotron halo

ABSTRACT A supernova remnant (SNR) candidate SRGe J0023+3625 = G116.6–26.1 was recently discovered in the SRG/eROSITA all-sky X-ray survey. This large (∼4° in diameter) SNR candidate lacks prominent counterparts in other bands. Here we report detection of radio emission from G116.6–26.1 in the LOFAR Two-metre Sky Surveyr (LoTSS-DR2). Radio images show a shell-like structure coincident with the X-ray boundary of the SNR. The measured surface brightness of radio emission from this SNR is very low. Extrapolation of the observed surface brightness to 1 GHz places G116.6-26.1 well below other objects in the Σ–D diagram. We argue that the detected radio flux might be consistent with the minimal level expected in the van der Laan adiabatic compression model, provided that the volume emissivity of the halo gas in the lofar band is ${\sim}10^{-42}\, {\rm Wm^{-3}\,Hz^{-1}\,sr^{-1}}$. If true, this SNR can be considered as a prototypical example of an evolved SNR in the Milky Way halo. In the X-ray and radio bands, such SNRs can be used as probes of thermal and non-thermal components constituting the Milky Way halo.