2017A&A...597A.116K

Context. More than 90% of the known Milky Way supernova remnants (SNRs) are within 5° of the Galactic plane. The discovery of the new high-latitude SNR G181.1+9.5 will give us the opportunity to learn more about the environment and magnetic field at the interface between disk and halo of our Galaxy.
Aims. We present the discovery of SNR G181.1+9.5, a new high-latitude SNR, serendipitously discovered in an ongoing survey of the Galactic anti-centre High-Velocity Cloud complex, observed with the DRAO Synthesis Telescope in the 21cm radio continuum and HI spectral line.
Methods. We use radio continuum observations (including the linearly polarized component) at 1420MHz (observed with the DRAO ST) and 4850MHz (observed with the Effelsberg 100-m radio telescope) to map G181.1+9.5 and determine its nature as a SNR. High-resolution 21cm HI line observations and HI emission and absorption spectra reveal the physical characteristics of its local interstellar environment. Finally, we estimate the basic physical parameters of G181.1+9.5 using models for highly-evolved SNRs.
Results. G181.1+9.5 has a circular shell-like morphology with a radius of about 16pc at a distance of 1.5kpc some 250pc above the mid-plane. The radio observations reveal highly linearly polarized emission with a non-thermal spectrum. Archival ROSAT X-ray data reveal high-energy emission from the interior of G181.1+9.5 indicative of the presence of shock-heated ejecta. The SNR is in the advanced radiative phase of SNR evolution, expanding into the HVC inter-cloud medium with a density of n_HI~=1cm^–3. Basic physical attributes of G181.1+9.5 calculated with radiative SNR models show an upper-limit age of 16000yr, a swept-up mass of more than 300M_☉, and an ambient density in agreement with that estimated from HI observations.
Conclusions. G181.1+9.5 shows all characteristics of a typical mature shell-type SNR, but its observed faintness is unusual and requires further study.