G181.1+9.5, a new high-latitude low-surface brightness supernova remnant

Context. More than 90% of the known Milky Way supernova remnants (SNRs) are within 5 ffi 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 21 cm radio continuum and H i spectral line. Methods. We use radio continuum observations (including the linearly polarized component) at 1420 MHz (observed with the DRAOST) and 4850 MHz (observed with the E ff elsberg 100-m radio telescope) to map G181.1+9.5 and determine its nature as a SNR. High-resolution 21 cm H i line observations and H i 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 16 pc at a distance of 1.5 kpc some 250 pc 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) approximate to 1 cm(-3). Basic physical attributes of G181.1+9.5 calculated with radiative SNR models show an upper-limit age of 16 000 yr, a swept-up mass of more than 300 M-circle dot, and an ambient density in agreement with that estimated from H i 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.