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

TL;DR

The paper reports the discovery and detailed analysis of G181.1+9.5, a high-latitude, low-surface brightness supernova remnant, using radio, HI, and X-ray observations, revealing its physical properties and evolutionary stage.

Contribution

It presents the first identification and comprehensive multi-wavelength characterization of a high-latitude, low-surface brightness supernova remnant, expanding understanding of SNR diversity.

Findings

G181.1+9.5 has a radius of about 16 pc at 1.5 kpc distance.

It exhibits highly polarized, non-thermal radio emission.

X-ray data indicate shock-heated ejecta, consistent with an advanced SNR.

Abstract

More than 90% of the known Milky Way supernova remnants are within 5 degrees of the Galactic Plane. We present the discovery of the supernova remnant 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 HI spectral line. We use radio continuum observations (including the linearly polarized component) at 1420~MHz (observed with the DRAO ST) and 4850~MHz (observed with the Effelsberg 100-m radio telescope) to map G181.1+9.5 and determine its nature as a SNR. High-resolution 21~cm 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. 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 1$~cm$^{-3}$. Basic physical attributes of G181.1+9.5 calculated with radiative SNR models show an upper-limit age of 16,000 years, a swept-up mass of more than 300 solar masses, and an ambient density in agreement with that estimated from HI observations. G181.1+9.5 shows all characteristics of a typical mature shell-type SNR, but its observed faintness is unusual and requires further study.