Supernova remnant G46.8-0.3, a new case of interaction with molecular material

Abstract: We present the first detailed study on the Galactic supernova remnant (SNR) G46.8-0.3 and its environment since its discovery more than 50 years ago. With new flux measurements from radio surveys plus a compilation from the literature we created an improved integrated radio spectrum of G46.8-0.3, which is well fitted by a simple power-law with an index alpha = -0.535 +/- 0.012, then excluding the presence of absorption by abundant ionised gas in the line of sight, or in the SNR's proximity. The analysis of local changes in the radio spectral index across G46.8-0.3 suggests a slight steepening at about 1 GHz, which does not impact on the integrated spectrum of the source. From atomic hydrogen (HI) spectra we placed the remnant at 8.7 +/- 1.0 kpc, and revisited the distance to the nearby HII region G046.495-00.241 to 7.3 +/- 1.2 kpc. From evolutionary models and our distance estimate, we conclude G46.8-0.3 is a middle-aged ($\sim$1 x 10^4 yr) SNR. We recognised several 12CO and 13CO molecular structures in the proximity of the remnant and used combined CO-HI profiles to derive the kinematic distances to these features and characterise their physical properties. We also provided compelling evidences for environmental molecular clouds physically linked to G46.8-0.3 at its centre, on its eastern edge, and towards the northern and southwestern rims on the far side of the SNR shell. Our study does not confirm that the remnant is embedded in a molecular cavity as previously suggested. G46.8-0.3 shows a line-of-sight coincidence with the gamma-ray source 4FGL J1918.1+1215c, detected at GeV energies by the space telescope Fermi. A rough analysis based on the properties of the interstellar matter close G46.8-0.3 indicates that the GeV gamma-rays photons detected in direction to the SNR can be plausibly attributed to hadronic collisions and/or bremsstrahlung radiation.