Supernova remnant G46.8-0.3: A new case of interaction with molecular material

SUPAN, L.; FISCHETTO G.; CASTELLETTI, G.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2022 vol. 664 p. 89 - 102

0004-6361

The Galactic supernova remnant (SNR) G46.8-0.3 has been known for more than fifty years, in spite of which no specific works about this source nor on its environment have been published to date. To make progress on this matter, we measured new flux densities from radio surveys and combined them with previous estimates carefully collected from the literature to create an improved and fully-populated version of the integrated radio spectrum for G46.8-0.3. The resulting spectrum exhibits a featureless power-law form with an exponent $alpha = -0.535 pm 0.012$. The lack of a spectral turnover at the lowest radio frequencies, observable in many other SNRs, excludes the presence of abundant ionised gas either proximate to the SNR itself, or along its line of sight. The analysis of local changes in the radio spectral index across G46.8-0.3 suggests a tendency to slightly steepen approximately at 1 GHz. Even if it is real, it does not impact on the integrated spectrum of the source. Deeper imaging of the radio structures of G46.8-0.3 and spectral maps constructed from matched raw data are needed to provide new insights into the local spectral properties of the remnant. On the basis of the spectral properties of the atomic gas we placed the remnant at 8.7 $pm$ 1.0 kpc and, as a bonus, we revisited the distance to the nearby HII region G046.495-00.241 to 7.3 $pm$ 1.2 kpc. From evolutionary models and our distance estimate, we conclude G46.8-0.3 is a middle-aged ($sim$1 $imes10^4$~yr) SNR. Furthermore, we recognised several 12CO and 13CO molecular structures in the proximity of the remnant. We used combined CO-HI profiles to derive the kinematic distances to these features and characterise their physical properties. We 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 of the molecular matter 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.