2024A&A...689A.281M

Supernova remnant (SNR) detection along the Galactic plane poses a number of challenges. A diffuse X-ray emission component emanating from unidentified sources on the Galactic plane further complicates such a detection in X-rays. Due to the presence of dense dust clouds along the Galactic plane, X-ray photons are also subject to high absorption. Similarly, diffuse signals from the Galactic plane cause γ-ray contamination from the signal of individual objects. The SNR G309.8+00.0 lies exactly on the Galactic plane, with its center coinciding with galactic latitude (b) = 0°. In this paper we report the first detection of the SNR G309.8+00.0 in X-rays and γ rays, using stacked data from the first four consecutive extended ROentgen Survey Imaging Telescope Array (eROSITA) - on board the Russian-German Spektrum Roentgen Gamma (SRG) - all-sky surveys (eRASS:4) and ∼15.5 yr of Pass 8 data recorded from Fermi-LAT, respectively. The SNR appears to have an elliptical shape of 0^ọ43 × 0^ọ32 in size in both radio synchrotron and X-ray data. The SNR's emission exhibits a shell-like morphology and good spatial correlation in both energy bands. The X-ray emission was solely detected in the 1-2 keV energy band (subject to strong absorption at soft X-rays) and the spectral analysis results of eRASS:4 data present a purely thermal SNR with a high absorption column density 3.1^+0.7_–0.5 ⋅ 10²² cm^–2 and a temperature of 0.34 ± 0.1 keV. Although the thermal plasma appears to be in equilibrium, the limited statistics do not allow us to exclude nonequilibrium models. The X-ray spectral analysis of the remnant resulted in the detection of relatively (given the limited statistics) prominent Mg triplet lines at 1.33-1.47 keV and silicon (Si XIII) at 1.74-1.9 keV energies. In combination with optical extinction data, the absorption column density values derived from the remnant's spectral analysis support a remnant's distance greater than 6 kpc, rather than a 3.12 kpc distance as reported in the literature, and yield an age of 1 - 3.5 ⋅ 10⁵ yr. Employing ∼15.5 yr of Fermi-LAT γ-ray data at and around the remnant's vicinity, we confirm the detection of the to-date unidentified 4FGL J1349.5-6206c source that can either be modeled as a single source or a conglomerate of multiple distinct source components. In the latter case, the detailed inspection of the Fermi-LAT γ-ray data in the direction of the remnant allowed us to decompose the 4FGL J1349.5-6206c source into four point-like components, among which one is spatially coincident with the SNR G309.8+00.0 shell. We detected the component that spatially coincides with the SNR with a significance of 5.8σ above 1 GeV with Fermi-LAT and thus argue that the SNR G309.8+00.0 likely represents at least a significant portion (if not all) of the emission from the 4FGL J1349.5-6206c γ-ray source, detected with 9.8σ significance > 1 GeV with Fermi-LAT.