Multiwavelength studies of G298.6-0.0: An old GeV supernova remnant interacting with molecular clouds

Hadronic γ-ray sources associated with supernova remnants (SNRs) can serve as stopwatches for the escape of cosmic rays from SNRs, which gradually develops from highest-energy particles to lowest-energy particles with time. In this work, we analyze the 13.7 yr Fermi-LAT data to investigate the γ-ray feature in/around the SNR G298.6-0.0 region. With γ-ray spatial analyses, we detect three point-like components. Among them, Src-NE is at the eastern SNR shell, and Src-NW is adjacent to the western edge of this SNR. Src-NE and Src-NW demonstrate spectral breaks at energies around/below 1.8 GeV, suggesting an old SNR age of >10 kyr. We also look into the X-ray emission from the G298.6-0.0 region, with the Chandra-ACIS data. We detected an extended keV source having a centrally filled structure inside the radio shell. The X-ray spectra are well fitted by a model which assumes a collisional ionization equilibrium of the thermal plasma, further supporting an old SNR age. Based on our analyses of the NANTEN CO- and ATCA-Parkes H I-line data, we determined a kinematic distance of ~10.1 kpc from us to G298.6-0.0. This distance entails a large physical radius of the SNR of ~15.5 pc, which is additional evidence for an old age of >10 kyr. Besides this, the CO data cube enables us to three-dimensionally locate the molecular clouds (MCs) which are potentially interacting with SNR G298.6-0.0 and could account for the hadronic γ-rays detected at Src-NE or Src-NW. Furthermore, the multiwavelength observational properties unanimously imply that the SNR-MC interaction occurs mainly in the north-eastern direction.