The interaction of SNR Kes 17 with the interstellar medium: Fresh view from radio and γ-rays

Abstract

This paper presents a comprehensive analysis of the Galactic supernova remnant (SNR) Kes 17 (G304.6+0.1). The focus is on its radio synchrotron emission, its surroundings, and the factors contributing to the observed γ rays. The fitting to the first obtained integrated radio continuum spectrum spanning from 88 to 8800 MHz yields an index α = −0.488 ± 0.023 (Sν ∝ να), which is indicative of a linear particle acceleration process at the shock front of the remnant. Accounting for the SNR radio shell size, the distribution of atomic hydrogen (nH ~ 10 cm−3), and assuming that the SNR is in the Sedov-Taylor stage of its evolution, we estimate that the remnant age is roughly 11 kyr. This result falls at the lower end of the wide range (~2–64 kyr) derived from previous analyses of the diffuse X-ray emission interior to the remnant. Furthermore, we used 12CO and 13CO (J = 1–0) emission line data as a proxy for molecular hydrogen and provide the first evidence that the eastern shell of Kes 17 is engulfing a molecular enhancement in the surrounding gas, with an average mass 4.2 × 104 M⊙ and density n ~ 300 cm−3. Towards the western boundary of Kes 17, no signatures of carbon monoxide emissions are detected above 3σ, despite previously reported infrared observations that revealed shocked molecular gas at this location. This suggests the existence of a CO-dark interacting molecular gas, a phenomenon also recorded in other Galactic SNRs (e.g. CTB 37A and RX J1713.7–3946) that reveals itself both in the infrared and γ-ray domains. Additionally, by analysing ~14.5 yr of continuously collected data from the Large Area Telescope on board the satellite Fermi, we determined that the best-fit power-law photon index for the 0.3–300 GeV γ-ray emission from the Kes 17 region is Γ = 2.39 ± 0.04−0.114+0.063 (±stat ±syst), in agreement with prior studies. The energy flux is (2.98 ± 0.14) × 10−11 erg cm−2 s−1, implying a luminosity (2.22 ± 0.45) × 1035 erg s−1 at ~8 kpc. Finally, we successfully modelled the multiwavelength spectral energy distribution by incorporating the radio-synchrotron spectrum and the new measurements of GeV γ-rays. Our analysis indicates that the observed γ-ray flux most likely originates from the interaction of Kes 17 with a western dark CO zone with a proton density np ~ 400 cm−3.