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) with a focus on its radio synchrotron emission, environs, and the factors contributing to the observed gamma rays. The fitting to the firstly-obtained integrated radio continuum spectrum spanning from 88 to 8800 MHz yields an index alpha = -0.488 +/- 0.023 (S propto nu^alpha), 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 the SNR is in the Sedov-Taylor stage of its evolution, we estimate the remnant’s age to be roughly 8.5 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 provided 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 x 10^4 Msun and density n ~ 300 cm-3. Towards the western boundary of Kes 17 there are not signatures of carbon monoxide emissions above 3sigma, despite previously reported infrared observations have revealed shocked molecular gas at that 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) brightening both in the infrared and gamma-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 gamma-ray emission from the Kes 17 region is Gamma = 2.39 +/- 0.04 +0.063 -0.114 (+/-stat +/-syst) in agreement with prior studies. The energy flux turns out to be (2.98 +/- 0.14) x 10^-11 erg cm-2 s-1 implying a luminosity (2.22 +/- 0.45) x 10^35 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 gamma-rays. Our analysis indicates that the observed gamma-ray flux most likely originates from the interaction of Kes 17 with western "dark" CO zone with a proton density np ~400 cm-3.