Possible Detection of X-Ray Emitting Circumstellar Material in the Synchrotron-dominated Supernova Remnant RX J1713.7-3946

We report on a discovery of an X-ray emitting circumstellar material (CSM) knot inside the synchrotron dominant supernova remnant RX J1713.7-3946. This knot was previously thought to be a Wolf-Rayet star (WR 85), but we realized that it is in fact ~40″ away from WR 85, indicating no relation to WR 85. We performed high-resolution X-ray spectroscopy with the Reflection Grating Spectrometer (RGS) on board XMM-Newton. The RGS spectrum clearly resolves a number of emission lines, such as N Lyα, O Lyα, Fe XVIII, Ne X, Mg XI, and Si XIII. The spectrum can be well represented by an absorbed thermal-emission model with a temperature of k _B T _e = 0.65 ± 0.02 keV. The elemental abundances are obtained to be ${\rm{N}}/{\rm{H}}=3.5\pm 0.8{\left({\rm{N}}/{\rm{H}}\right)}_{\odot }$ N/H=3.5±0.8N/H⊙ , ${\rm{O}}/{\rm{H}}=0.5\pm 0.1{\left({\rm{O}}/{\rm{H}}\right)}_{\odot }$ O/H=0.5±0.1O/H⊙ , $\mathrm{Ne}/{\rm{H}}=0.9\pm 0.1{\left(\mathrm{Ne}/{\rm{H}}\right)}_{\odot }$ Ne/H=0.9±0.1Ne/H⊙ , $\mathrm{Mg}/{\rm{H}}=1.0\pm 0.1{\left(\mathrm{Mg}/{\rm{H}}\right)}_{\odot }$ Mg/H=1.0±0.1Mg/H⊙ , $\mathrm{Si}/{\rm{H}}=1.0\pm 0.2{\left(\mathrm{Si}/{\rm{H}}\right)}_{\odot }$ Si/H=1.0±0.2Si/H⊙ , and $\mathrm{Fe}/{\rm{H}}=1.3\pm 0.1{\left(\mathrm{Fe}/{\rm{H}}\right)}_{\odot }$ Fe/H=1.3±0.1Fe/H⊙ . The enhanced N abundance with others being about the solar values allows us to infer that this knot is CSM ejected when the progenitor star evolved into a red supergiant. The abundance ratio of N to O is obtained to be ${\rm{N}}/{\rm{O}}={6.8}_{-2.1}^{+2.5}{\left({\rm{N}}/{\rm{O}}\right)}_{\odot }$ N/O=6.8-2.1+2.5N/O⊙ . By comparing this to those in outer layers of red supergiant stars expected from stellar evolution simulations, we estimate the initial mass of the progenitor star to be 15 M _⊙ ≲ M ≲ 20 M _⊙.