We report the first detection of thermal X-ray line emission from the supernova remnant ( SNR ) RX J1713.7-3946 , the prototype of the small class of synchrotron dominated SNRs .
A softness-ratio map generated using XMM-Newton data shows that faint interior regions are softer than bright shell regions .
Using Suzaku and deep XMM-Newton observations , we have extracted X-ray spectra from the softest area , finding clear line features at E _ { ph } \sim 1 keV and \sim 1.35 keV .
These lines can be best explained as Ne Ly \alpha and Mg He \alpha from a thermal emission component .
Since the abundance ratios of metals to Fe are much higher than solar values in the thermal component , we attribute the thermal emission to reverse-shocked SN ejecta .
The measured Mg/Ne , Si/Ne , and Fe/Ne ratios of 2.0–2.6 , 1.5–2.0 , and < 0.05 solar suggest that the progenitor star of RX J1713.7-3946 was a relatively low-mass star ( \lesssim 20 M _ { \odot } ) , consistent with a previous inference based on the effect of stellar winds of the progenitor star on the surrounding medium .
Since the mean blastwave speed of \sim 6000 km s ^ { -1 } ( the radius of 9.6 pc divided by the age of 1600 yr ) is relatively fast compared with other core-collapse SNRs , we propose that RX J1713.7-3946 is a result of a Type Ib/c supernova whose progenitor was a member of an interacting binary .
While our analysis provides strong evidence for X-ray line emission , our interpretation of its nature as thermal emission from SN ejecta requires further confirmation especially through future precision spectroscopic measurements using ASTRO-H .