  Type IIb Supernova (SN) 2011dh, with conclusive detection of an unprecedented
Yellow Supergiant (YSG) progenitor, provides an excellent opportunity to deepen
our understanding on the massive star evolution in the final centuries toward
the SN explosion. In this paper, we report on detection and analyses of thermal
X-ray emission from SN IIb 2011dh at ~500 days after the explosion on Chandra
archival data, providing a solidly derived mass loss rate of an YSG progenitor
for the first time. We find that the circumstellar media (CSM) should be dense,
more than that expected from a Wolf-Rayet (WR) star by one order of magnitude.
The emission is powered by a reverse shock penetrating into an outer envelope,
fully consistent with the YSG progenitor but not with a WR progenitor. The
density distribution at the outermost ejecta is much steeper than that expected
from a compact WR star, and this finding must be taken into account in modeling
the early UV/optical emission from SNe IIb. The derived mass loss rate is 3 x
10^{-6} Msun/year for the mass loss velocity of ~20 km/s in the final ~1,300
years before the explosion. The derived mass loss properties are largely
consistent with the standard wind mass loss expected for a giant star. This is
not sufficient to be a main driver to expel nearly all the hydrogen envelope.
Therefore, the binary interaction, with a huge mass transfer having taken place
at >1,300 years before the explosion, is a likely scenario to produce the YSG
progenitor.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics