  DR21 south filament (DR21SF) is a unique component of the giant network of
filamentary molecular clouds in the north region of Cygnus X complex. Unlike
the highly fragmented and star-forming active environment it resides, DR21SF
exhibits a coherent profile in the column density map with very few star
formation signposts, even though the previously reported linear density of the
filament is an order of magnitude higher than the thermal stable threshold. We
derive the size (3.6~pc by 0.13~pc), temperature (10 to 15~K), and mass
(1048~\textit{M$_\odot$}) of DR21SF from Shanghai 65 m TianMa Radio Telescope
(TMRT) observations of NH$_3$ (1, 1) and (2, 2) inversion lines in conjunction
with the column density map from our previous work. Star-forming sites are
identified along the filament where gas temperature excesses. We find clear
gradients in radial velocity and intrinsic line-width along the spine of the
filament. The gradients can be well interpreted with a scenario of an accretion
flow feeding DR 21 at a mass transfer rate of $1.1 \times
10^{-3}$~\textit{M$_\odot$} yr$^{-1}$. Based on the analysis of its kinematic
temperature, intrinsic line-width and mass distribution, we conclude that
DR21SF is in an overall trans-critical status, which indicates an early
evolutionary stage.
