We report on the first proper motion measurement in the supernova remnant RX J1713.7 - 3946 using the XMM-Newton X-ray telescope on a 13 year time interval .
This expansion measurement is carried out in the south-east region of the remnant , where two sharp filament structures are observed .
For the outermost filament , the proper motion is 0.75 ^ { +0.05 } _ { -0.06 } \pm 0.069 _ { syst } arcsec yr ^ { -1 } which is equivalent to a shock speed of \sim 3 500 km s ^ { -1 } at a distance of 1 kpc .
In contrast with the bright north-west region , where the shock is interacting with the border of the cavity , the shock in the south-east region is probably expanding in the original ambient medium carved by the progenitor and can be used to derive the current density at the shock and the age of the remnant .
In the case where the shock is evolving in a wind profile ( \rho \propto r ^ { - s } , s = 2 ) or in a uniform medium ( s = 0 ) , we estimate an age of \sim 2 300 yrs and \sim 1 800 yrs respectively for an ejecta power-law index of n = 9 .
The specific case of an ejecta power-law index of n = 7 , and s = 0 , yields an age of \sim 1 500 yrs , which would reconcile RX J1713.7 - 3946 with the historical records of SN 393 .
In all scenarios , we derive similar upstream densities of the order of 0.01 cm ^ { -3 } , compatible with the lack of thermal X-rays from the shocked ambient medium .