Measuring the shock speed in RX J1713.7-3946

These expansion measurements are a powerful way to probe the basic, but oftenunknown, parameters of the SNR such as age, distance and ambient medium densityin which the shock is expanding. In particular, determining the ambient densityis a crucial parameter for constraining the fraction of kinetic energytransferred to hadrons accelerated at the front shock of the SNR. In theprototypical accelerator RX J1713.73946, no thermal Xray emission (from theshocked medium) has been observed and proper motion measurement is probably thebest way to constrain the ambient density. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]