The plasma structure of the north-east rim of the Cygnus Loop as observed with ASCA

We present the first results from Advanced Satellite for Cosmology and Astrophysics (ASCAS) observations of the Cygnus Loop. A 22 min square of the north-east rim was observed. The imaging capability of ASCA permits clear indentification of the shock front structure. The energy resolving power of ASCA in the energy band between 0.5 keV to 10 keV reveals several emission lines from oxygen to silicon. The intensity ratio between the O VIII and O VII emission lines demonstrates that the plasma has not reached collisional ionization equilibrium. A nonequilibrium ionization model is applied to determine the gradients both in electron temperature T(sub e), and in electron density n(sub e). T(sub e) ranges from 0.22 keV at the shock front to 0.30 keV at 17 min inside the shock front. n(sub e) can be represented as a power law function of radius with an index of about 6 at an ambient gas density of 0.5 cm(exp -3). The plasma parameter n(sub e)t is about 2 x 10(exp 11) cm(exp -3), resulting in an age of about 20000 yr for the Loop.