GRB 160821A is the third most energetic gamma ray burst observed by the Fermi gamma-ray space telescope .
Based on the observations made by Cadmium Zinc Telluride Imager ( CZTI ) on board AstroSat , here we report the most conclusive evidence to date of ( i ) high linear polarization ( 66 ^ { +26 } _ { -27 } \% ; 5.3 \sigma detection ) , and ( ii ) variation of its polarization angle with time happening twice during the rise and decay phase of the burst at 3.5 \sigma and 3.1 \sigma detections respectively .
All confidence levels are reported for two parameters of interest .
These observations strongly suggest synchrotron radiation produced in magnetic field lines which are highly ordered on angular scales of 1 / \Gamma , where \Gamma is the Lorentz factor of the outflow .