A gamma-ray telescope intercepts the pulse of gamma radiation from a magnetar, a kind of star having a spectacularly big magnetic field. The pulse lasts 0.25 s and delivers 8.60*10^-6 J of energy perpendicularly to the 74-m2 surface area of telescope's detector. The magnetar is thought to be 4.50*10^20 m (around 50000 light-years) from earth, and to encompass a radius of 8.70*10^3 m. Determine the magnitude of the rms magnetic field of the gamma-ray pulse at the surface of magnetar, supposing that the pulse radiates evenly outward in all directions.