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 seconds and delivers 7.80 x 10-6 J of energy perpendicular to the 77-m2 surface area of the telescope's detector. The magnetar is considered to be 4.50 x 1020 m (around 50 000 light-years) from earth, and to encompass a radius of 8.10 x 103 m. Determine the magnitude of the rms magnetic field of the gamma-ray pulse at the surface of the magnetar, supposing that the pulse radiates uniformly outward in all the directions.