A gamma-ray telescope intercepts a pulse of gamma radiation from a magnetar, a type of star with a spectacularly large magnetic field. The pulse lasts 0.24 s and delivers 8.4 × 10-6 J of energy perpendicularly to the 75-m2 surface area of the telescope's detector. The magnetar is thought to be 4.5 × 1020 m (about 50 000 light-years) from earth, and to have a radius of 9.0 × 103 m. Find the magnitude of the rms magnetic field of the gamma-ray pulse at the surface of the magnetar, assuming that the pulse radiates uniformly outward in all directions.