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 0.29 s and delivers 7.9 × 10⁻⁶ J of energy perpendicularly to the 56 m² surface area of the telescope's detector. The magnetar is thought to be 5.76 × 10²⁰ m (about 6100 light-years) from Earth, and to have a radius of 7.2 × 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. (Assume a year is 365.25 days.)