[thrash]

I haven't found a plausible mechanism for directly killing off the humans while still leaving the atmosphere intact, which is why I've been taking "sterilized" to specifically mean "subjected to lethal doses of solar UV." GRBs are too short to affect more than one hemisphere at a time, although the possibility exists for "aftershocks." A supernova event might last a full 24 hours and cover the entire globe, but in most scenarios the damage would either be limited to the upper atmosphere or strip the atmosphere entire.