:That's the point, though  nearly all life was not killed off.  Why would a coronal mass ejection target a city, but not the area (presumably a suburb) where the man lived?  Would a gamma ray burst do that?  I doubt it  it would affect the entire surface of the earth uniformly, city, suburb, and country. 
 :We come back to the point that the story does not require a precise description.  The important part is that it happened.  That said, the attack seems to have come from a manmade source, as the cities seem to have been the focus of the disaster.  (Please sign your comments.)