 One active region on the Sun may produce several flares over its lifetime, and the effects of such a sequence would be aggregated over the one-year period covered by a single 14C measurement; however, the total effect would still be ten times greater than anything observed in a similar period in modern times.Solar flares are not the only possibility for producing the cosmogenic isotopes.A long or short gamma-ray burst has been initially proposed as a possible cause of the AD 774/5 event.