A solar super-flare as cause for the 14C variation in AD 774/5 ?
  We present further considerations regarding the strong 14C variation in AD 774/5. For its cause, either a solar super-flare or a short Gamma-Ray Burst were suggested. We show that all kinds of stellar or neutron star flares would be too weak for the observed energy input at Earth in AD 774/5. Even though Maehara et al. (2012) present two super-flares with 10e35 erg of presumably solar-type stars, we would like to caution: These two stars are poorly studied and may well be close binaries, and/or having a M-type dwarf companion, and/or may be much younger and/or much more magnetic than the Sun - in any such case, they might not be true solar analog stars. From the frequency of large stellar flares averaged over all stellar activity phases (maybe obtained only during grand activity maxima), one can derive (a limit of) the probability for a large solar flare at a random time of normal activity: We find the probability for one flare within 3000 years to be possibly as low as 0.3 to 0.008 considering the full 1 sigma error range. Given the energy estimate in Miyake et al. (2012) for the AD 774/5 event, it would need to be \sim 2000 stronger than the Carrington event as solar super-flare. If the AD 774/5 event as solar flare would be beamed (to an angle of only 24 deg), 100 times lower energy would be needed. A new AD 774/5 energy estimate by Usoskin et al. (2013) with a different carbon cycle model, yielding 4 or 6 time lower 14C production, predicts 4-6 times less energy. If both reductions are applied, the AD 774/5 event would need to be only 4 times stronger than the Carrington event in 1859 (if both had similar spectra). However, neither 14C nor 10Be peaks were found around AD 1859. Hence ... 
Ralph Neuhaeuser and Valeri Hambaryan (U Jena)
astro-ph.SR physics.hist-ph