Fast, Extreme X-ray Flaring Found Serendipitously in X-ray Archives

The X-ray archives provide a fantastic resource for finding unusual extreme X-ray flaring from objects that were not the intended target of the pointed observation. Twenty+ years of serendipitous discoveries has led to new insight into highly energetic flaring activity in a variety of astronomical objects. While most of these serendipitous flares are from hitherto unknown X-ray active late-type Milky Way stars, there are now a growing number of more exotic sources, both Galactic and extragalactic, that flare by large factors in a short period of time. Flux changes of hundreds to tens of thousands on sub-minute time scales emanating from brown dwarfs, super-Eddington compact objects in globular clusters, supernova shock break-outs, suspected binary neutron star mergers at cosmological distances, and X-ray flares with no counterpart at any other wavelength challenge our understanding of flare generation mechanisms. Here we will present some of the more extreme and interesting cases of X-ray flaring we have found in our search of X-ray archives with a view of interpreting the flare process and what can be learned in their connection to stellar magnetic fields, compact object accretion modes, supernova shock wave physics, and compact object mergers.