A Potentially New Type of Fast, Extreme X-ray Flaring

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. Here, we present two examples found serendipitously in the Chandra archive that flare in X-rays by factors of 4,000-25,000 on time scales of less than 10 seconds. The first object flared in a 2012 Chandra observation but was not discovered until 2015 and was of unknown origin until recently, but newer NIR data indicate that it could be a nearby Milky Way brown dwarf. The second object shows nearly identical X-ray flare characteristics as the first source, yet shows no optical/NIR counterpart in deep Gemini imaging, calling into question a brown dwarf explanation. Curiously, the second source lies in the direction of the Large Magellanic Cloud which would imply a peak luminosity of >10³⁹ erg/s if at the distance of the LMC. In this talk we will compare and contrast both sources and discuss the implications of such violent flares in brown dwarfs, or alternatively, a new type of X-ray flare in the LMC for the second source.