Instead, they saw a flare similar in power to a small flare from our own Sun. We didn't expect to see flaring from such a lightweight object. The dwarf then suddenly brightened at x-ray wavelengths, then faded away over the following two hours.
Astronomers hypothesize that the flare was generated by activity beneath the brown dwarf's surface.
Young brown dwarfs do have coronae, generated in part by flares similar to the one observed by Chandra that heat up the ionized gas in the dwarf's atmosphere. However, as the dwarf cools, the ions in the atmosphere recombine into neutral atoms that are unaffected by magnetic field that helps generate the flares. We've shown that older brown dwarfs don't have coronae, but the flare tells us they still have magnetic fields and also that subsurface flares occasionally punch through into the atmosphere.