We didn't yet have the technology to notice, but Sagittarius A*, the supermassive black hole at the center of our galaxy, threw one hell of a tantrum a few hundred years ago. It's the only explanation for mysterious x-ray anomalies.
The center of the Milky Way is full of X-ray sources, many of which seem to originate from clouds that could not possible produce them. Astronomers figure that what we're really look at here are the aftershocks of a more severe event that occurred at some earlier point, before we had telescopes capable of noticing this original flare. Previous analysis of these X-ray emissions suggest that they indeed do have an earlier origin, but until now we couldn't know for sure what that was.
There are two main possibilities. If the original flare was only slightly stronger than the X-ray flares we now observe, then the initial event could have come from any ordinary binary star system, specifically one with a normal star and either a white dwarf or neutron star. Above a certain energy level - about 10^35 ergs - it becomes impossible for any single star system to create such a massive flare, and there's only one possible explanation.
Sagittarius A* (pronounced Sagittarius A-star) is the only object in the Milky Way powerful enough to create this particular X-ray phenomenon. And now new research from Kyoto University has confirmed that fact. Astrophysicist Masayoshi Nobukawa and his team determined that the behavior of the current emissions must be the echoes of a particularly severe flare, perhaps as much as 10^39 ergs. That's way, way beyond what any star system could create. The supermassive black hole is the only explanation.
The current X-ray emissions are located in molecular clouds only a few hundred light-years from Sagittarius A*, so logically the original flare must have happened a few hundred years ago, likely sometime between the 14th and 17th centuries. Sagittarius A* continues to emit a steady stream of massive X-ray bursts, but none on the scale of 10^39 ergs. In our years of observation of the black hole, 10^34 ergs is about the upper limit, and even that is relatively rare.
The question then is how rare this X-ray blast really was. It appears that it's at least several hundred years between bursts - otherwise, we'd likely already have seen evidence of a more recent one - and it might be an altogether more cosmic scale than that. Perhaps this was a once in a thousand year event, maybe even once in a million years. It's probably a lot closer in frequency to every thousand years than every million, but still - I'm guessing Galileo and Kepler are kicking themselves for missing out on this once-in-a-lifetime cosmic event back when it actually happened.